# How to find out if a galaxy is in a specific cluster

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I am trying to find out how to find out if a certain galaxy is located inside a cluster. For the famous ones this is quite easy (sometimes even on wikipedia!), but I haven't been able to do it for some less-known galaxies. I have tried to look them up on Simbad, but I couldn't find it written down anywhere (even for the ones I know to be in clusters). Also generic "googling" has not helped me. Do you have any suggestions?

Your question is not specific, so I'm afraid the answer is general too.

Using Simbad/NED check to see what observations/survey data exists for you galaxies. Almost all large galaxy surveys, and a lot of smaller ones, have corresponding "environment catalogues" where the galaxies are associated to their corresponding group. By cross-matching the coordinates (ra,dec) and redshift (or v) of your galaxies with the members of groups and clusters in one or more of these catalogues you should be able to find a match.

First: You can use all galaxies in the line of sight of the cluster to construct their color-magnitude diagram. Those galaxies on the Red Sequence are more likely to be in the cluster (not always true).

Second: You should be able to determine their redshift, and compare with the mean cluster redshift. To be able to make cut, see next step.

Third: You should run some algorithms in order to see whether the redshifts/velocities you have are within the clusters mean velocity dispersion (usually in the clustercentric-velocity space), as used in these papers for example: https://arxiv.org/pdf/1601.06080.pdf

You want to use NED (NASA Extragalactic Database) rather than SIMBAD, since the former has more galaxy- and cluster-specific information.

I'm going to assume you have a reasonably accurate position for your galaxy as well as its redshift (in km/s). The basic idea is to identify candidate galaxy clusters which are close enough to your galaxy on the sky and in redshift so that the galaxy could potentially be a member of one of them.

1. Determine what angular radius $R$ corresponds to 1 or 2 megaparsecs at the redshift of your galaxy (e.g., using Ned Wright's Cosmology Calculator)

2. Do a search in NED for Galaxy Clusters within R of the coordinates of your galaxy. If your galaxy has a name, you can skip the coordinate lookup and use the "Near name" search form; if you only have coordinates, there's a "Near position" search form.

A. Put the estimated angular radius $R$ (converted to arc minutes) in the "Search Radius" box;

B. Set the "Selection in Redshift" form to "Between" and give it lower and upper limits of $V_{ m gal} - 2000$ and $V_{ m gal} + 2000$ in km/s, where $V_{ m gal}$ is the redshift (in km/s) of your galaxy;

C. Under "Selection by Object Type", select "Galaxy Clusters" in the "Classified Extragalactic Objects" list.

That should give you a list of galaxy clusters within the specified radius of your galaxy which are at a similar redshift. Note that the same cluster may come up multiple times under different names (i.e., from different cluster catalogs), but it should be clear from the coordinates and the redshifts which are the same clusters.

Now the question is whether your galaxy is plausibly a member of a given candidate cluster or not. Ideally you should find some measurement of each cluster's velocity dispersion $sigma$; if the galaxy's redshift isn't within $pm 3 sigma$ of the cluster's redshift $V_{ m cluster}$, then you can ignore that cluster, since it's very unlikely that a galaxy with that relative velocity with respect to the cluster is actually gravitationally bound to the cluster.

You can't really do better than a probabilistic assessment: roughly how likely it is that the galaxy is a member of a given cluster. Galaxies which are members of a cluster are more likely to be close to the center (on the sky); member galaxies closer to the center can have a higher range of relative velocities, while member galaxies further away should have velocities closer to the cluster redshift.

A quick example: Is NGC 4889 ($V_{ m gal} = 6495$ km/s) a member of a cluster? At its redshift, 1 Mpc = 33 arc minutes; if we search on NED for galaxy clusters within 33 arcmin of NGC 4889 with redshift limits of 4500-8500 km/s, we get 6 results, 4 of which are just the Coma Cluster in different catalogs, with $V_{ m cluster} sim 6900$ km/s. Since the various distances ("Separ. arcmin") translate to $sim$ 100-300 kpc from the cluster center, and since the Coma cluster has a velocity dispersion of $sim$ 1000 km/s and the difference between NGC 4889 and the cluster is only $sim$ 400 km/s, it's a very good bet that this galaxy is a member of the Coma Cluster. (It's actually one of the two cD galaxies in the cluster center.)

## Astronomy Picture of the Day

Discover the cosmos! Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer.

2019 February 26
Simulation TNG50: A Galaxy Cluster Forms
Video Credit: IllustrisTNG Project Visualization: Dylan Nelson (Max Planck Institute for Astrophysics) et al.
Music: Symphony No. 5 (Ludwig van Beethoven), via YouTube Audio Library

Explanation: How do clusters of galaxies form? Since our universe moves too slowly to watch, faster-moving computer simulations are created to help find out. A recent effort is TNG50 from IllustrisTNG, an upgrade of the famous Illustris Simulation. The first part of the featured video tracks cosmic gas (mostly hydrogen) as it evolves into galaxies and galaxy clusters from the early universe to today, with brighter colors marking faster moving gas. As the universe matures, gas falls into gravitational wells, galaxies forms, galaxies spin, galaxies collide and merge, all while black holes form in galaxy centers and expel surrounding gas at high speeds. The second half of the video switches to tracking stars, showing a galaxy cluster coming together complete with tidal tails and stellar streams. The outflow from black holes in TNG50 is surprisingly complex and details are being compared with our real universe. Studying how gas coalesced in the early universe helps humanity better understand how our Earth, Sun, and Solar System originally formed.

## Finding Capricornus

To locate Capricornus, simply look for the constellation Sagittarius. It's in the southern skies for observers located north of the equator, and higher in the northern sky for folks south of the equator. Capricornus looks very much like a squashed-looking triangle. Some charts, like the one shown here, depict it as two triangles arranged along a long line. It lies along the ecliptic, which is the path the Sun appears to take across the sky throughout the year. The Moon and planets also appear to move roughly along the ecliptic.

## Tips for galaxy viewing

Galaxies are extremely far away, ranging from a few million to billions of light-years away, so they appear extremely faint despite emitting the combined light of billions of stars. The keys to seeing them include a very dark night sky, away from the city lights and during moonless nights, and a good-size telescope (6 inches and larger to get the best results). A telescope acts as a "light bucket," collecting the photons (light particles) that reach it from the sky, and the wider its lens or mirror, the more photons it will concentrate into your eyepiece.

Your peripheral vision is more sensitive to faint light. If you think you have a galaxy in your telescope's field of view, try directing your gaze to one area and noticing the rest of the field visible in the scope. The averted-vision technique takes a bit of practice, but it works wonders. A gentle tap on the telescope will set a faint object wiggling, rendering it much easier to notice. If you are using a computerized GoTo telescope, slew it a little ways off the target, and watch the object slide into view as you command the telescope to recenter the target. [Best Telescopes for the Money - Reviews and Guide]

## Astronomers use a Galaxy Cluster as an Extremely Powerful “Natural Telescope” to Peer Even Farther into the Universe

When it comes to studying some of the most distant and oldest galaxies in the Universe, a number of challenges present themselves. In addition to being billions of light years away, these galaxies are often too faint to see clearly. Luckily, astronomers have come to rely on a technique known as Gravitational Lensing, where the gravitational force of a large object (like a galactic cluster) is used to enhance the light of these fainter galaxies.

Using this technique, an international team of astronomers recently discovered a distant and quiet galaxy that would have otherwise gone unnoticed. Led by researchers from the University of Hawaii at Manoa, the team used the Hubble Space Telescope to conduct the most extreme case of gravitational lensing to date, which allowed them to observe the faint galaxy known as eMACSJ1341-QG-1.

The study that describes their findings recently appeared in The Astrophysical Journal Letters under the title “Thirty-fold: Extreme Gravitational Lensing of a Quiescent Galaxy at z = 1.6″. Led by Harald Ebeling, an astronomer from the University of Hawaii at Manoa, the team included members from the Niels Bohr Institute, the Centre Nationale de Recherche Scientifique (CNRS), the Space Telescope Science Institute, and the European Southern Observatory (ESO).

The quiescent galaxy eMACSJ1341-QG-1 as seen by the Hubble Space Telescope. The yellow dotted line traces the boundaries of the galaxy’s gravitationally lensed image. The inset on the upper left shows what eMACSJ1341-QG-1 would look like if we observed it directly, without the cluster lens. Credit: Harald Ebeling/UH IfA

The team began by taking images of the faint galaxy with the Hubble and then conducting follow-up spectroscopic observations using the ESO/X-Shooter spectrograph – which is part of the Very Large Telescope (VLT) at the Paranal Observatory in Chile. Based on their estimates, the team determined that they were able to amplify the background galaxy by a factor of 30 for the primary image, and a factor of six for the two remaining images.

This makes eMACSJ1341-QG-1 the most strongly amplified quiescent galaxy discovered to date, and by a rather large margin! As Johan Richard – an assistant astronomer at the University of Lyon who performed the lensing calculations, and a co-author on the study – indicated in a University of Hawaii News release:

“The very high magnification of this image provides us with a rare opportunity to investigate the stellar populations of this distant object and, ultimately, to reconstruct its undistorted shape and properties.”

A spiral galaxy ablaze in the blue light of young stars from ongoing star formation (left) and an elliptical galaxy bathed in the red light of old stars (right). Credit: Sloan Digital Sky Survey, CC BY-NC.

Although other extreme magnifications have been conducted before, this discovery has set a new record for the magnification of a rare quiescent background galaxy. These older galaxies are not only very difficult to detect because of their lower luminosity the study of them can reveal some very interesting things about the formation and evolution of galaxies in our Universe.

“We specialize in finding extremely massive clusters that act as natural telescopes and have already discovered many exciting cases of gravitational lensing. This discovery stands out, though, as the huge magnification provided by eMACSJ1341 allows us to study in detail a very rare type of galaxy.”

“[A]s we look at more distant galaxies, we are also looking back in time, so we are seeing objects that are younger and should not yet have used up their gas supply. Understanding why this galaxy has already stopped forming stars may give us critical clues about the processes that govern how galaxies evolve.”

An artist’s impression of the accretion disc around the supermassive black hole that powers an active galaxy. Credit: NASA/Dana Berry, SkyWorks Digital

In a similar vein, recent studies have been conducted that suggest that the presence of a Supermassive Black Hole (SMBH) could be what is responsible for galaxies becoming quiescent. As the powerful jets these black holes create begin to drain the core of galaxies of their dust and gas, potential stars find themselves starved of the material they would need to undergo gravitational collapse.

In the meantime, follow-up observations of eMACSJ1341-QG1 are being conducted using telescopes at the Paranal Observatory in Chile and the Maunakea Observatories in Hawaii. What these observations reveal is sure to tell us much about what will become of our own Milky Way Galaxy someday, when the last of the dust and gas is depleted and all its stars become red giants and long-lived red dwarfs.

## CFHT and W.M. Keck Observatories help NASA space telescopes find Galaxy Cluster with a Vibrant Heart

A massive cluster of galaxies, called SpARCS1049+56, can be seen in this multi-wavelength view from NASA's Hubble and Spitzer space telescopes. At the middle of the picture is the largest, central member of the family of galaxies (upper right red dot of central pair). Unlike other central galaxies in clusters, this one is bursting with the birth of new stars.

Scientists say this star birth was triggered by a collision between a smaller galaxy and the giant, central galaxy. The smaller galaxy's wispy, shredded parts, called a tidal tail, can be seen coming out below the larger galaxy. Throughout this region are features called "beads on a string," which are areas where gas has clumped to form new stars.

This type of "feeding" mechanism for galaxy clusters -- where gas from the merging of galaxies is converted to new stars -- is rare.

The Hubble data in this image show infrared light with a wavelength of 1 micron in blue, and 1.6 microns in green. The Spitzer data show infrared light of 3.6 microns in red.

Image credit: NASA/STScI/ESA/JPL-Caltech/McGill

## Astronomers have discovered a rare beast of a galaxy cluster whose heart is bursting with new stars. The unexpected find, made with NASA's Spitzer and Hubble space telescopes, MOSFIRE on Keck Observatory and MegaCam on CFHT, suggests that behemoth galaxies at the cores of these massive clusters can grow significantly by feeding off gas stolen from another galaxy.

"Usually, the stars at the centers of galaxies clusters are old and dead, essentially fossils," said Tracy Webb of McGill University, Montreal, Canada, lead author of a new paper on the findings accepted for publication in the Astrophysical Journal. "But we think the giant galaxy at the center of this cluster is furiously making new stars after merging with a smaller galaxy."

Galaxy clusters are vast families of galaxies bound by the ties of gravity. Our own Milky Way resides in a small galaxy group, called the Local Group, which itself is on the periphery of the vast Laniakea supercluster of 100,000 galaxies (Laniakea is Hawaiian for "immeasurable heaven.")

The cluster in the new study, referred to by astronomers as SpARCS1049+56, has at least 27 galaxy members, probably more, and a combined mass equal to nearly 400 trillion suns. It is located 4 billion light-years away in the Ursa Major constellation. The object was initially discovered using Spitzer and the Canada-France-Hawaii telescope, located on Maunakea in Hawaii, and confirmed using MOSFIRE on Keck Observatory.

What makes this cluster unique is its luminous heart of new stars. At the core of most massive galaxy clusters lies one hulking galaxy. The galaxy dominating the cluster SpARCS1049+56 is rapidly spitting out an enormous number of stars -- about 860 new ones a year. For reference, our Milky Way makes only about one to two stars per year.

"With Spitzer's infrared camera, we can actually see the ferocious heat from all these hot young stars," said co-author Jason Surace from NASA's Spitzer Science Center at the California Institute of Technology in Pasadena, California. Spitzer detects infrared light, so it can see the warm glow of hidden, dusty regions where stars form.

Follow-up studies with Hubble in visible-light helped confirm the source of the fuel, or gas, for the new stars. A smaller galaxy seems to have recently merged with the monster galaxy in the middle of the cluster, lending its gas to the larger galaxy and igniting a fury of new stars.

"Hubble found a train wreck of a merger at the center of this galaxy," said Webb.

Hubble specifically detected features called beads on a string, which are pockets of gas that condense where new stars are forming. Beads on a string are telltale signs of collisions between gas-rich galaxies, a phenomenon known to astronomers as wet mergers, where "wet" refers to the presence of gas. In these smash-ups, the gas is quickly converted to new stars. Dry mergers, by contrast, occur when galaxies with little gas collide and no new stars are formed.

Typically, galaxy clusters grow in mass either through dry mergers at their core, or by siphoning gas into their centers, as is the case with the megatropolis of a galaxy cluster known as the Phoenix cluster.

The new discovery is one of the first known cases of a wet merger at the core of a distant galaxy cluster. Hubble previously discovered another closer galaxy cluster containing a wet merger, but it wasn't forming stars as vigorously.

The researchers are planning more studies to find out how common galaxy clusters like SpARCS1049+56 are. The cluster may be an outlier -- or it may represent an early time in our universe when gobbling up gas-rich galaxies was the norm.

NASA's Jet Propulsion Laboratory, Pasadena, California, manages the Spitzer Space Telescope mission for NASA's Science Mission Directorate, Washington. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena. Spacecraft operations are based at Lockheed Martin Space Systems Company, Littleton, Colorado. Data are archived at the Infrared Science Archive housed at the Infrared Processing and Analysis Center at Caltech. Caltech manages JPL for NASA.

The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency). NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore, Maryland, conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy in Washington.

## When a Galaxy Meets a Cluster

Space is big and even the regions between one galaxy and its neighbor can seem empty once you get “out there.” So, what happens when two objects get close together in space? You get collisions and close approaches.

The birth of our own Moon was likely the result of a collision between the infant Earth and a Mars-sized world called “Theia”. Of course, one galaxy can collide with another. That’s what astronomers predict will happen between the Milky Way and Andromeda Galaxies in a few billion years. The image above shows what it might look like to an observer on a planet inside one of the two galaxies. But, what about other kinds of collisions and near-misses?

Astronomers using the Canada-France-Hawaii Telescope on the Big Island of Hawai’i took a look at a nearby globular cluster called M92. It’s about 27,000 light-years from Earth and can be spotted just at the top of the constellation Hercules. What the astronomers “see” is a stream of stars being pulled out of the cluster. They’re not immediately obvious to the casual observer, but in specific wavelengths of light, they stand right out, as shown here in a plot of data from the observations.

These long, thin lines of stars exist because M92 is too close to the Milky Way galaxy. The immense gravitational pull of the Milky Way is ripping the smaller conglomerations of stars apart. It’s not a fast process. Some of the stellar streams caused by such interactions can last for billions of years. In the case of the stream from M92, it’s been around for about 500 million years. And, that short length of time leads to some interesting questions.

### Why Such a Young Stream?

The observations from CFHT and the Pan-STARRS1 survey telescope at Haleakala on Maui provided a lot of data to help astronomers figure out how long the M92 stream has been around. The data also give some idea of the cluster’s origin. It might also help astronomers figure out the distribution of dark matter in the Milky Way and its role in corraling a globular cluster as it passes by.

The M92 cluster itself is about 1.1 billion years old, but the stream is 500 million yeas old. So, something happened ‘recently’ to cause the cluster to lose stars to the stream. That “something” is gravitational interaction with the Milky Way as the cluster passed by. But from where? That raises questions about where M92 originally formed. If it formed elsewhere and only started losing stars as it got closer to the Milky Way, then perhaps astronomers can use the information about its stars to figure out where it came from originally.

Typically, the globular clusters like M92 orbit the central region of the Milky Way. Such clusters contain stars tightly bound together in a spherical shape. How and where these clusters form is still an unfinished astronomy story. In many clusters, most stars are about the same age. That means they all formed about the same time. However, some have stars of varying ages, which suggests they formed in “waves” of starbirth. If we look at various galaxies that are undergoing starburst activity, it’s easy to see many globulars forming in such areas. And, some of those galaxies are in collisions or close interactions, which spurs the starburst activity.

M92 itself is fairly young, compared to the galaxy. At 1.1 billion years, it’s about 10 billion years younger than the Milky Way. Our galaxy began forming shortly after the birth of the universe, or about 13.4 billion years ago. Could the Milky Way have undergone a large starburst epoch about the time M92 was born? It’s not likely. So, it looks pretty likely that M92 formed somewhere else and got caught up in the gravitational tug of our galaxy and its dark matter halo.

Today, the Milky Way is actively consuming (cannibalizing) several smaller galaxies, including the nearby Large and Small Magellanic Clouds. That’s how the Milky Way has grown over time. It does have other globulars that are much older than M92, so they likely formed along with our galaxy. But, there is little evidence showing that the Milky has interacted with other large galaxies in the past 10 billion years. So, that avenue of globular creation doesn’t seem to be how M92 was created. Perhaps it was born in another collision and migrated here? That would be a fascinating story!

### Questions about Our Galaxy and M92

Astronomers want to find out where M92 formed and what the conditions were when it did. What was going on 1.1 billion years ago when it was born? And, what were conditions like when it wandered too close to the core of our galaxy and began losing its stars? Those questions await more observations and answers.

## How to find out if a galaxy is in a specific cluster - Astronomy

A cluster of galaxies imaged in X-ray light by Chandra.

The central region of spiral galaxy M81 in X-ray light by Chandra.

Astro-H has been designed with several key science questions in mind. The questions astronomers would like Astro-H to help answer include:

• How do structures such as galaxies and clusters of galaxies form and evolve? How do they create and disperse metals? Astro-H will measure the emission lines from galaxies, clusters of galaxies, and supernova remnants to deduce the abundances of different chemical elements along with determining other properties such as turbulence.
• How do supermassive black holes feed back energy into galaxies and clusters? How do neutron stars and white dwarfs accrete and eject material? Astro-H will observe supermassive black holes and X-ray binary systems to look for iron emission and absorption lines. By looking at the shape of the emission or absorption line astronomers can figure out how material near the black hole is moving.
• What is the nature of dark matter and dark energy? Astro-H will look at galaxy clusters and compare the data to models from cosmologists to find which models the data supports.

To find out more about X-ray astronomy in general and some of the specific objects that astronomers study using X-ray information, visit the main Science page.

## The Lion’s Share of Galaxies

The Universe is built in layers. The fundamental units, you could argue, are stars. Some are solitary (like the Sun), some orbit each other as binary stars. By the hundreds or thousands they comprise clusters, and if you have a few billion to a few hundred billion, you get a galaxy.

Our Milky Way is part of a small group of about 50 other galaxies, most of which are smallish dwarfs. The next step up from a group is a galaxy cluster, which can contain hundreds to thousands of galaxies.

One of the nearest galaxy clusters is Abell 1367, more commonly called the Leo Cluster. A search of the astronomical literature shows it’s unclear how many galaxies can be considered Leo citizens there are at least 70 major galaxies and perhaps many more. It’s 300 million light-years away in the constellation of Leo (of course).

And it’s gorgeous. Astronomer Adam Block took an amazing image of the central region of the cluster using the 0.81 meter Schulman telescope on Mt. Lemmon in Arizona:

That’s just one part of the cluster, the core click the picture to see the whole magnificent image Adam created. As you can see, there’s one big fuzzy elliptical galaxy dominating the cluster core. That’s NGC 3842, and it sits at the center of mass of the cluster. It probably grew that large by cannibalizing other galaxies, eating them and adding to its girth.

Most of the galaxies in the cluster look yellow, even the spirals. That happens a lot in clusters. There is gas between the galaxies, and as they plow through it at high speed, attracted to each other by their mutual gravity, the gas inside the galaxies is stripped away. (The analogy I like is opening the windows in a car to air out a, um, foul odor.) If it had stayed in the galaxies, that gas would’ve gone into making new stars, so in those stripped galaxies no stars have been made in a long time. The stars we see in them are old. Blue stars don’t live long, so over time the galaxies redden.

The exception is that edge-on blue galaxy, called UGC 6697. It appears to be undergoing a burst of star formation, and that’s likely due to the gas inside the galaxy getting compressed as it passes through the cluster gas. It lies on the outskirts of the cluster (it’s superposed on the core at the moment), where the amount of gas is just enough to trigger star birth but not enough to sweep away the galaxy’s gas completely.

Closer view of a handful of distorted spiral galaxies in the Leo Cluster.

Surveying the rest of the image yields more wonders. Near the top is a gorgeous spiral galaxy, canted to our line of sight, along with some smaller disturbed galaxies. These look like they’ve recently suffered collisions with other galaxies, another common occurrence in the busy environment of clusters. The face-on galaxy, cut off at the top of the frame, is a complete mess. Collisions between galaxies can really play havoc on their structure … as we’ll find out in a mere 4 billion years, when our Milky Way collides and merges with the massive Andromeda galaxy.

Three disk galaxies, all gassed out.

I also like this little trio of galaxies in the cluster. All three are spirals, but the one at the top seems to have no obvious arms (in that case it’s just called more generically a “disk galaxy”). The other two have strong bars, linear streams of stars across their nuclei. Our own galaxy is a barred spiral, though very different than these the Milky Way is still busily churning out young stars, so our spiral arms are very obvious, and very blue.

The Leo Cluster is roughly the same distance away from the Earth as the far larger and richer Coma Cluster. Together, along with some smaller ones like the Hercules Cluster, they form the Coma Supercluster, the next layer of the hierarchy.

And it goes up from there. The Coma Supercluster, along with many others, forms a structure called the Great Wall, a vast complex hundreds of millions light-years long. It’s one of the largest structures in the entire Universe, but there are many like it.

It’s incredible, mind-numbing. The scale of the Universe crushes our sense of size, fills our capacity of awe to overflowing. I know some people despair when they think about this, but it has the opposite effect on me: I am uplifted. Not only is it wonderful enough that such things exist at all, but how astonishing is it that we can see them, study them, understand them? Perhaps not completely, of course, not yet and perhaps never in their entirety.

But we can try. And that makes us important, even at the tiny, tiny scale of this vast, vast Universe we inhabit.

## The evolution of massive galaxy clusters

A multi-wavelength image of the distant massive galaxy cluster, IDCS J1426.5+3508 (X-rays from Chandra in blue, visible light from Hubble in green, and infrared data from Spitzer in red). A new millimeter wavelength study of massive clusters with the South Pole Telescope has found good agreement with current ideas about galaxy cluster evolution. Credit: NASA Chandra, Spitzer, Hubble

Galaxy clusters have long been recognized as important laboratories for the study of galaxy formation and evolution. The advent of the new generation of millimeter and submillimeter wave survey telescopes, like the South Pole Telescope (SPT), has made it possible to identify faint galaxy clusters over large fractions of the sky using an effect first recognized by Rashid Sunyaev and Yakov Zel'dovich in 1969: When hot electrons in the cluster gas interact with light from the ubiquitous cosmic microwave background they increase its brightness very slightly.

SAO is a partner institution in the South Pole Telescope, which has been conducting a large survey covering about six percent of the whole sky with a sensitivity and angular resolution suitable for spotting galaxy clusters as far away as those from the epoch about four billion years after the big bang. One advantage of studying this sample of clusters is that because they have been identified from their hot gas signatures (rather than from the starlight of their member galaxies), the evolution of the cluster and its ensemble population is easier to disentangle.

CfA astronomer Brian Stalder and a team of colleagues used the SPT survey data to identify twenty-six of the most massive clusters known, each with a mass of over about a million billion solar-masses. They find that the clusters are broadly in agreement with the current thinking about the evolution of massive clusters and the stars in these galaxies. The models suggest a generally passive evolution (that is, without unusual disruptions by collisions or nuclear black hole feedback) and imply that most of the star formation and galaxy merging took place at an even earlier epoch than this sample covers. The scientists note, however, that a larger sample is needed to extend the conclusions, and it is currently being undertaken using other large optical telescopes including the Magellan twin 6.5 meter telescopes in Chile of which SAO is also a leading partner.