Table of contents

We examine how coupled dark matter and dark energy modify the development of Zel'dovich pancakes. We study how the various effects of these theories, such as a fifth force in the dark sector and a modified particle Hubble drag, produce variations in the redshifts of caustic formation and the present-day density profiles of pancakes. We compare our results in direct simulation to a perturbation theory approach for the dark energy scalar field. We determine the range of initial scalar field amplitudes for which perturbation theory is accurate in describing the development of the pancakes. Notably, we find that perturbative methods which neglect kinetic terms in the scalar field equation of motion are not valid for arbitrarily small perturbations. We also examine whether models that have been tuned to match the constraints of current observations can produce new observable effects in the nonlinear structure of pancakes. Our results suggest that a fully realistic three-dimensional simulation will produce significant new observable features, such as modifications to the mass function and halo radial density profile shapes, that can be used to distinguish these models from standard concordance cosmology and from each other.

The Hubble constant estimated from the combined analysis of the Sunyaev-Zel'dovich effect and X-ray observations of galaxy clusters is systematically lower than estimates from other methods by 10%-15%. We examine the origin of the systematic underestimate using an analytic model of the intracluster medium (ICM), and compare the prediction with idealistic triaxial models and with clusters extracted from cosmological hydrodynamic simulations. We identify three important sources for the systematic errors; density and temperature inhomogeneities in the ICM, departures from isothermality, and asphericity. In particular, the combination of the first two leads to the systematic underestimate of the ICM spectroscopic temperature relative to its emission-weighted one. We find that these three systematics reproduce well both the observed bias and the intrinsic dispersions of the Hubble constant estimated from the Sunyaev-Zel'dovich effect.

QUaD is a bolometric CMB polarimeter sited at the South Pole, operating at frequencies of 100 and 150 GHz. In this paper we report preliminary results from the first season of operation (austral winter 2005). All six CMB power spectra are presented derived as cross spectra between the 100 and 150 GHz maps using 67 days of observation in a low foreground region of approximately 60 deg². These data are a small fraction of the data acquired to date. The measured spectra are consistent with the ΛCDM cosmological model. We perform jackknife tests that indicate that the observed signal has negligible contamination from instrumental systematics. In addition, by using a frequency jackknife we find no evidence for foreground contamination.

Theoretical arguments and indirect observational evidence suggest that the stellar IMF may evolve with time, such that it is more weighted toward high-mass stars at higher redshift. Here we test this idea by comparing the rate of luminosity evolution of massive early-type galaxies in clusters at 0.02 ⩽ z⩽ 0.83 to the rate of their color evolution. A combined fit to the rest-frame U − V color evolution and the previously measured evolution of the M/L_B ratio gives x = − 0.3^{+ 0.4}_−0.7 for the logarithmic slope of the IMF in the region around 1 M_☉, significantly flatter than the present-day value in the Milky Way disk of x = 1.3 ± 0.3. The best-fitting luminosity-weighted formation redshift of the stars in massive cluster galaxies is 3.7^{+ 2.3}_−0.8, and a possible interpretation is that the characteristic mass m_c had a value of ~2 M_☉ at z ∼ 4 (compared to m_c ∼ 0.1 M_☉ today), in qualitative agreement with models in which the characteristic mass is a function of the Jeans mass in molecular clouds. Such a "bottom-light" IMF for massive cluster galaxies has significant implications for the interpretation of measurements of galaxy formation and evolution. Applying a simple form of IMF evolution to literature data, we find that the volume-averaged SFR at high redshift may have been overestimated (by a factor of 3-4 at z > 4), and the cosmic star formation history may have a fairly well defined peak at z ∼ 1.5. The M/L_V ratios of galaxies are less affected than their SFRs, and future data on the stellar mass density at z > 3 will provide further constraints on IMF evolution. The formal errors likely underestimate the uncertainties, and confirmation of these results requires a larger sample of clusters and the inclusion of redder rest-frame colors in the analysis.

We analyze the mean rest-frame ultraviolet (UV) spectrum of Type Ia Supernovae (SNe) and its dispersion using high signal-to-noise ratio Keck-I/LRIS-B spectroscopy for a sample of 36 events at intermediate redshift ( = 0.5) discovered by the Canada-France-Hawaii Telescope Supernova Legacy Survey (SNLS). We introduce a new method for removing host galaxy contamination in our spectra, exploiting the comprehensive photometric coverage of the SNLS SNe and their host galaxies, thereby providing the first quantitative view of the UV spectral properties of a large sample of distant SNe Ia. Although the mean SN Ia spectrum has not evolved significantly over the past 40% of cosmic history, precise evolutionary constraints are limited by the absence of a comparable sample of high-quality local spectra. The mean UV spectrum of our z≃ 0.5 SNe Ia and its dispersion is tabulated for use in future applications. Within the high-redshift sample, we discover significant UV spectral variations and exclude dust extinction as the primary cause by examining trends with the optical SN color. Although progenitor metallicity may drive some of these trends, the variations we see are much larger than predicted in recent models and do not follow expected patterns. An interesting new result is a variation seen in the wavelength of selected UV features with phase. We also demonstrate systematic differences in the SN Ia spectral features with SN light curve width in both the UV and the optical. We show that these intrinsic variations could represent a statistical limitation in the future use of high-redshift SNe Ia for precision cosmology. We conclude that further detailed studies are needed, both locally and at moderate redshift where the rest-frame UV can be studied precisely, in order that future missions can confidently be planned to fully exploit SNe Ia as cosmological probes.

We investigate the stellar populations of a sample of 162 Lyα-emitting galaxies (LAEs) at z = 3.1 in the Extended Chandra Deep Field South, using deep Spitzer IRAC data available from the GOODS and SIMPLE surveys to derive reliable stellar population estimates. We divide the LAEs according to their rest-frame near-IR luminosities into IRAC-detected and IRAC-undetected samples. About 70% of the LAEs are undetected in 3.6 μm down to m_3.6 = 25.2 AB. Stacking analysis reveals that the average stellar population of the IRAC-undetected sample has an age of ~200 Myr and a mass of ~3 × 10⁸M_☉, consistent with the expectation that LAEs are mostly young and low-mass galaxies. On the other hand, the IRAC-detected LAEs are on average significantly older and more massive, with an average age ≳1 Gyr and mass ~10¹⁰M_☉. Comparing the IRAC colors and magnitudes of the LAEs to z ∼ 3 Lyman break galaxies (LBGs) shows that the IRAC-detected LAEs lie at the faint blue end of the LBG color-magnitude distribution, suggesting that IRAC-detected LAEs may be the low-mass extension of the LBG population. We also present tentative evidence for a small fraction (~5%) of obscured AGNs within the LAE sample. Our results suggest that LAEs posses a wide range of ages and masses. In addition, the presence of evolved stellar populations inside LAEs suggests that the Lyα-luminous phase of galaxies may be either a long-lasting or recurring phenomenon.

The phenomenon of cosmic shear, or distortion of images of distant sources unaccompanied by magnification, is an effective way of probing the content and state of the foreground universe, because light rays do not have to pass through matter clumps in order to be sheared. It is shown that the delay in the arrival times between two simultaneously emitted photons that appear to be arriving from a pair of images of a strongly lensed cosmological source contains not only information about the Hubble constant, but also the long-range gravitational effect of galactic-scale mass clumps located away from the light paths in question. This is therefore also a method of detecting shear. Data on time delays among a sample of strongly lensed sources can provide crucial information about whether extra dynamics beyond gravity and dark energy are responsible for the global flatness of space. If the standard ΛCDM model is correct, there should be a large dispersion in the value of H₀ as inferred from the delay data by (the usual procedure of) ignoring the shear from all other mass clumps except the strong lens itself. The fact that there has not been any report of a significant deviation from the h = 0.7 mark during any of the H₀ determinations by this technique may already be pointing to the absence of the random effect discussed here.

We present Hubble Space Telescope ACS images of 13 dust-reddened type 1 quasars selected from the FIRST/2MASS Red Quasar Survey. These quasars have high intrinsic luminosities after correction for dust obscuration (–23.5 ⩾ M_B ⩾ − 26.2 from K-magnitude). The images show strong evidence of recent or ongoing interaction in 11 of the 13 cases, even before the quasar nucleus is subtracted. None of the host galaxies are well fit by a simple elliptical profile. The fraction of quasars showing interaction is significantly higher than the 30% seen in samples of host galaxies of normal, unobscured quasars. There is a weak correlation between the amount of dust reddening and the magnitude of interaction in the host galaxy, measured using the Gini coefficient and the concentration index. Although few host galaxy studies of normal quasars are matched to ours in intrinsic quasar luminosity, no evidence has been found for a strong dependence of merger activity on host luminosity in samples of the host galaxies of normal quasars. We thus believe that the high merger fraction in our sample is related to their obscured nature, with a significant amount of reddening occurring in the host galaxy. The red quasar phenomenon seems to have an evolutionary explanation, with the young quasar spending the early part of its lifetime enshrouded in an interacting galaxy. This might be further indication of a link between AGNs and starburst galaxies.

We present three-dimensional hydrodynamical simulations of gas flows in the vicinity of an active galactic nucleus (AGN) powered by a precessing accretion disk. We consider the effects of the radiation force from such a disk on its environment on a relatively large scale (up to ~10 pc). We implicitly include the precessing disk by forcing the disk radiation field to precess around a symmetry axis with a given period (P) and a tilt angle (Θ). We study the time evolution of the flows irradiated by the disk and investigate basic dependencies of the flow morphology, mass flux, and angular momentum on different combinations of Θ and P. As this is our first attempt to model such three-dimensional gas flows, we consider a simplest form of radiation force, i.e., force due to electron scattering, and neglect the forces due to line and dust scattering/absorption. Furthermore, the gas is assumed to be nearly isothermal. We find that the gas flow settles into a configuration with two components, (1) an equatorial inflow and (2) a bipolar inflow/outflow, with the outflow leaving the system along the poles (the directions of disk normals). However, the flow does not always reach a steady state. We find that the maximum outflow velocity and the kinetic outflow power at the outer boundary can be reduced significantly with increasing Θ. We also find that the mass inflow rate across the inner boundary does not change significantly with increasing Θ. The amount of the density-weighted mean specific angular momentum deposited in the environment by the precessing disk increases as P approaches the gas free-fall time (t_ff) and then decreases as P becomes much larger than t_ff. Generally, the characteristics of the flows are closely related to a combination of P and Θ, but not to P and Θ individually. Our models exhibit helical structures in the weakly collimated outflows. Although on different scales, the model reproduces the - or -shaped density morphology of gas outflows, which are often seen in radio observations of AGNs.

We use a complete sample of active galactic nuclei (AGN) selected on the basis of relativistically beamed 15 GHz radio flux density (MOJAVE: Monitoring of Jets in AGN with VLBA Experiments) to derive the parent radio luminosity function (RLF) of bright radio-selected blazar cores. We use a maximum likelihood method to fit a beamed RLF to the observed data and thereby recover the parameters of the intrinsic (unbeamed) RLF. We analyze two subsamples of the MOJAVE sample: the first contains only objects of known FR II class, with a total of 103 sources, and the second subsample adds 24 objects of uncertain FR class for a total of 127 sources. Both subsamples exclude four known FR I radio galaxies and two gigahertz-peaked spectrum sources. We obtain good fits to both subsamples using a single power law intrinsic RLF and a pure density evolution function of the form z^mexp{−1/2[(z − z₀)/σ]²}. We find that a previously reported break in the observed MOJAVE RLF actually arises from using incomplete bins (because of the luminosity cutoff) across a steep and strongly evolving RLF, and does not reflect a break in the intrinsic RLF. The derived space density of the parent population of the FR II sources from the MOJAVE sample (with L_{15 GHz} ⩾ 1.3 × 10²⁵ W Hz⁻¹) is approximately 1.6 × 10³ Gpc ⁻³.

Luminous X-ray outbursts with variability amplitudes as high as ~1000 have been detected from a small number of galactic nuclei. These events are likely associated with transient fueling of nuclear supermassive black holes. In this paper we constrain X-ray outbursts with harder spectra, higher redshifts, and lower luminosities than have been studied previously. We performed a systematic survey of 24,668 optical galaxies in the Chandra Deep Fields to search for such X-ray outbursts; the median redshift of these galaxies is ~0.8. The survey spans 798 days for the Chandra Deep Field-North, and 1828 days for the Chandra Deep Field-South. No outbursts were found, and thus we set upper limits on the rate of such events in the universe, which depend on the adopted outburst X-ray luminosity. For an outburst with X-ray luminosity ≳10⁴³ ergs s⁻¹ and a duration of 6 months, the upper limit on its event rate is ~10⁻⁴ galaxy⁻¹ yr⁻¹, roughly consistent with theoretical predictions. Compared to previous survey results, our harder band and deeper survey suggests that the outburst rate may increase by a maximum factor of 10 when considering both obscured X-ray outbursts and redshift evolution from z ∼ 0 to z ∼ 0.8. Our results also suggest that the X-ray luminosity function for moderate-luminosity active galactic nuclei is not primarily due to stellar tidal disruptions.

We report on the detection of Fe Kα emission in F04103–2838, an ultraluminous infrared galaxy [ULIRG; log (L_IR/L_☉) ⩾ 12] optically classified as a LINER. Previous Chandra observations suggested the presence of both a starburst and an active galactic nucleus (AGN) in this source. A deeper (~20 ks) XMM-Newton spectrum reveals an Fe Kα line at rest-frame energy ~6.4 keV, which is consistent with cold neutral iron. The best-fit spectral model indicates that the Fe Kα line has an equivalent width of ~1.6 keV. The hard X-ray emission is dominated by a Compton-thick AGN with an intrinsic 0.2-10 keV luminosity of ~10⁴⁴ ergs s⁻¹, while the soft X-ray emission is from ~0.1 keV gas attributed to the starburst. The X-ray spectrum of this source bears a striking resemblance to that of the archetypal luminous infrared galaxy NGC 6240, despite differences in merger state and infrared properties.

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC 6872/IC 4970 in the Pavo galaxy group and show that the smaller companion galaxy IC 4970 hosts a highly obscured active galactic nucleus (AGN). The 0.5-10 keV X-ray luminosity of the nucleus is variable, increasing by a factor 2.9 to a luminosity of 1.7 × 10⁴² erg s⁻¹ (bright state) on ~100 ks timescales. The X-ray spectrum of the bright state is heavily absorbed (N_H = 3 × 10²³ cm⁻² for power-law models with Γ = 1.5–2.0) and shows a clear 6.4 keV Fe Kα line with equivalent width of 144-195 eV. Limits on the diffuse emission in IC 4970 from Chandra X-ray data suggest that the available power from Bondi accretion of IC 4970's hot interstellar gas may be an order of magnitude too small to power the AGN. Spitzer images show that 8 μm nonstellar emission is concentrated in the central 1 kpc of IC 4970, consistent with high obscuration in this region. The mid-infrared colors of the nucleus are consistent with those expected for a highly obscured AGN. Taken together these data suggest that the nucleus of IC 4970 is a Seyfert 2, triggered and fueled by cold material supplied to the central supermassive black hole as a result of the off-axis collision of IC 4970 with the cold-gas-rich spiral galaxy NGC 6872.

Motivated by the increasing use of the Kennicutt-Schmidt (K-S) star formation law to interpret observations of high-redshift galaxies, the importance of gas accretion to galaxy formation, and the recent observations of chemical abundances in galaxies at z ∼ 2-3, I use simple analytical models to assess the consistency of these processes of galaxy evolution with observations and with each other. I derive the time dependence of star formation implied by the K-S law and show that the sustained high star formation rates observed in galaxies at z ∼ 2-3 require the accretion of additional gas. A model in which the gas accretion rate is approximately equal to the combined star formation and outflow rates broadly reproduces the observed trends of star formation rate with galaxy age. Using an analytical description of chemical evolution, I also show that this model, further constrained to have an outflow rate roughly equal to the star formation rate, reproduces the observed mass-metallicity relation at z ∼ 2.

We have performed a series of three-dimensional simulations of a starburst-driven wind in an inhomogeneous interstellar medium. The introduction of an inhomogeneous disk leads to differences in the formation of the wind, most noticeably the absence of the "blow-out" effect seen in homogeneous models. A wind forms from a series of small bubbles that propagate into the tenuous gas between dense clouds in the disk. These bubbles merge and follow the path of least resistance out of the disk before flowing freely into the halo. Filaments are formed from disk gas that is broken up and accelerated into the outflow. These filaments are distributed throughout a biconical structure within a more spherically distributed hot wind. The distribution of the inhomogeneous interstellar medium in the disk is important in determining the morphology of this wind, as well as the distribution of the filaments. While higher resolution simulations are required in order to ascertain the importance of mixing processes, we find that soft X-ray emission arises from gas that has been mass-loaded from clouds in the disk, as well as from bow shocks upstream of the clouds driven into the flow by the ram pressure of the wind, and the interaction between these shocks.

Luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs) dominate the star formation rate budget of the universe at z≳ 1, yet no local measurements of their heavy-element abundances exist. We measure nuclear or near-nuclear oxygen abundances in a sample of 100 star-forming LIRGs and ULIRGs using new, previously published, and archival spectroscopy of strong emission lines (including [O II] λλ3726, 3729) in galaxies with redshifts ⟨ z⟩ ∼ 0.1. When compared to local emission-line galaxies of similar luminosity and mass (using the near-infrared luminosity-metallicity and mass-metallicity relations), we find that LIRGs and ULIRGs are underabundant by a factor of 2 on average. As a corollary, LIRGs and ULIRGs also have smaller effective yields. We conclude that the observed underabundance results from the combination of a decrease of abundance with increasing radius in the progenitor galaxies and strong, interaction- or merger-induced gas inflow into the galaxy nucleus. This conclusion demonstrates that local abundance scaling relations are not universal, a fact that must be accounted for when interpreting abundances earlier in the universe's history, when merger-induced star formation was the dominant mode. We use our local sample to compare to high-redshift samples and assess abundance evolution in LIRGs and ULIRGs. We find that abundances in these systems increased by ~0.2 dex from z ∼ 0.6 to z ∼ 0.1. Evolution from z ∼ 2 submillimeter galaxies to z ∼ 0.1 ULIRGs also appears to be present, although uncertainty due to spectroscopic limitations is large.

The strong spectral features near 2.2 μm in early-type galaxies remain relatively unexplored. Yet, they open a tightly focused window on the coolest giant stars in these galaxies—a window that can be used to explore both age and metallicity effects. Here, new measurements of K-band spectral features are presented for 11 early-type galaxies in the nearby Fornax galaxy cluster. Based on these measurements, the following conclusions have been reached: (1) in galaxies with no signatures of a young stellar component, the K-band Na I index is highly correlated with both the optical metallicity indicator [MgFe]' and the central velocity dispersion σ; (2) in the same galaxies, the K-band Fe features saturate in galaxies with σ > 150 km s⁻¹, while Na I (and [MgFe]') continues to increase; (3) [Si/Fe] (and possibly [Na/Fe]) is larger in all observed Fornax galaxies than in Galactic open clusters with near-solar metallicity; (4) in various near-IR diagnostic diagrams, galaxies with signatures of a young stellar component (strong Hβ, weak [MgFe]') are clearly separated from galaxies with purely old stellar populations; furthermore, this separation is consistent with the presence of an increased number of M-giant stars (most likely to be thermally pulsating AGB stars); (5) the near-IR Na I versus σ or ⟨Fe I⟩ versus σ diagrams discussed here seem as efficient for detecting putatively young stellar components in early-type galaxies as the more commonly used age/metallicity diagnostic plots using optical indices (e.g., Hβ vs. [MgFe]'). The combination of these spectral indices near 2.2 μm with high spatial resolution spectroscopy from ground-based or space-based observatories promises to provide new insights into the nature of stellar populations in the central regions of distant early-type galaxies.

We have obtained an XMM-Newton spectrum of the diffuse X-ray emission toward (ℓ ,b) = (111.14°,1.11°) , a line of sight with a relatively simple distribution of absorbing clouds: >9 × 10¹⁹ cm⁻² at R > 170 pc, a 6 × 10²¹ cm⁻² molecular cloud at 2.5-3.3 kpc, and a total column of 1.2 × 10²² cm⁻². We find that the analysis of the XMM-Newton spectrum in conjunction with the ROSAT All-Sky Survey (RASS) spectral energy distribution for the same direction requires three thermal components to be well fit: a "standard" Local Hot Bubble component with kT = 0.095, a component beyond the molecular cloud with kT = 0.57, and a component before the molecular cloud with kT = 0.24. The strength of the O VII 0.56 keV line from the Local Hot Bubble, 1.75 ± 0.7 photons cm⁻² s⁻¹ sr⁻¹, is consistent with other recent measures. The 0.24 keV component has an emission measure of 0.0021 ± 0.0006 cm⁻⁶ pc and is not localized save as diffuse emission within the Galactic plane. Rough calculations show that only about a quarter of this component is due to unresolved stellar emission; it is the best candidate for a pervasive hot medium. The spatial separation of the ~0.2 keV component from the ~0.6 keV component suggests that the spectral decompositions of the emission from late-type spiral disks found in the literature do represent real temperature components rather than reflecting more complex temperature distributions.

A halo model is presented that possesses a constant phase-space density (Q) core followed by a radial CDM-like power-law decrease in Q. The motivation for the core is the allowance for a possible primordial phase-space density limit such as the Tremaine-Gunn upper bound. The space density profile derived from this model has a constant-density core and falls off rapidly beyond it. The new model is shown to improve the fits to the observations of LSB galaxy rotation curves, naturally provides a model that has been shown to result in a lengthened dynamical friction timescale for the Fornax dwarf spheroidal galaxy, and predicts a flattening of the density profile within the Einstein radius of galaxy clusters. A constant gas entropy floor is predicted whose adiabatic constant provides a lower limit in accord with observed galaxy cluster values. While "observable-sized" cores are not seen in standard cold dark matter (CDM) simulations, phase-space considerations suggest that they could appear in warm dark matter (WDM) cosmological simulations and in certain hierarchically consistent SuperWIMP scenarios.

Ongoing accretion onto galactic disks has been recently theorized to progress via the unstable cooling of the baryonic halo into condensed clouds. These clouds have been identified as analogous to the high-velocity clouds (HVCs) observed in H I in our Galaxy. Here we compare the distribution of HVCs observed around our own Galaxy and extraplanar gas around the Andromeda galaxy to these possible HVC analogs in a simulation of galaxy formation that naturally generates these condensed clouds. We find a very good correspondence between these observations and the simulation in terms of number, angular size, velocity distribution, overall flux, and flux distribution of the clouds. We show that condensed cloud accretion accounts for only ~0.2 M_☉ yr⁻¹ of the current overall Galactic accretion in the simulations. We also find that the simulated halo clouds accelerate and become more massive as they fall toward the disk. The parameter space of the simulated clouds is consistent with all of the observed HVC complexes that have distance constraints, except the Magellanic Stream, which is known to have a different origin. We also find that nearly half of these simulated halo clouds would be indistinguishable from lower velocity gas and that this effect is strongest farther from the disk of the galaxy, thus indicating a possible missing population of HVCs. These results indicate that the majority of HVCs are consistent with being infalling, condensed clouds that are a remnant of Galaxy formation.

Absolute proper motions for six new globular clusters have recently been determined. This motivated us to obtain the Galactic orbits of these six clusters both in an axisymmetric Galactic potential and in a barred potential, such as that of our Galaxy. Orbits are also obtained for a Galactic potential that includes spiral arms. The orbital characteristics are compared and discussed for these three cases. Tidal radii and destruction rates are also computed and discussed.

We have obtained velocity-resolved spectra of the H₂ 1-0 S(1) (λ = 2.1218 μm) emission line at 2^'' angular resolution (or ~0.08 pc spatial resolution) in four regions within the central 10 pc of the Galaxy where the supernova-like remnant Sgr A East is colliding with molecular clouds. To investigate the kinematic, physical, and positional relationships between the important gaseous components in the center, we compared the H₂ data cube with previously published NH₃ data. The projected interaction boundary of Sgr A East is determined to be an ellipse with its center offset ~1.5 pc from Sgr A* and dimensions of 10.8 pc × 7.6 pc . This H₂ boundary is larger than the synchrotron emission shell but consistent with the dust ring, which is believed to trace the shock front of Sgr A East. Since Sgr A East is driving shocks into its nearby molecular clouds, we can determine their positional relationships using the shock directions as indicators. As a result, we suggest a revised model for the three-dimensional structure of the central 10 pc. The actual contact between Sgr A East and all of the surrounding molecular material, including the circumnuclear disk and the southern streamer, makes the hypothesis of infall into the nucleus and feeding of Sgr A* very likely.

We apply a wind model, driven by combined cosmic-ray and thermal-gas pressure, to the Milky Way, and show that the observed Galactic diffuse soft X-ray emission can be better explained by a wind than by previous static gas models. We find that cosmic-ray pressure is essential to driving the observed wind. Having thus defined a "best-fit" model for a Galactic wind, we explore variations in the base parameters and show how the wind's properties vary with changes in gas pressure, cosmic-ray pressure, and density. We demonstrate the importance of cosmic rays in launching winds, and the effect cosmic rays have on wind dynamics. In addition, this model adds support to the hypothesis of Breitschwerdt and collaborators that such a wind may help to explain the relatively small gradient observed in γ-ray emission as a function of galactocentric radius.

Over the last decade, considerable effort has been made to measure the proper motions of the pulsars B1757–24 and B1951+32 in order to establish or refute associations with nearby supernova remnants and to understand better the complicated geometries of their surrounding nebulae. We present proper motion measurements of both pulsars with the Very Large Array, increasing the time baselines of the measurements from 3.9 yr to 6.5 yr and from 12.0 yr to 14.5 yr, respectively, compared to previous observations. We confirm the nondetection of proper motion of PSR B1757–24, and our measurement of (μ_α,μ_δ) = (− 11 ± 9, − 1 ± 15) mas yr⁻¹ confirms that the association of PSR B1757–24 with SNR G5.4–1.2 is unlikely for the pulsar characteristic age of 15.5 kyr, although an association cannot be excluded for a significantly larger age. For PSR B1951+32, we measure a proper motion of (μ_α,μ_δ) = (− 28.8 ± 0.9, − 14.7 ± 0.9) mas yr⁻¹, reducing the uncertainty in the proper motion by a factor of 2 compared to previous results. After correcting to the local standard of rest, the proper motion indicates a kinetic age of ~51 kyr for the pulsar, assuming it was born near the geometric center of the supernova remnant. The radio-bright arc of emission along the pulsar proper motion vector shows time-variable structure, but moves with the pulsar at an approximately constant separation ~2.5^'', lending weight to its interpretation as a shock structure driven by the pulsar.

We present the angular distribution of gamma rays produced by proton-proton interactions in parameterized formulae to facilitate calculations in astrophysical environments. The parameterization is derived from Monte Carlo simulations of the up-to-date proton-proton interaction model and its extension by Kamae and coworkers. This model includes the logarithmically rising inelastic cross section, the diffraction dissociation process, and the Feynman scaling violation. The extension adds two baryon resonance contributions: one representing the Δ(1232) and the other representing multiple resonances around 1600 MeV/c². We demonstrate the use of the formulae by calculating the predicted gamma-ray spectrum for two different cases: the first is a pencil beam of protons following a power law, and the second is a fanned proton jet with a Gaussian intensity profile impinging on the surrounding material. In both cases we find the predicted gamma-ray spectrum to be dependent on the viewing angle.

We have used the Arecibo telescope to measure the H I absorption spectra of eight pulsars. We show how kinematic distance measurements depend on the values of the Galactic constants R₀ and Θ₀, and we select our preferred current values from the literature. We then derive kinematic distances for the low-latitude pulsars in our sample and electron densities along their lines of sight. We combine these measurements with all others in the inner Galactic plane visible from Arecibo to study the electron density in this region. The electron density in the interarm range 48° < l < 70° is cm⁻³. This is of the value calculated by the Galactic electron density model of Cordes & Lazio. The model agrees more closely with electron density measurements toward Arecibo pulsars lying closer to the Galactic center, at 30° < l < 48°. Our analysis leads to the best current estimate of the distance of the relativistic binary pulsar B1913+16: d = 9.0 ± 3 kpc. We use the high-latitude pulsars to search for small-scale structure in the interstellar hydrogen observed in absorption over multiple epochs. PSR B0301+19 exhibited significant changes in its absorption spectrum over 22 yr, indicating H I structure on a ~500 AU scale.

We present VLBA observations of the Zeeman effect in H₂O masers in the high-mass star-forming region OH 43.8-0.1, where we observed 116 maser features. These masers may be arranged in several groups: the most prominent are an arc-shaped structure to the north, a central cluster, two groups located symmetrically around the central cluster to its northeast and southwest, and a group in the extreme south. The highest velocity (redshifted) masers are in the center of the northern arc. The observed morphology of masers in OH 43.8-0.1 suggests a stellar object (or objects) located within the central cluster of masers, driving outflows to the north and south; the redshifted and blueshifted group in the northern arc may represent the leading edge of two or more such outflows. The two groups located symmetrically around the central cluster may suggest a circumstellar disk of diameter 3000 AU. Seven masers in OH 43.8-0.1 are above our Zeeman detection limit. We detected magnetic fields in the range 10-20 mG in four of these masers and imposed sensitive upper limits on the other three. Three detections are for masers in the northern arc; the fourth is in the central cluster. We find no significant difference between the magnetic field strengths in these two groups. In the northern arc we detect a magnetic field reversal over a scale as small as 170 AU. We use our Zeeman-effect results to examine connections between the pre- and postshock magnetic fields and densities. The predicted preshock magnetic field strength and density are consistent with the fields and densities observed in typical preshock regions. The predicted postshock density also appears to be in the regime for optimal H₂O maser pumping. Finally, we find that the magnetic and kinetic energy densities are likely in equilibrium in both pre- and postshock regions, meaning that the magnetic field must affect significantly the outflow dynamics.

We present the results of a detailed study of interstellar polarization efficiency (as measured by the ratio p_λ/τ_λ) toward molecular clouds, with the aim of discriminating between grain alignment mechanisms in dense regions of the interstellar medium. The data set includes both continuum measurements in the K (2.2 μm) passband and values based on ice and silicate spectral features. Background field stars are used to probe polarization efficiency in quiescent regions of dark clouds, yielding a dependence on visual extinction well-represented by a power law (p_λ/τ_λ ∝ [ A_V]^−0.52), in agreement with previous work. No significant change in this behavior is observed in the transition region between the diffuse outer layers and dense inner regions of clouds, where icy mantles are formed, and we conclude that mantle formation has little or no effect on the efficiency of grain alignment. The field-star data are used as a template for comparison with results for embedded young stellar objects (YSOs). The latter generally exhibit greater polarization efficiency compared with field stars at comparable extinctions, some displaying enhancements in p_λ/τ_λ by factors of up to ~6 with respect to the power-law fit. Of the proposed alignment mechanisms, that based on radiative torques appears best able to explain the data. The attenuated external radiation field appears adequate to account for the observed polarization in quiescent regions for extinctions up to A_V ∼ 10 mag. Radiation from the embedded stars themselves may enhance alignment in the lines of sight to YSOs. Enhancements in p_λ/τ_λ observed in the ice features toward several YSOs are of greatest significance, as they demonstrate efficient alignment in cold molecular clouds associated with star formation.

We study numerically the formation of molecular clouds in large-scale colliding flows including self-gravity. The models emphasize the competition between the effects of gravity on global and local scales in an isolated cloud. Global gravity builds up large-scale filaments, while local gravity, triggered by a combination of strong thermal and dynamical instabilities, causes cores to form. The dynamical instabilities give rise to a local focusing of the colliding flows, facilitating the rapid formation of massive protostellar cores of a few hundred M_☉. The forming clouds do not reach an equilibrium state, although the motions within the clouds appear to be comparable to virial. The self-similar core mass distributions derived from models with and without self-gravity indicate that the core mass distribution is set very early on during the cloud formation process, predominantly by a combination of thermal and dynamical instabilities rather than by self-gravity.

We report the discovery of a pre-main-sequence (PMS), low-mass, double-lined, spectroscopic, eclipsing binary in the Orion star-forming region. We present our observations, including radial velocities derived from optical high-resolution spectroscopy, and present an orbit solution that permits the determination of precise empirical masses for both components of the system. We find that Par 1802 is composed of two equal-mass (0.39 ± 0.03, 0.40 ± 0.03 M_☉) stars in a circular, 4.7 day orbit. There is strong evidence, such as the system exhibiting strong Li lines and a center-of-mass velocity consistent with cluster membership, that this system is a member of the Orion star-forming region and quite possibly the Orion Nebula Cluster, and therefore has an age of only a few million years. As there are currently only a few empirical mass and radius measurements for low-mass, PMS stars, this system presents an interesting test for the predictions of current theoretical models of PMS stellar evolution.

We present a comprehensive analysis of structure in the young, embedded cluster, NGC 1333 using members identified with Spitzer and 2MASS photometry based on their IR-excess emission. A total of 137 members are identified in this way, composed of 39 protostars and 98 more evolved pre-main-sequence stars with disks. Of the latter class, four are transition/debris disk candidates. The fraction of exposed pre-main-sequence stars with disks is 83% ± 11% , showing that there is a measurable diskless pre-main-sequence population. The sources in each of the Class I and II evolutionary states are shown to have very different spatial distributions relative to the distribution of the dense gas in their natal cloud. However, the distribution of nearest neighbor spacings among these two groups of sources are found to be quite similar, with a strong peak at spacings of 0.045 pc. Radial and azimuthal density profiles and surface density maps computed from the identified YSOs show that NGC 1333 is elongated and not strongly centrally concentrated, confirming previous claims in the literature. We interpret these new results as signs of a low velocity dispersion, extremely young cluster that is not in virial equilibrium.

We present the results of the analysis of neutrino observations by the Antarctic Muon and Neutrino Detector Array (AMANDA) correlated with photon observations of more than 400 gamma-ray bursts (GRBs) in the northern hemisphere from 1997 to 2003. During this time period, AMANDA's effective collection area for muon neutrinos was larger than that of any other existing detector. After the application of various selection criteria to our data, we expect ~1 neutrino event and <2 background events. Based on our observations of zero events during and immediately prior to the GRBs in the data set, we set the most stringent upper limit on muon neutrino emission correlated with GRBs. Assuming a Waxman-Bahcall spectrum and incorporating all systematic uncertainties, our flux upper limit has a normalization at 1 PeV of E²Φ_ν ⩽ 6.3 × 10⁻⁹ GeV cm⁻² s⁻¹ sr⁻¹, with 90% of the events expected within the energy range of ~10 TeV to ~3 PeV. The impact of this limit on several theoretical models of GRBs is discussed, as well as the future potential for detection of GRBs by next-generation neutrino telescopes. Finally, we briefly describe several modifications to this analysis in order to apply it to other types of transient point sources.

We present spectral fits to five epochs of the typical Type Ib supernova 1999dn using the generalized, non-LTE, stellar atmospheres code PHOENIX. Our goal is threefold: to determine basic physical properties of the supernova ejecta, such as velocity, temperature, and density gradients; to reproduce He I absorption lines by invoking nonthermal excitation; and to investigate possible spectral signatures of hydrogen, especially a feature around 6200 Å, which has been attributed to high-velocity Hα. Our models assume an atmosphere with uniform composition devoid of any hydrogen. Our model spectra fit the observed spectra well, successfully reproducing most of the features, including the prominent He I absorptions. The most plausible alternative to Hα as the source of the 6200 Å feature is a blend of Fe II and Si II lines, which can be made stronger in order to better fit the observed feature by increasing the metallicity of the ejecta. High-metallicity models fit well at early epochs, but not as well as solar-metallicity models after maximum light. While this blend of metal lines is a reasonable explanation of the source of the 6200 Å feature, it is still important to investigate hydrogen as the source; therefore, a second paper will present models that include a thin shell of hydrogen around the main composition structure.

We study the long term evolution of magnetic fields generated by an initially unmagnetized collisionless relativistic e⁺e⁻ shock. Our two-dimensional particle-in-cell numerical simulations show that downstream of such a Weibel-mediated shock, particle distributions are approximately isotropic, relativistic Maxwellians, and the magnetic turbulence is highly intermittent spatially. The nonpropagating magnetic fields decay in amplitude and do not merge. The fields start with magnetic energy density ~ 0.1-0.2 of equipartition, but rapid downstream decay drives the fields to much smaller values, below ~10⁻³ of equipartition after ~10³ skin depths. To construct a theory to follow field decay to these smaller values, we hypothesize that the observed damping is a variant of Landau damping. The model is based on the small value of the downstream magnetic energy density, which only weakly perturbs particle orbits, for homogeneous turbulence. Using linear kinetic theory, we find a simple analytic form for the damping rates for small-amplitude, subluminous electromagnetic fields. Our theory predicts that overall magnetic energy decays as (ω_pt)^−q with q ∼ 1, which compares with simulations. However, our theory predicts overly rapid damping of short-wavelength modes. Magnetic trapping of particles within the highly spatially intermittent downstream magnetic structures may be the origin of this discrepancy and may allow for some of this initial magnetic energy to persist. Absent additional physical processes that create longer wavelength, more persistent fields, we conclude that initially unmagnetized relativistic shocks in electron-positron plasmas are unable to form persistent downstream magnetic fields. These results put interesting constraints on synchrotron models for the prompt and afterglow emission from GRBs. We also comment on the relevance of these results for relativistic electron-ion shocks.

We describe the results of numerical ``2.5-dimensional" MHD simulations of an initially unmagnetized disk orbiting a central point mass and responding to the continual generation of poloidal magnetic field by a secular source emulating the Poynting-Robertson (PR) drag on electrons in the vicinity of a luminous stellar or compact accreting object. The secular PR term has dual purpose both as the magnetic field source and the trigger of magnetorotational instability (MRI). The disk and surrounding hotter atmosphere fluids have finite resistivity, allowing the magnetic field to diffuse out of its generation sites, while at the same time the disk differential rotation twists the poloidal field and induces a substantial toroidal-field component. For moderate disk resistivity (diffusion timescales up to ~16 local dynamical times) and after ~100 orbits, the MRI allows the fluid of the disk inner edge along with its magnetic flux to flow toward the central point mass where a new, magnetized, nuclear disk forms. The toroidal field in this nuclear disk is amplified by differential rotation and, when near equipartition, unwinds vertically, producing episodic jetlike outflows. For diffusion times longer than the flow time, the poloidal field in the inner region cannot diffuse out; it grows linearly in time, as the outer sections of new poloidal loops are drawn outward by the MRI while their inner sections continue to accumulate onto the compact inner disk. However, for low resistivity (diffusion timescales larger than ~16 local dynamical times), the inflowing matter does not form a nuclear disk or jets and the linear growth of the poloidal magnetic field is interrupted after ~20 orbits because of magnetic reconnection and asymmetric outflows.

We investigate the behavior of the magnetic Prandtl number (the ratio of microscopic viscosity to resistivity) for accretion sources. Generally this number is very small in standard accretion disk models, but it can become larger than unity within ~50 Schwarzschild radii of the central mass. Recent numerical investigations suggest a marked dependence of the level of MHD turbulence on the value of the Prandtl number. Hence, black hole and neutron star accretors, i.e., compact X-ray sources, are affected. The astrophysical consequences of this could be significant, including a possible route to understanding the mysterious state changes that have long characterized these sources.

Using Suzaku observations of three neutron star low-mass X-ray binaries (Ser X–1, 4U 1820–30, and GX 349+2) we have found broad, asymmetric, relativistic Fe K emission lines in all three objects. These Fe K lines can be well fit by a model for lines from a relativistic accretion disk ("diskline"), allowing a measurement of the inner radius of the accretion disk and hence an upper limit on the neutron star radius. These upper limits correspond to 14.5-16.5 km for a 1.4 M_☉ neutron star. The inner disk radii that we measure with Fe K lines are in good agreement with the inner disk radii implied by kHz QPOs observed in both 4U 1820–30 and GX 349+2, supporting the inner disk origin for kHz QPOs. In addition, the Fe K lines observed in these neutron stars are narrower than those in the black holes that are thought to be close to maximally spinning, as one would expect if inferences for spin are robust.

The nature of the excess near-infrared emission associated with the magnetic white dwarf commonly known as SDSS 1212 is investigated primarily through spectroscopy, and also via photometry. The inferred low-mass secondary in this system has been previously detected by the emission and variation of Hα, and the 1-2.5 μm spectral data presented here are consistent with the presence of a late L or early T dwarf. The excess flux seen beyond 1.5 μm in the phase-averaged spectrum is adequately modeled with an L8 dwarf substellar companion and cyclotron emission in a 7 MG magnetic field. This interesting system manifests several observational properties typical of polars, and is most likely an old interacting binary with a magnetic white dwarf and a substellar donor in an extended low state.

This paper presents the results of a Spitzer IRAC 3-8 μm photometric search for warm dust orbiting 17 nearby, metal-rich white dwarfs, 15 of which apparently have hydrogen-dominated atmospheres (type DAZ). G166-58, G29-38, and GD 362 manifest excess emission in their IRAC fluxes and the latter two are known to harbor dust grains warm enough to radiate detectable emission at near-infrared wavelengths as short as 2 μm. Their IRAC fluxes display differences compatible with a relatively larger amount of cooler dust at GD 362. G166-58 is presently unique in that it appears to exhibit excess flux only at wavelengths longer than about 5 μm. Evidence is presented that this mid-infrared emission is most likely associated with the white dwarf, indicating that G166-58 bears circumstellar dust no warmer than T ∼ 400 K. The remaining 14 targets reveal no reliable mid-infrared excess, indicating the majority of DAZ stars do not have warm debris disks sufficiently opaque to be detected by IRAC.

M85 Optical Transient 2006-1 (M85 OT 2006-1) is the most luminous member of the small family of V838 Mon-like objects, whose nature is still a mystery. This event took place in the Virgo Cluster of galaxies and peaked at an absolute magnitude of M_I ≈ − 13. Here we present Hubble Space Telescope images of M85 OT 2006-1 and its environment, taken before and after the eruption, along with a spectrum of the host galaxy at the transient location. We find that the progenitor of M85 OT 2006-1 was not associated with any star-forming region. The g- and z-band absolute magnitudes of the progenitor were fainter than about –4 and –6 mag, respectively. Therefore, we can set a lower limit of ~50 Myr on the age of the youngest stars at the location of the progenitor that corresponds to a mass of <7 M_☉. Previously published line indices suggest that M85 has a mean stellar age of 1.6 ± 0.3 Gyr. If this mean age is representative of the progenitor of M85 OT 2006-1, then we can further constrain its mass to be less than 2 M_☉. We compare the energetics and mass limit derived for the M85 OT 2006-1 progenitor with those expected from a simple model of violent stellar mergers. Combined with further modeling, these new clues may ultimately reveal the true nature of these puzzling events.

Optical and near-infrared spectroscopy of the newly discovered peculiar L dwarf 2MASS J11263991–5003550 are presented. Folkes et al. classified this source as a high proper motion L9±1 dwarf based on its strong H₂O absorption at 1.4 μ m . We find that the optical spectrum of 2MASS J1126–5003 is in fact consistent with that of a normal L4.5 dwarf with notably enhanced FeH absorption at 9896 Å. However, its near-infrared spectrum is unusually blue, with strong H₂O and weak CO bands similar in character to several recently identified "blue L dwarfs." Using 2MASS J1126–5003 as a case study, and guided by trends in the condensate cloud models of Burrows et al. and Marley et al., we find that the observed spectral peculiarities of these sources can be adequately explained by the presence of thin and/or large-grained condensate clouds as compared to normal field L dwarfs. Atypical surface gravities or metallicities alone cannot reproduce the observed peculiarities, although they may be partly responsible for the unusual condensate properties. We also rule out unresolved multiplicity as a cause for the spectral peculiarities of 2MASS J1126–5003. Our analysis is supported by examination of Spitzer mid-infrared spectral data from Cushing et al. which show that bluer L dwarfs tend to have weaker 10 μ m absorption, a feature tentatively associated with silicate oxide grains. With their unique spectral properties, blue L dwarfs like 2MASS J1126–5003 should prove useful in studying the formation and properties of condensates and condensate clouds in low-temperature atmospheres.

We examine the implications for the distribution of extrasolar planets based on the null results from two of the largest direct imaging surveys published to date. Combining the measured contrast curves from 22 of the stars observed with the VLT NACO adaptive optics system by Masciadri and coworkers and 48 of the stars observed with the VLT NACO SDI and MMT SDI devices by Biller and coworkers (for a total of 60 unique stars), we consider what distributions of planet masses and semimajor axes can be ruled out by these data, based on Monte Carlo simulations of planet populations. We can set the following upper limit with 95% confidence: the fraction of stars with planets with semimajor axis between 20 and 100 AU, and mass above 4 M_Jup, is 20% or less. Also, with a distribution of planet mass of dN/dM ∝ M^−1.16 in the range of 0.5-13 M_Jup, we can rule out a power-law distribution for semimajor axis (dN/da ∝ a^α) with index 0 and upper cutoff of 18 AU, and index -0.5 with an upper cutoff of 48 AU. For the distribution suggested by Cumming et al., a power-law of index –0.61, we can place an upper limit of 75 AU on the semimajor axis distribution. In general, we find that even null results from direct imaging surveys are very powerful in constraining the distributions of giant planets (0.5-13 M_Jup) at large separations, but more work needs to be done to close the gap between planets that can be detected by direct imaging, and those to which the radial velocity method is sensitive.

We report the spectroscopic detection of mid-infrared emission from the transiting exoplanet HD 209458b. Using archive data taken with the Spitzer IRS instrument, we have determined the spectrum of HD 209458b between 7.46 and 15.25 μm. We have used two independent methods to determine the planet spectrum, one differential in wavelength and one absolute, and find the results are in good agreement. Over much of this spectral range, the planet spectrum is consistent with featureless thermal emission. Between 7.5 and 8.5 μm, we find evidence for an unidentified spectral feature. If this spectral modulation is due to absorption, it implies that the dayside vertical temperature profile of the planetary atmosphere is not entirely isothermal. Using the IRS data, we have determined the broadband eclipse depth to be 0.00315 ± 0.000315, implying significant redistribution of heat from the dayside to the nightside. This work required the development of improved methods for Spitzer IRS data calibration that increase the achievable absolute calibration precision and dynamic range for observations of bright point sources.

Meridional flows with velocities of a few meters per second are observed in the uppermost regions of the solar convection zone. The amplitude and pattern of the flows deeper in the solar interior, in particular near the top of the radiative region, are of crucial importance to a wide range of solar magnetohydrodynamical processes. In this paper, we provide a systematic study of the penetration of large-scale meridional flows from the convection zone into the radiative zone. In particular, we study the effects of the assumed boundary conditions applied at the radiative-convective interface on the deeper flows. Using simplified analytical models in conjunction with more complete numerical methods, we show that penetration of the convectively driven meridional flows into the deeper interior is not necessarily limited to a shallow Ekman depth but can penetrate much deeper.

We present a refinement of the Fisk-Parker hybrid field of Burger and Hitge which now includes a region bordering the solar rotational equator where magnetic field footpoint motion occurs only through diffusive reconnection. The hybrid field, therefore, only occurs above a certain latitude in a given hemisphere, and in the equatorial region the field is a pure Parker field. We also propose a simple qualitative model for the solar cycle dependence of the hybrid field, taking into account changes in the tilt angle of the heliospheric current sheet and the latitudinal extend of the polar coronal hole on the photosphere and on the source surface over the course of a solar activity cycle. We find that the amplitude of magnetic field fluctuations for assumed solar minimum parameters would not be observable above the background noise (see Roberts and coworkers). We also show that for these parameters, periodicities associated with differential footpoint motion would be barely distinguishable from rigid rotation at the solar equatorial rate. We point out that the question of periodicities in magnetic field data is perhaps more complicated than previously thought. We confirm the result of Burger and Hitge that a Fisk-type heliospheric magnetic field provides a natural explanation for the observed linear relationship between the amplitude of the recurrent cosmic-ray variations and the global latitude gradient (see Zhang). We show that this relationship holds for helium, protons, and electrons. Moreover, we show that the constant of proportionality is larger when qA > 0 than when qA < 0, as inferred from observations by Richardson and coworkers.

We investigate the origin of small-scale flux concentrations in the quiet Sun. In apparent violation of the physical requirement for flux balance, 94% of the features containing newly detected flux are unipolar at a resolution of 1.2''. We analyze 2619 of these apparent unipolar emergences in an image sequence from the SOHO MDI magnetograph and compare the ensemble average to a model of asymmetric bipolar emergence that could in principle hide opposing flux under the noise floor of MDI. We examine the statistical consequences of this mechanism and find that it cannot be responsible for more than a small fraction of the unipolar emergences. We conclude that the majority of the newly detected flux in the quiet Sun is instead due to the coalescence of previously existing but unresolved flux into concentrations that are large and strong enough to be detected. We estimate the rate of coalescence into arcsecond-scale magnetic features averaged over the solar surface to be 7 × 10²¹ Mx hr⁻¹, comparable to the reported flux injection rate due to ephemeral regions. This implies that most flux in the solar network has been processed by very small scale shredding, emergence, cancellation, and/or coalescence that is not resolved at 1.2'', and it suggests that currently unresolved emergences may be at least as important as ephemeral region emergences to the overall flux budget.

In this study we investigate for the first time the fractal dimension of solar flares and find that the flare area observed in EUV wavelengths exhibits a fractal scaling. We measure the area fractal dimension D₂, also called the Hausdorff dimension, with a box-counting method, which describes the fractal area as A(L) ∝ L^D₂. We apply the fractal analysis to a statistical sample of 20 GOES X- and M-class flares, including the Bastille Day 2000 July 14 flare, one of the largest flares ever recorded. We find that the fractal area (normalized by the time-integrated flare area A_f) varies from near zero at the beginning of the flare to a maximum of A(t)/A_f = 0.65 ± 0.12 after the peak time of the flare, which corresponds to an area fractal dimension in the range of 1.0≲ D₂(t) ≲ 1.89 ± 0.05. We find that the total EUV flux F_tot(t) is linearly correlated with the fractal area A(t) . From the area fractal dimension D₂, the volume fractal dimension D₃ can be inferred (subject of Paper II), which is crucial to inferring a realistic volume filling factor, which affects the derived electron densities, thermal energies, and cooling times of solar and stellar flares.

Based on the area fractal dimension D₂ of solar flares measured in Paper I, we carry out modeling of the three-dimensional (3D) flare volume here and derive an analytical relation between the volume fractal scaling V(L) ∝ L^D₃ and the area fractal scaling A(L) ∝ L^D₂. The 3D volume model captures a flare arcade with a variable number of flare loops; its fractal structure is not isotropic, but consists of aligned one-dimensional substructures. The geometry of the arcade model has three free parameters and makes some simplifying assumptions, such as semicircular loops, east-west orientation, location near the equator, and no magnetic shear. The analytical model predicts the scaling of the area filling factor q_A(n_loop) and volumetric filling factor q_V(n_loop) as a function of the number of loops n_loop, and allows one to predict the volume filling factor q_V(q_A) and volume fractal dimension D₃(D₂) from the observationally measured parameters q_A and D₂. We also corroborate the analytical model with numerical simulations. We apply this fractal model to the 20 flares analyzed in Paper I and find maximum volume filling factors with a median range of q_V ≈ 0.03–0.08 (assuming solid filling for loop widths of ≲1 Mm). The fractal nature of the flare volume has important consequences for correcting electron densities determined from flare volume emission measures and density-dependent physical quantities, such as the thermal energy or radiative cooling time. The fractal scaling has also far-reaching consequences for frequency distributions and scaling laws of solar and stellar flares.

Photospheric magnetic flux emergence has been often observed in solar active regions and considered to be closely related to solar explosive phenomena such as flares and CMEs. However, in view of the fact that photospheric flux emergence and associated flares or CMEs differ remarkably in timescale, their relationship must be implemented by a catastrophic process. To demonstrate this point, we investigate the catastrophe of a coronal flux rope system caused by photospheric flux emergence. The initial magnetic field is taken to be a two-dimensional force-free field in spherical geometry, consisting of an isolated flux rope and a bipolar background field surrounding it. A flux emergence is then introduced somewhere on the photosphere, and it causes a variation of the background field and the formation of a current sheet between the emerging arcade and the original background field. It is shown that as the total flux of the emerging arcade increases, a catastrophe may take place for the system, depending on the location and field orientation of the emerging arcade and whether there exists a magnetic reconnection across the newly formed current sheet. Although the main conclusions are superficially similar to those obtained by previous studies, we stress an entirely new concept that the photospheric flux emergence plays the role of a trigger rather than a driver of the associated flare and CME by forcing a highly complex system to reach its catastrophic point.

We report on the properties of radio-quiet (RQ) and radio-loud (RL) coronal mass ejections (CMEs) that are fast and wide (FW). RQ CMEs lack type II radio bursts in the metric and decameter-hectometric (DH) wavelengths. RL CMEs are associated with metric or DH type II bursts. We found that ~40% of the FW CMEs from 1996 to 2005 were RQ. The RQ CMEs had an average speed of 1117 km s⁻¹ compared to 1438 km s⁻¹ for the RL, bracketing the average speed of all FW CMEs (1303 km s⁻¹). The fraction of full halo CMEs (apparent width = 360°) was the largest for the RL CMEs (60%), smallest for the RQ CMEs (16%), and intermediate for all FW CMEs (42%). The median soft X-ray flare size for the RQ CMEs (C6.9) was also smaller than that for the RL CMEs (M3.9). About 55% of RQ CMEs were back sided, while the front-sided ones originated close to the limb. The RL CMEs originated generally on the disk with only ~25% being back sided. The RQ FW CMEs suggest that the Alfvén speed in the low-latitude outer corona can often exceed 1000 km s⁻¹ and can vary over a factor of ≥3. None of the RQ CMEs was associated with large solar energetic particle events, which is useful information for space weather applications.

McConnell et al. have reported preliminary results of hard X-ray polarization measured by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) in an intense solar flare on 2002 July 23. The magnitude of the reported polarization is broadly consistent with the predictions of existing solar flare models which invoke the precipitation of a nonthermal electron beam into a dense chromospheric target. However, the orientation of the polarization vector lies at a substantial angle to the local solar radial direction. This is inconsistent with model predictions of a polarization vector along the local radial direction, a prediction that is a direct consequence of the assumption of a vertical guiding magnetic field. Smith et al., in a study of the same 2002 July 23 event, have suggested that the magnetic structure in which the flare occurs is tilted relative to the local vertical. Motivated by this observation, and by the preliminary nonradial polarization vector reported by McConnell et al., we explore the effect of tilt of the flaring loop on the magnitude and orientation of the predicted polarization vector. We find that allowing loops tilted from the local solar vertical does indeed permit a much wider range of polarization vector orientations than allowed by purely vertical loop geometries. In particular, adding tilt of the magnitude inferred by Smith et al. for the 2002 July 23 event can in principle account for both the magnitude and direction of the polarization vector reported by McConnell et al. for that event.

We report on a statistical analysis of 96 CME-associated EUV coronal dimmings between 1998 and 2000. We investigate the size and location of the events and characterize how these events evolve with time. The durations typically range from 3 to 12 hr. The dimmings appear most frequently within the belt of active regions (20°-50° latitude). Dimming events are generally symmetric in latitude and longitude with some tendency to be broader in latitude. The temporal profiles of most events are characterized by a sharp rise and a gradual recovery. Although the majority of cases are well fit by a single recovery slope, a large minority of events have a two-part decay with an initial decaying slope that is similar in magnitude to the rising slope and a secondary, flatter, decay lasting several hours.

We examine the early phases of two near-limb filament destabilizations involved in coronal mass ejections (CMEs) on 2005 June 16 and July 27, using high-resolution, high-cadence observations made with the Transition Region and Coronal Explorer (TRACE), complemented by coronagraphic observations by the Mauna Loa Solar Observatory (MLSO) and the Solar and Heliospheric Observatory (SOHO). The filaments' heights above the solar limb in their rapid-acceleration phases are best characterized by a height dependence h(t) ∝ t^m with m near, or slightly above, 3 for both events. Such profiles are incompatible with published results for breakout, MHD-instability, and catastrophe models. We show numerical simulations of the torus instability that approximate this height evolution in case a substantial initial velocity perturbation is applied to the developing instability. We argue that the sensitivity of magnetic instabilities to initial and boundary conditions requires higher fidelity modeling of all proposed mechanisms if observations of rise profiles are to be used to differentiate between them. The observations show no significant delays between the motions of the filament and of overlying loops: the filaments seem to move as part of the overall coronal field until several minutes after the onset of the rapid-acceleration phase.

The reliability of quiet-Sun magnetic field diagnostics based on the Fe I lines at 6302 Å has been questioned by recent work. Here we present the results of a thorough study of high-resolution multiline observations taken with the new spectropolarimeter SPINOR, comprising the 5250 and 6302 Å spectral domains. The observations were analyzed using several inversion algorithms, including Milne-Eddington, LTE with 1 and 2 components, and MISMA codes. We find that the line-ratio technique applied to the 5250 Å lines is not sufficiently reliable to provide a direct magnetic diagnostic in the presence of thermal fluctuations and variable line broadening. In general, one needs to resort to inversion algorithms, ideally with realistic magnetohydrodynamic constrains. When this is done, the 5250 Å lines do not seem to provide any significant advantage over those at 6302 Å. In fact, our results point toward a better performance with the latter (in the presence of turbulent line broadening). In any case, for very weak flux concentrations, neither spectral region alone provides sufficient constraints to fully disentangle the intrinsic field strengths. Instead, we advocate for a combined analysis of both spectral ranges, which yields a better determination of the quiet-Sun magnetic properties. Finally, we propose the use of two other Fe I lines (at 4122 and 9000 Å) with identical line opacities that seem to work much better than the others.

The solar argon abundance cannot be directly derived by spectroscopic observations of the solar photosphere. The solar argon abundance is evaluated from solar wind measurements, nucleosynthetic arguments, observations of B stars, H II regions, planetary nebulae, and noble gas abundances measured in Jupiter's atmosphere. These data lead to a recommended argon abundance of N(Ar) = 91,200 ± 23,700 (on a scale where Si = 10⁶ atoms). The recommended abundance for the solar photosphere [on a scale where log N(H) = 12] is A(Ar)_photo = 6.50 ± 0.10, and taking element settling into account, the solar system (protosolar) abundance is A(Ar)_solsys = 6.57 ± 0.10.

We present the mass functions of actively accreting supermassive black holes over the redshift range 0.3 ≤ z ≤ 5 for a well-defined, homogeneous sample of 15,180 quasars from the Sloan Digital Sky Survey Data Release 3 (SDSS DR3) within an effective area of 1644 deg². This sample is the most uniform statistically significant subset available for the DR3 quasar sample. It was used for the DR3 quasar luminosity function, presented by Richards et al., and is the only sample suitable for the determination of the SDSS quasar black hole mass function. The sample extends from i = 15 to i = 19.1 at z≲ 3 and to i = 20.2 for z≳ 3. The mass functions display a rise and fall in the space density distribution of active black holes at all epochs. Within the uncertainties the high-mass decline is consistent with a constant slope of β ≈ − 3.3 at all epochs. This slope is similar to the bright-end slope of the luminosity function for epochs below z = 4. Our tests suggest that the downturn toward lower mass values is due to incompleteness of the quasar sample with respect to black hole mass. Further details and analysis of these mass functions will be presented in forthcoming papers.

We have analyzed the angular clustering of X-ray-selected active galactic nuclei (AGNs) in different flux-limited subsamples of the Chandra Deep Field North (CDF-N) and South (CDF-S) surveys. We find a strong dependence of the clustering strength on the subsample flux limit, a fact that explains most of the disparate clustering results of different XMM and Chandra surveys. Using Limber's equation, we find that the inverted CDF-N and CDF-S spatial clustering lengths are consistent with direct spatial clustering measures found in the literature, while at higher flux limits the clustering length increases considerably; for example, at f_x,limit ∼ 10⁻¹⁵ ergs s⁻¹ cm⁻², we obtain r₀≃ 17 ± 5 and 18 ± 3 h⁻¹ Mpc for the CDF-N and CDF-S, respectively. We show that the observed flux limit clustering trend hints toward an X-ray luminosity dependent clustering of X-ray-selected, z ∼ 1, AGNs.

We present relations of the black hole mass and the optical luminosity with the velocity dispersion and the luminosity of the [Ne V] and the [O IV] high-ionization lines in the mid-infrared (MIR) for 28 reverberation-mapped active galactic nuclei. We used high-resolution Spitzer Infrared Spectrograph and Infrared Space Observatory Short Wavelength Spectrometer data to fit the profiles of these MIR emission lines that originate from the narrow-line region of the nucleus. We find that the lines are often resolved and that the velocity dispersion of [Ne V] and [O IV] follows a relation similar to that between the black hole mass and the bulge stellar velocity dispersion found for local galaxies. The luminosity of the [Ne V] and the [O IV] lines in these sources is correlated with that of the optical 5100 Å continuum and with the black hole mass. Our results provide a means to derive black hole properties in various types of active galactic nuclei, including highly obscured systems.

We compare environmental effects in two analogous samples of galaxies, one from the Sloan Digital Sky Survey (SDSS) and the other from a semianalytic model (SAM) based on the Millennium Simulation (MS), to test to what extent current SAMs of galaxy formation are reproducing environmental effects. We estimate the large-scale environment of each galaxy using a Bayesian density estimator based on distances to all 10 nearest neighbors, and we compare broadband photometric properties of the two samples as a function of environment. The feedbacks implemented in the semianalytic model produce a qualitatively correct galaxy population with similar environmental dependence as that seen in SDSS galaxies. In detail, however, the colors of MS galaxies exhibit an exaggerated dependence on environment: the field contains too many blue galaxies, whereas clusters contain too many red galaxies, compared to the SDSS sample. We also find that the MS contains a population of highly clustered, relatively faint red galaxies with velocity dispersions comparable to their Hubble flow. Such high-density galaxies, if they exist, would be overlooked in any low-redshift survey, since their membership to a cluster cannot be determined because of the "fingers-of-God" effect.

The line-of-sight velocities and [O III] 5007 Å expansion velocities are measured for 11 planetary nebulae (PNs) in the Virgo Cluster core, at 15 Mpc distance, with the FLAMES spectrograph on the ESO VLT. These PNs are located about halfway between the two giant elliptical galaxies M87 and M86. From the [O III] 5007 Å line profile widths, the average half-width at half-maximum expansion velocity for this sample of 11 PNs is _HWHM = 16.5 km s⁻¹ (rms = 2.6 km s⁻¹). We use the PN subsample bound to M87 to remove the distance uncertainties and the resulting [O III] 5007 Å luminosities to derive the central star masses. We find these masses to be at least 0.6 M_☉ and obtain PN observable lifetimes t_PN < 2000 yr, which imply that the bright PNs detected in the Virgo Cluster core are compact, high-density nebulae. We finally discuss several scenarios for explaining the high central star masses in these bright M87 halo PNs.

A new scenario for the emission of high-energy γ-rays from dark matter annihilation around massive black holes is presented. A black hole can leave its parent halo, by means of gravitational radiation recoil, in a merger event or in the asymmetric collapse of its progenitor star. A recoiled black hole which moves on an almost radial orbit outside the virial radius of its central halo, in the cold dark matter background, reaches its apapsis in a finite time. Near or at the apapsis passage, a high-density wake extending over a large radius of influence forms around the black hole. Significant γ-ray emission can result from the enhancement of neutralino annihilation in these wakes. At its apapsis passage, a black hole produces a flash of high-energy γ-rays whose duration is determined by the mass of the black hole and the redshift at which it is ejected. The ensemble of such black holes in the Hubble volume is shown to produce a diffuse high-energy γ-ray background whose magnitude is compared to the diffuse emission from dark matter halos alone.

If a class of stars orbits the central black hole in our galaxy in short-period (~0.1 yr), high-eccentricity (~0.9) orbits, they will experience precessions of their orbital planes induced by both relativistic frame dragging and the quadrupolar gravity of the hole, at levels that could be as large as 10 μas per year, if the black hole is rotating faster than half of its maximum rotation rate. Astrometric observations of the orbits of at least two such stars can in principle lead to a determination of the angular momentum vector of the black hole and its quadrupole moment Q₂. This could lead to a test of the general relativistic no-hair theorems, which demand that Q₂ = − J²/M. Future high-precision adaptive infrared optics instruments may make such a fundamental test of the black hole paradigm possible.

Determining the final spin of a black hole (BH) binary is a question of key importance in astrophysics. Modeling this quantity in general is made difficult by the fact that it depends on the seven-dimensional space of parameters characterizing the two initial black holes. However, in special cases, when symmetries can be exploited, the description can become simpler. For BH binaries with unequal masses but with equal spins which are aligned with the orbital angular momentum, we show that the use of recent simulations and basic but exact constraints derived from the extreme mass-ratio limit allow us to model this quantity with a simple analytic expression. Despite the simple dependence, the expression models very accurately all of the available estimates, with errors of a couple of percent at most. We also discuss how to use the fit to predict when a Schwarzschild BH is produced by the merger of two spinning BHs, when the total angular momentum of the spacetime "flips" sign, or under what conditions the final BH is "spun up" by the merger. Finally, we suggest an extension of the fit to include unequal-spin binaries, thus potentially providing a complete description of the final spin from the coalescence of generic BH binaries with spins aligned to the orbital angular momentum.

Over a timescale of a few years, an observed change in the optically thick radio continuum flux can indicate whether an unresolved H II region around a newly formed massive star is changing in size. In this Letter we report on a study of archival VLA observations of the hypercompact H II region G24.78+0.08 A1 that shows a decrease of ~45% in the 6 cm flux over a 5 yr period. Such a decrease indicates a contraction of ~25% in the ionized radius and could be caused by an increase in the ionized gas density if the size of the H II region is determined by a balance between photoionization and recombination. This finding is not compatible with continuous expansion of the H II region after the end of accretion onto the ionizing star, but is consistent with the hypothesis of gravitational trapping and ionized accretion flows if the mass accretion rate is not steady.

Delay due to multipath scattering in the interstellar medium is a concern for high-precision pulsar timing, particularly if it is not constant over time. We report on 36 weekly observations of the pulsar PSR B1737+13 with the Arecibo telescope that monitored the time variability of the scattering delay. At a frequency of 1380 MHz, the interstellar delay varied between 0.2 and 2.1 μ s (±0.1 μ s ) over 270 days of observation. The delay was consistent over four observing bands with center frequencies from 1175 to 1470 MHz and scaled as τ ∝ ν^{−3.6 ± 0.2}, which differs from the ν^−4.4 scaling expected for Kolmogorov turbulence. We show that another estimation technique is feasible for weaker pulsars or smaller telescopes, although it underestimates the delay during episodes of extra scattering detectable through a full secondary spectrum analysis. An array of pulsars distributed around the sky can be used as a sensitive detector of long-wavelength (~ several light-years) gravitational radiation, and such pulsar timing array observations have been initiated by several groups worldwide. To reach interesting sensitivity levels it is necessary to reduce the sources of error to below 1 μ s , and 100 ns is a target precision level. Correction for interstellar scattering delay will be an important step in achieving long-term, submicrosecond timing precision.

We report the discovery of an episode of coherent millisecond X-ray pulsation in the neutron star low-mass X-ray binary Aql X-1. The episode lasts for slightly more than 150 s, during which the pulse frequency is consistent with being constant. No X-ray burst or other evidence of thermonuclear burning activity is seen in correspondence with the pulsation, which can thus be identified as occurring in the persistent emission. The pulsation frequency is 550.27 Hz, very close (0.5 Hz higher) to the maximum reported frequency from burst oscillations in this source. Hence we identify this frequency with the neutron star spin frequency. The pulsed fraction is strongly energy dependent, ranging from <1% at 3-5 keV to >10% at 16-30 keV. We discuss possible physical interpretations and their consequences for our understanding of the lack of pulsation in most neutron star low-mass X-ray binaries. If interpreted as accretion-powered pulsation, Aql X-1 might play a key role in understanding the differences between pulsating and nonpulsating sources.

We report on intermittent X-ray pulsations with a frequency of 442.36 Hz from the neutron star X-ray binary SAX J1748.9–2021 in the globular cluster NGC 6440. The pulsations were seen during both 2001 and 2005 outbursts of the source, but only intermittently, appearing and disappearing on timescales of hundreds of seconds. We find a suggestive relation between the occurrence of type I X-ray bursts and the appearance of the pulsations, but the relation is not strict. This behavior is very similar to that of the intermittent accreting millisecond X-ray pulsar HETE J1900.1–2455. The reason for the intermittence of the pulsations remains unclear. However, it is now evident that a strict division between pulsating and nonpulsating neutron star systems does not exist. By studying the Doppler shift of the pulsation frequency we determine an orbit with a period of 8.7 hr and a projected semimajor axis of 0.39 lt-s. The companion star might be a main-sequence or a slightly evolved star with a mass of ~1 M_☉. Therefore, SAX J1748.9–2021 has a longer period and may have a more massive companion star than all the other accreting millisecond X-ray pulsars except for Aql X-1.

I suggest a theoretical quantitative definition for the termination of the asymptotic giant branch (AGB) phase and the beginning of the post-AGB phase. I suggest that the transition will be taken to occur when the ratio of the dynamical timescale to the envelope thermal timescale, Q ≡ τ_dyn/τ_KH-env , reaches its maximum value. Time average values are used for the different quantities, as the criterion does not refer to the short-timescale variations occurring on the AGB and post-AGB, e.g., thermal pulses (helium shell flashes) and magnetic activity. Along the entire AGB the value of Q increases, even when the star starts to contract. Only when a rapid contraction starts does the value of Q start to decrease. This criterion captures the essence of the transition from the AGB to the post-AGB phase, because Q is connected to the stellar effective temperature, reaching its maximum value at T≃ 4000–6000 K , it is related to the mass loss properties, and it reaches its maximum value when rapid contraction starts and envelope mass is very low.

Mass estimates of K giants are generally very uncertain. Traditionally, stellar masses of single field stars are determined by comparing their location in the Hertzsprung-Russell diagram with stellar evolutionary models. Applying an additional method to determine the mass is therefore of significant interest for understanding stellar evolution. We present the time series analysis of 11 K giants recently observed with the WIRE satellite. With this comprehensive sample, we report the first confirmation that the characteristic acoustic frequency, ν_max, can be predicted for K giants by scaling from the solar acoustic cutoff frequency. We are further able to utilize our measurements of ν_max to determine an asteroseismic mass for each star with a lower uncertainty compared to the traditional method, for most stars in our sample. This indicates good prospects for the application of our method on the vast amounts of data that will soon come from the COROT and Kepler space missions.

We use a numerical simulation method for recovering the photospheric velocity field from the vector magnetograms. The traditional method is local correlation tracking (LCT), which is based on measuring the relative displacements of features in blocks of pixels between successive white-light images or magnetograms. Within this method, there are a variety of implementations. One of recently developed implementations is induction local correlation tracking (ILCT) as described by Welsch and coworkers. They employ the normal component of magnetic induction equation as a constraint to assure consistent solutions. Our numerical method uses the fully three-dimensional MHD equations to recover the photospheric velocity field with individual vector magnetograms. We compare our method to the ILCT method using NOAA AR 8210 as an example. The differences and similarities are discussed in detail.