Galaxy Cluster SPT-CL J0615−5746 as seen by Webb

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An international team of astronomers have used the James Webb Space Telescope to discover gravitationally bound star clusters when the Universe was 460 million years old. This is the first discovery of star clusters in an infant galaxy less than 500 million years after the Big bang.

Young galaxies in the early Universe underwent significant burst phases of star formation, generating substantial amounts of ionising radiation. However, because of their cosmological distances, direct studies of their stellar content have proven challenging. Using Webb, an international team of astronomers have now detected five young massive star clusters in the Cosmic Gems arc (SPT0615-JD1), a strongly-lensed galaxy emitting light when the Universe was roughly 460 million years old, looking back across 97% of cosmic time.

The Cosmic Gems arc was initially discovered in Hubble Space Telescope images obtained by the RELICS (Reionization Lensing Cluster Survey) programme of the lensing galaxy cluster SPT-CL J0615−5746.

With Webb, the science team can now see where stars formed and how they are distributed, in a similar way to how the Hubble Space Telescope is used to study local galaxies. Webb's view provides a unique opportunity to study star formation and the inner workings of infant galaxies at such an unprecedented distance.

Image Credit: ESA/Webb, NASA and CSA, L. Bradley (STScI), A. Adamo (Stockholm University) and the Cosmic Spring collaboration

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell 2390 as seen by Euclid

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Abell 2390 is a galaxy cluster, a giant conglomeration of many galaxies like the Milky Way. More than 50 000 galaxies are seen here, the distances to which can be measured thanks to these new observations. Such clusters contain huge amounts of mass (up to 10 trillion times that of the Sun), with much of this being in the form of dark matter – a form of matter that we can’t observe directly, but is purported to together with dark energy make up the bulk of the contents of the Universe. Galaxy clusters like Abell 2390 are large repositories of dark matter, making them ideal astrophysical laboratories for studying its properties. Once Euclid begins its main survey it will capture many thousands of galaxy clusters over around one-third of the sky, obtaining information we can use to make unprecedented constraints on the dark Universe.

Euclid's new view of the cluster showcases one of the telescope's key techniques for exploring this dark Universe: indirectly measuring the amount and distribution of dark matter in a galaxy cluster via gravitational lensing, a phenomenon where the light travelling to us from more distant galaxies is bent and distorted by this mysterious matter. Thanks to Euclid's advanced instruments we can see an especially beautiful display of lensing in Abell 2390, with multiple giant curved arcs, some of which are actually multiple views of the same distant object.

Alongside understanding more about dark matter, scientists are using Euclid data to measure how the masses and number of galaxy clusters on the sky change over cosmic time, revealing more about the evolution of the Universe (and by extension more about dark energy, which is thought to influence this evolution). Euclid's cutout view of Abell 2390 also shows the faint "intracluster light" emitted by stars that have been ripped away from their parent galaxies into intergalactic space (the light has been enhanced in the cutout image to make it more clearly visible). Viewing this light is a specialty of Euclid, and these stellar orphans may allow us to "see" where dark matter lies.

Euclid captures light ranging from the visible to the near-infrared using its VIS (visible) and NISP (near-infrared) cameras. These can operate simultaneously, imaging wide areas of the sky to create images hundreds of times larger than comparable ones from other space telescopes. This wide field-of-view lets us take pictures of extended objects like Abell 2390 in a single shot, rather than having to take many pictures and stitch them together.

Observing a galaxy cluster in both visible and infrared light allows us to see galaxies at a greater range of distances than using either visible or infrared alone – crucial if we want to observe both the galaxies in a relatively nearby cluster and the galaxies lying behind it (far further from us). Euclid can take these types of deep, wide, high-resolution images hundreds of times faster than other telescopes.

Abell 2390 lies 2.7 billion light-years away in the constellation of Pegasus.

Image Credit: ESA/Euclid/Euclid Consortium/NASA

Image processing: Jean-Charles Cuillandre (CEA Paris-Saclay) and Giovanni Anselmi

Image enhancement: Jean-Baptiste Faure

Quasar RX J1131-1231 and its Gravitational Lensing

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This new picture features the gravitational lensing of the quasar known as RX J1131-1231, located roughly 6 billion light-years from Earth in the constellation Crater. It is considered one of the best lensed quasars discovered to date, as the foreground galaxy smears the image of the background quasar into a bright arc and creates four images of the object.

Gravitational lensing, first predicted by Einstein, offers a rare opportunity to study regions close to the black hole in distant quasars, by acting as a natural telescope and magnifying the light from these sources. All matter in the Universe warps the space around itself, with larger masses producing a more pronounced effect. Around very massive objects, such as galaxies, light that passes close by follows this warped space, appearing to bend away from its original path by a clearly visible amount. One of the consequential effects of gravitational lensing is that it can magnify distant astronomical objects, letting astronomers study objects that would otherwise be too faint or far away.

Measurements of the X-ray emission from quasars can provide an indication of how fast the central black hole is spinning, which can provide researchers important clues about how black holes grow over time. For example, if a black hole grows primarily from collisions and mergers between galaxies, it should accumulate material in a stable disc, and the steady supply of new material from the disc should lead to a rapidly spinning black hole. On the other hand, if the black hole grew through many small accretion episodes, it would accumulate material from random directions. Observations have indicated that the black hole in this particular quasar is spinning at over half the speed of light, which suggests that this black hole has grown via mergers, rather than pulling material in from different directions.

This image was captured with Webb's MIRI (Mid-Infrared Instrument) as part of an observation programme to study dark matter. Dark matter is an invisible form of matter that accounts for most of the Universe's mass. Webb's observations of quasars are allowing astronomers to probe the nature of dark matter at smaller scales than ever before.

Image Credit: ESA/Webb, NASA and CSA, A. Nierenberg

The Fornax Galaxy Cluster

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The Fornax Galaxy Cluster is one of the closest of such groupings beyond our Local Group of galaxies. This new VLT Survey Telescope image shows the central part of the cluster in great detail. At the lower-right is the elegant barred-spiral galaxy NGC 1365 and to the left the big elliptical NGC 1399.

It has an estimated mass of (7±2)×1013 solar masses, making it the second richest galaxy cluster within 100 million light-years, after the considerably larger Virgo Cluster. It may be associated with the nearby Eridanus Group. It lies primarily in the constellation Fornax, with its southern boundaries partially crossing into the constellation of Eridanus, and covers an area of sky about 6° across or about 28 sq degrees.

The Fornax Galaxy Cluster is a particularly valuable source of information about the evolution of such clusters due to its relatively close proximity to the Sun. It also shows the gravitational effects of a merger of a galaxy subgroup with the main galaxy group, which in turn lends clues about the associated galactic superstructure. At the center of the cluster lies NGC 1399. Other cluster members include NGC 1316 (the group's brightest galaxy), NGC 1365, NGC 1427A, NGC 1427 and NGC 1404.

Image Credit: ESO

Acknowledgement: Aniello Grado and Luca Limatola

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster SDSS J1226+2152

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The vast galaxy cluster SDSS J1226+2152 in the constellation Coma Berenices is distorting the images of distant background galaxies into streaks and smears of light in this image from the James Webb Space Telescope. This is a spectacular example of gravitational lensing, a phenomenon which occurs when a massive celestial object such as a galaxy cluster deforms spacetime and causes the path of light from more distant galaxies to be deflected, almost as if a monumental lens was redirecting it. This image is from a set of early science observations with Webb.

One of the most notable lensed galaxies in this rich field is named SGAS J12265.3+215220. In this image, it's the innermost lensed galaxy, just above and to the right of the central galaxy. This lies far beyond the foreground cluster in distance, giving us a view into the galaxy roughly two billion years after the big bang. Astronomers are now using this eagerly-awaited hoard of bright, gravitationally-lensed galaxies from Webb to explore star formation in distant galaxies.

Just like their optical namesakes, gravitational lenses can magnify as well as distort distant galaxies. This allows astronomers to observe the finer details of galaxies that would usually be too distant to clearly resolve. In the case of SGAS J122651.3+215220, the combination of gravitational lensing and Webb's unprecedented observational capabilities will allow astronomers to measure where, and how fast, stars are forming and also to gain an insight into the environments which support star formation in lensed galaxies.

Amid this spectacular display of gravitational lensing, a menagerie of spiral and elliptical galaxies in all shapes and sizes surround the galaxy cluster. Webb's sensitive infrared instruments have proven prodigious in picking out distant galaxies from the darkness of space. None of the tiny pinpricks in the patch of sky captured here is a star: each one is a galaxy. The variety of colours of the small, dim galaxies gives us hints at what we are seeing: many of the paler white galaxies will date back to the period of intense star formation known as cosmic noon, some two to three billion years after the big bang, while the few small orange and red systems are probably from even earlier in the Universe's history.

Image Credit: ESA/Webb, NASA and CSA, J. Rigby and the JWST TEMPLATES team

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster MACS0416

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This panchromatic view of galaxy cluster MACS0416 was created by combining infrared observations from the James Webb Space Telescope with visible-light data from the Hubble Space Telescope. To make the image, in general the shortest wavelengths of light were colour-coded blue, the longest wavelengths red, and intermediate wavelengths green. The resulting wavelength coverage, from 0.4 to 5 microns, reveals a vivid landscape of galaxies that could be described as one of the most colourful views of the universe ever created.

MACS0416 is a galaxy cluster located about 4.3 billion light-years from Earth, meaning that the light from it that we see now left the cluster shortly after the formation of our Solar System. This cluster magnifies the light from more distant background galaxies through gravitational lensing. As a result, the research team has been able to identify magnified supernovae and even very highly magnified individual stars.

Those colours give clues to galaxy distances: the bluest galaxies are relatively nearby and often show intense star formation, as best detected by Hubble, while the redder galaxies tend to be more distant, or else contain copious amounts of dust, as best detected by Webb. The image reveals a wealth of details that it is only possible to capture by combining the power of both space telescopes.

In this image, blue represents data at wavelengths of 0.435, 0.606, 0.814, and 1.05 microns (Hubble filters F435W, F606W, F814W, and F105W). Green combines data at 0.90, 1.15, 1.5, 1.6, 2.0, and 2.77 microns (Hubble filter F160W and Webb filters F090W, F115W, F150W, F200W, and F277W). Red represents data at 3.56, 4.1, and 4.44 microns (Webb filters F356W, F410M and F444W).

Image Credit: NASA, ESA, CSA, STScI, J. Diego (Instituto de Física de Cantabria, Spain), J. D’Silva (U. Western Australia), A. Koekemoer (STScI), J. Summers & R. Windhorst (ASU), and H. Yan (U. Missouri)

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster SPT-CL J0019-2026

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A massive galaxy cluster in the constellation Cetus dominates the center of this image from the Hubble Space Telescope. This image is populated with a serene collection of elliptical and spiral galaxies, but galaxies surrounding the central cluster – which is named SPT-CL J0019-2026 – appear stretched into bright arcs, as if distorted by a gargantuan magnifying glass. This cosmic contortion is called gravitational lensing, and it occurs when a massive object like a galaxy cluster has a sufficiently powerful gravitational field to distort and magnify the light from background objects. Gravitational lenses magnify light from objects that would usually be too distant and faint to observe, and so these lenses can extend Hubble’s view even deeper into the Universe.

This observation is part of an ongoing project to fill short gaps in Hubble's observing schedule by systematically exploring the most massive galaxy clusters in the distant Universe, in the hopes of identifying promising targets for further study with both Hubble and the James Webb Space Telescope. This particular galaxy cluster lies at a vast distance of 4.6 billion light years from Earth.

Each year, the Space Telescope Science Institute is inundated with observing proposals for Hubble, in which astronomers suggest targets for observation. Even after selecting only the very best proposals, scheduling observations of all of Hubble's targets for a year is a formidable task. There is sometimes a small fraction of observing time left unused in Hubble's schedule, so in its ‘spare time’ the telescope has a collection of objects to explore – including the lensing galaxy cluster shown in this image.

Image Credit: ESA/Hubble and NASA, H. Ebeling

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell 1351

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The massive galaxy cluster Abell 1351 is captured in this image by the Hubble Space Telescope's Wide Field Camera 3 and Advanced Camera for Surveys. This galaxy cluster lies in the constellation Ursa Major in the northern hemisphere.

This image is filled with streaks of light, which are actually the images of distant galaxies. The streaks are the result of gravitational lensing, an astrophysical phenomenon that occurs when a massive celestial body such as a galaxy cluster distorts spacetime sufficiently strongly to affect the path of light passing through it – almost as if the light were passing through a gigantic lens. Gravitational lensing comes in two varieties – strong and weak – and both can give astronomers an insight into the distribution of mass within a lensing galaxy cluster such as Abell 1351.

This observation is part of an astronomical album comprising snapshots of some of the most massive galaxy clusters. This menagerie of massive clusters demonstrates interesting astrophysical phenomena such as strong gravitational lensing, as well as showcasing spectacular examples of violent galaxy evolution. To obtain this astronomical album, astronomers proposed a Snapshot Program to be slotted into Hubble's packed observing schedule. These Snapshot Programs are lists of separate, relatively short exposures which can fit into gaps between longer Hubble observations. Having a large pool of Snapshot candidates to dip into allows Hubble to use every second of observing time possible and to maximise the scientific output of the observatory.

Image Credit: ESA/Hubble and NASA, H. Ebeling

Acknowledgement: L. Shatz

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster MACSJ0138.0-2155

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The center of this image from the Hubble Space Telescope is framed by the tell-tale arcs that result from strong gravitational lensing, a striking astronomical phenomenon which can warp, magnify, or even duplicate the appearance of distant galaxies.

Gravitational lensing occurs when light from a distant galaxy is subtly distorted by the gravitational pull of an intervening astronomical object. In this case, the relatively nearby galaxy cluster MACSJ0138.0-2155 has lensed a significantly more distant quiescent galaxy – a slumbering giant known as MRG-M0138 which has run out of the gas required to form new stars and is located 10 billion light years away. Astronomers can use gravitational lensing as a natural magnifying glass, allowing them to inspect objects like distant quiescent galaxies which would usually be too difficult for even Hubble to resolve.

This image was made using observations from eight different infrared filters spread across two of Hubble's most advanced astronomical instruments: the Advanced Camera for Surveys and the Wide Field Camera 3. These instruments were installed by astronauts during the final two servicing missions to Hubble, and provide astronomers with superbly detailed observations across a large area of sky and a wide range of wavelengths.

Image Credit: ESA/Hubble and NASA, A. Newman, M. Akhshik, K. Whitaker

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster ACO S 295

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This ESA/Hubble picture showcases the galaxy cluster ACO S 295, as well as a jostling crowd of background galaxies and foreground stars. Galaxies of all shapes and sizes populate this image, ranging from stately spirals to fuzzy ellipticals. As well as a range of sizes, this galactic menagerie boasts a range of orientations, with spiral galaxies such as the one at the center of this image appearing almost face on, and some edge-on spiral galaxies visible only as thin slivers of light.

The cluster dominates the center of this image, both visually and physically. The huge mass of the galaxy cluster has gravitationally lensed the background galaxies, distorting and smearing their shapes. As well as providing astronomers with a natural magnifying glass with which to study distant galaxies, gravitational lensing has subtly framed the center of this image, producing a visually striking scene.

Image Credit: ESA/Hubble and NASA, F. Pacaud, D. Coe

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell 2813

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This extraordinary image from the Hubble Space Telescope of the galaxy cluster Abell 2813 (also known as ACO 2813) has an almost delicate beauty, which also illustrates the remarkable physics at work within it. The image spectacularly demonstrates the concept of gravitational lensing.

In amongst the tiny dots, spirals and ovals that are the galaxies that belong to the cluster, there are several distinct crescent shapes. These curved arcs of light are strong examples of a phenomenon known as gravitational lensing. The image was compiled using observations taken with the Hubble Space Telescope's Advanced Camera for Surveys (ACS) and Wide Field Camera 3 (WFC3).

Gravitational lensing occurs when an object's mass causes light to bend. The curved crescents and s-shapes of light in this image are not curved galaxies, but are light from galaxies that actually lie beyond Abell 2813. The galaxy cluster has so much mass that it acts as a gravitational lens, causing light from more distant galaxies to bend around it. These distortions can appear as many different shapes, such as long lines or arcs. This very visual evidence that mass causes light to bend has been famously used as a proof of one of the most famous scientific theories: Einstein’s theory of general relativity.

Image Credit: ESA/Hubble and NASA, D. Coe

Image enhancement: Jean-Baptiste Faure

Einstein Ring GAL-CLUS-022058s

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The narrow galaxy elegantly curving around its spherical companion in this image is a fantastic example of a truly strange and very rare phenomenon. This image, taken with the Hubble Space Telescope, depicts GAL-CLUS-022058s, located in the southern hemisphere constellation of Fornax (The Furnace). GAL-CLUS-022058s is the largest and one of the most complete Einstein rings ever discovered in our Universe. The object has been nicknamed by the Principal Investigator and his team who are studying this Einstein ring as the "Molten Ring", which alludes to its appearance and host constellation.

First theorised to exist by Einstein in his general theory of relativity, this object's unusual shape can be explained by a process called gravitational lensing, which causes light shining from far away to be bent and pulled by the gravity of an object between its source and the observer. In this case, the light from the background galaxy has been distorted into the curve we see by the gravity of the galaxy cluster sitting in front of it.

The near exact alignment of the background galaxy with the central elliptical galaxy of the cluster, seen in the middle of this image, has warped and magnified the image of the background galaxy around itself into an almost perfect ring. The gravity from other galaxies in the cluster is soon to cause additional distortions. Objects like these are the ideal laboratory in which to research galaxies too faint and distant to otherwise see.

Image Credit: ESA/Hubble and NASA, S. Jha

Acknowledgement: L. Shatz

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell 370 and Asteroids

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As if this Hubble Space Telescope picture isn't cluttered enough with myriad galaxies, nearby asteroids photobomb the image, their trails sometimes mimicking background astronomical phenomena.

The stunningly beautiful galaxy cluster Abell 370 contains an astounding assortment of several hundred galaxies tied together by the mutual pull of gravity. Located approximately 4 billion light-years away in the constellation Cetus, the Sea Monster, this immense cluster is a rich mix of a variety of galaxy shapes.

Entangled among the galaxies are thin, white trails that look like curved or S-shaped streaks. These are trails from asteroids that reside, on average, only about 260 million kilometers from Earth – right around the corner in astronomical terms. The trails appear in multiple Hubble exposures that have been combined into one image. Of the 22 total asteroid sightings for this field, five are unique objects. These asteroids are so faint that they were not previously identified.

The asteroid trails look curved due to an observational effect called parallax. As Hubble orbits around Earth, an asteroid will appear to move along an arc with respect to the vastly more distant background stars and galaxies. The motion of Earth around the Sun, and the motion of the asteroids along their orbits, are other contributing factors to the apparent skewing of asteroid paths.

All the asteroids were found manually, the majority by "blinking" consecutive exposures to capture apparent asteroid motion. Astronomers found a unique asteroid for every 10 to 20 hours of exposure time. These asteroid trails should not be confused with the mysterious-looking arcs of blue light that are actually distorted images of distant galaxies behind the cluster. Many of these far-flung galaxies are too faint for Hubble to see directly. Instead, in a dramatic example of "gravitational lensing", the cluster functions as a natural telescope, warping space and affecting light traveling through the cluster toward Earth.

The Frontier Fields program is a collaboration among several space telescopes and ground-based observatories to study six massive galaxy clusters and their effects. Using a different camera, pointing in a slightly different direction, Hubble photographed six so-called "parallel fields" at the same time it photographed the massive galaxy clusters. This maximised Hubble's observational efficiency in doing deep space exposures. These parallel fields are similar in depth to the famous Hubble Deep Field, and include galaxies about four-billion times fainter than can be seen by the human eye.

This image was assembled from several exposures taken in visible and infrared light. The field's position on the sky is near the ecliptic, the plane of our Solar System. This is the zone in which most asteroids reside, which is why Hubble astronomers saw so many crossings. Hubble deep-sky observations taken along a line-of-sight near the plane of our Solar System commonly record asteroid trails.

Image Credit: NASA, ESA, and B. Sunnquist and J. Mack (STScI)

Acknowledgment: NASA, ESA, and J. Lotz (STScI) and the HFF Team

Image enhancement: Jean-Baptiste Faure

Gravitationally-lensed Galaxy SDSS J090122.37+181432.3

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This Hubble Space Telescope image features the galaxy SDSS J090122.37+181432.3, also known as LRG-3-817. The galaxy, its image distorted by the effects of gravitational lensing, appears as a long arc to the left of the central galaxy cluster.

Gravitational lensing occurs when a large distribution of matter, such as a galaxy cluster, sits between Earth and a distant light source. As space is warped by massive objects, the light from the distant object bends as it travels to us and we see a distorted image of it. This effect was first predicted by Einstein’s general theory of relativity.

Strong gravitational lenses provide an opportunity for studying properties of distant galaxies, since Hubble can resolve details within the multiple arcs that are one of the main results of gravitational lensing. An important consequence of lensing distortion is magnification, allowing us to observe objects that would otherwise be too far away and too faint to be seen. Hubble makes use of this magnification effect to study objects beyond the sensitivity of its 2.4-meter-diameter primary mirror, showing us the most distant galaxies humanity has ever encountered.

This lensed galaxy was found as part of the Sloan Bright Arcs Survey, which discovered some of the brightest gravitationally lensed high-redshift galaxies in the night sky.

Image Credit: ESA/Hubble and NASA, S. Allam et al.

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell 2744 in X-rays and visible light

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This image is of galaxy cluster Abell 2744. Galaxy clusters are the largest objects in the universe held together by gravity. They contain enormous amounts of superheated gas, with temperatures of tens of millions of degrees, which glows brightly in X-rays, and can be observed across millions of light years between the galaxies. This image combines X-rays from the Chandra X-Ray Observatory (diffuse blue emission) with optical light data from the Hubble Space Telescope (red, green, and blue).

Image Credit: NASA/CXC; Optical: NASA/STScI

Image enhancement: Jean-Baptiste Faure

Star Orbiting Supermassive Black Hole

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Observations made with ESO's Very Large Telescope (VLT) have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's theory of general relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This artist's impression illustrates the precession of the star's orbit, with the effect exaggerated for easier visualisation.

Image Credit: ESO/L. Calçada

Colliding Galaxy Clusters MACS J0416.1-2403

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At first glance, this cosmic kaleidoscope of purple, blue and pink offers a strikingly beautiful – and serene – snapshot of the cosmos. However, this multi-coloured haze actually marks the site of two colliding galaxy clusters, forming a single object known as MACS J0416.1-2403 (or MACS J0416 for short).

MACS J0416 is located about 4.3 billion light-years from Earth, in the constellation of Eridanus. This new image of the cluster combines data from three different telescopes: the NASA/ESA Hubble Space Telescope (showing the galaxies and stars), the NASA Chandra X-ray Observatory (diffuse emission in blue), and the NRAO Jansky Very Large Array (diffuse emission in pink). Each telescope shows a different element of the cluster, allowing astronomers to study MACS J0416 in detail.

As with all galaxy clusters, MACS J0416 contains a significant amount of dark matter, which leaves a detectable imprint in visible light by distorting the images of background galaxies. In this image, this dark matter appears to align well with the blue-hued hot gas, suggesting that the two clusters have not yet collided; if the clusters had already smashed into one another, the dark matter and gas would have separated. MACS J0416 also contains other features – such as a compact core of hot gas – that would likely have been disrupted had a collision already occurred.

Together with five other galaxy clusters, MACS J0416 is playing a leading role in the Hubble Frontier Fields programme, for which this data was obtained. Owing to its huge mass, the cluster is in fact bending the light of background objects, acting as a magnifying lens. Astronomers can use this phenomenon to find galaxies that existed only hundreds of million years after the big bang.

Image Credit: NASA, ESA, CXC, NRAO/AUI/NSF, STScI, and G. Ogrean (Stanford University)

Acknowledgment: NASA, ESA, and J. Lotz (STScI), and the HFF team

Galaxy Cluster SDSS J0333+0651

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At first glance, this image is dominated by the vibrant glow of the swirling spiral to the lower left of the frame. However, this galaxy is far from the most interesting spectacle here – behind it sits a galaxy cluster. Galaxies are not randomly distributed in space; they swarm together, gathered up by the unyielding hand of gravity, to form groups and clusters. The Milky Way is a member of the Local Group, which is part of the Virgo Cluster, which in turn is part of the 100,000-galaxy-strong Laniakea Supercluster, or Local Supercluster.

The galaxy cluster seen in this image is known as SDSS J0333+0651. Clusters such as this can help astronomers understand the distant – and therefore early – Universe. SDSS J0333+0651 was imaged as part of a study of star formation in far-flung galaxies. Star-forming regions are typically not very large, stretching out for a few hundred light-years at most, so it is difficult for telescopes to resolve them at a distance. Even using its most sensitive and highest-resolution cameras, Hubble cannot resolve very distant star-forming regions, so astronomers use a cosmic trick: they search instead for galaxy clusters, which have a gravitational influence so immense that they warp the spacetime around them. This distortion acts like a lens, magnifying the light of galaxies sitting far behind the cluster and producing elongated arcs like the one seen to the left of center in this image.

Image Credit: ESA/Hubble and NASA

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster PSZ1 G311.65-18.48

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This image, taken with the Hubble Space Telescope, shows PSZ1 G311.65-18.48, a massive galaxy cluster, about 4.6 billion light-years away. Along its borders four bright arcs are visible; these are copies of the same distant galaxy, nicknamed the Sunburst Arc.

The Sunburst Arc galaxy is almost 11 billion light-years away and the light from it is being lensed into multiple images by gravitational lensing. The Sunburst Arc is among the brightest lensed galaxies known and its image is visible at least 12 times within the four arcs. Three arcs are visible in the top right of the image, the fourth arc in the lower left. The last one is partially obscured by a bright foreground star, which is located in the Milky Way.

Hubble uses these cosmic magnifying glasses to study objects otherwise too faint and too small for even its extraordinarily sensitive instruments. The Sunburst Arc is no exception, despite being one of the brightest gravitationally lensed galaxies known. The lens makes various images of the Sunburst Arc between 10 and 30 times brighter. This allows Hubble to view structures as small as 520 light-years across – a rare detailed observation for an object that distant. This compares reasonably well with star forming regions in galaxies in the local Universe, allowing astronomers to study the galaxy and its environment in great detail.

Hubble's observations showed that the Sunburst Arc is an analogue of galaxies which existed at a much earlier time in the history of the Universe: a period known as the epoch of reionisation – an era which began only 150 million years after the Big Bang.

The epoch of reionisation was a key era in the early Universe, one which ended the "dark ages", the epoch before the first stars were created when the Universe was dark and filled with neutral hydrogen. Once the first stars formed, they started to radiate light, producing the high-energy photons required to ionise the neutral hydrogen. This converted the intergalactic matter into the mostly ionised form in which it exists today. However, to ionise intergalactic hydrogen, high-energy radiation from these early stars would have had to escape their host galaxies without first being absorbed by interstellar matter. So far only a small number of galaxies have been found to "leak" high-energy photons into deep space. How this light escaped from the early galaxies remains a mystery.

The analysis of the Sunburst Arc helps astronomers to add another piece to the puzzle – it seems that at least some photons can leave the galaxy through narrow channels in a gas rich neutral medium. This is the first observation of a long-theorised process. While this process is unlikely to be the main mechanism that led the Universe to become reionised, it may very well have provided a decisive push.

Image Credit: ESA/Hubble, NASA, Rivera-Thorsen et al.

Image enhancement: Jean-Baptiste Faure

Galaxy Cluster Abell S1063

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Abell S1063, a galaxy cluster, was observed by the Hubble Space Telescope as part of the Frontier Fields programme. The huge mass of the cluster – containing both baryonic matter and dark matter – acts as cosmic magnification glass and deforms objects behind it. In the past astronomers used this gravitational lensing effect to calculate the distribution of dark matter in galaxy clusters. A more accurate and faster way, however, is to study the intracluster light (visible in blue), which follows the distribution of dark matter.

In recent decades astronomers have tried to understand the true nature of the mysterious substance that makes up most of the matter in the Universe – dark matter – and to map its distribution in the Universe. Intracluster light is a byproduct of interactions between galaxies. In the course of these interactions, individual stars are stripped from their galaxies and float freely within the cluster. Once free from their galaxies, they end up where the majority of the mass of the cluster, mostly dark matter, resides.

Currently, all that is known about dark matter is that it appears to interact with regular matter gravitationally, but not in any other way. To find that it self-interacts would place significant constraints on its identity.

Image Credit: NASA, ESA, and M. Montes (University of New South Wales, Sydney, Australia)

Image enhancement: Jean-Baptiste Faure