The Evolution of Galaxy

Home , Coma Cluster, Galaxy group, List of Abell clusters

by J. Patrick Henry, Ulrich G. Briel and Hans Böhringer

he royal Ferret of Comets was not to be misled by them during his real blages of galaxies in roughly the same busy tracking his prey. On the work, the search for comets. Later he way that galaxies are assemblages of night of April 15, 1779, commented that a small region on the stars. On the cosmic organizational T — Charles Messier watched from his Paris Virgo-Coma border contained 13 of chart, they are the vice presidents only observatory as the Comet of 1779 slow- the 109 stationary splotches that he, one level below the universe itself. In ly passed between the Virgo and Coma with the aid of Pierre Mechain, eventu- fact, they are more massive relative to a Berenices constellations on its long ally identiﬁed—the Messier objects well human being than a human being is rel- journey through the solar system. known to amateur and professional as- ative to a subatomic particle. Messier’s renown in comet spotting had tronomers today. In many ways, clusters are the closest inspired the furry moniker from King As so often happens in astronomy, that astronomers can get to studying the Louis XV, but on this night he took his Messier found something completely universe from the outside. Because a place in astronomy history books for a different from what he was seeking. He cluster contains stars and galaxies of ev- different reason. He noticed three fuzzy had discovered the ﬁrst example of the ery age and type, it represents an aver- patches that looked like comets yet did most massive things in the universe age sample of cosmic material—includ- not move from night to night; he added held together by their own gravity: ing the dark matter that choreographs them to his list of such impostors so as clusters of galaxies. Clusters are assem- the movements of celestial objects yet

52 Scientific American December 1998 The Evolution of Galaxy Clusters Copyright 1998 Scientific American, Inc. The most massive objects in the universe are huge clusters of galaxies and gas that have slowly congregated over billions of years. The process of agglomeration may now be ending

to three of the most fundamental issues dark matter, astronomers have learned in cosmology: the composition, organi- a bit more about it since the days of zation and ultimate fate of the universe. Zwicky. But they are still in the uncom- A few years after Messier’s observa- fortable position of not knowing what tions in Paris, William Herschel and his most of the universe is made of [see sister, Caroline, began to examine the “Dark Matter in the Universe,” by Messier objects from their garden in Eng- Lawrence M. Krauss; Scientiﬁc Amer- land. Intrigued, they decided to search ican, December 1986]. for others. Using substantially better telescopes than their French predecessor Light from Dark Matter had, they found more than 2,000 fuzzy spots—including 300 in the Virgo cluster mpelled by these mysteries, the pace alone. Both William and his son, John, Iof discovery in the study of clusters noticed the lumpy arrangement of these has accelerated over the past 40 years. E SCIENCE INSTITUTE

OP objects on the sky. What organized these Astronomers now know of some 10,000 objects (which we now know to be gal- of them. American astronomer George

CE TELESC axies) into the patterns they saw? Abell compiled the ﬁrst large list in the A A second question emerged in the early 1950s, based on photographs of mid-1930s, when astronomers Fritz the entire northern sky taken at Palomar Zwicky and Sinclair Smith measured Observatory in California. By the 1970s the speeds of galaxies in the Virgo clus- astronomers felt they at least understood E WFPC TEAM AND SP

OP ter and in a slightly more distant cluster the basic properties of clusters: They in Coma. Just as the planets orbit about consisted of speeding galaxies bound to- CE TELESC

A the center of mass of the solar system, gether by huge amounts of dark matter. galaxies orbit about the center of mass They were stable and immutable objects. of their cluster. But the galaxies were Then came 1970. In that year a new , HUBBLE SP

on orbiting so fast that their collective satellite, named Uhuru (“freedom” in mass could not provide enough gravity Swahili) in honor of its launch from ashingt to hold them all together. The clusters Kenya, began observing a form of radi- y of W

Clusters ersit had to be nearly 100 times as heavy as ation hitherto nearly inaccessible to as- niv

U the visible galaxies, or else the galaxies tronomers: x-rays. Edwin M. Kellogg, UM

A would have torn out of the clusters Herbert Gursky and their colleagues at long ago. The inescapable conclusion American Science and Engineering, a was that the clusters were mostly made small company in Massachusetts, point- WILLIAM A. B of unseen, or “dark,” matter. But what ed Uhuru at the Virgo and Coma clus- TWO BRIGHT GALAXIES in the Coma was this matter? ters. They found that the clusters consist cluster, one elliptical (top left) and the other These two mysteries—the uneven dis- not only of galaxies but also of huge spiral (top right), appear in this composite tribution of galaxies in space and the amounts of gas threading the space be- Hubble Space Telescope image taken in 1994. The Coma cluster, located some 300 unknown nature of dark matter—con- tween the galaxies. The gas is too tenu- million light-years away, was one of the first tinue to confound astronomers. The ous to be seen in visible light, but it is so galaxy clusters identified by astronomers. former became especially puzzling after hot—more than 25 million degrees Cel- Most of the other splotches in the image are the discovery in the mid-1960s of the sius—that it pours out x-rays. galaxies at even greater distances. cosmic microwave background radia- In short, astronomers had found tion. The radiation, a snapshot of the some of the dark matter—20 percent of universe after the big bang and before it by mass. Although the gas is not the formation of stars and galaxies, is enough to solve the dark matter mys- cannot be seen by human eyes. And be- almost perfectly smooth. Its tiny imper- tery completely, it does account for cause a cluster is the result of gravity fections somehow grew to the struc- more mass than all the galaxies put to- acting on immense scales, its structure tures that exist today, but the process is gether. In a way, the term “clusters of and evolution are tied to the structure still not clear [see “Very Large Structures galaxies” is inaccurate. These objects and evolution of the universe itself. in the Universe,” by Jack O. Burns; Sci- are balls of gas in which galaxies are Thus, the study of clusters offers clues entiﬁc American, July 1986]. As for embedded like seeds in a watermelon

The Evolution of Galaxy Clusters Scientific American December 1998 53 Copyright 1998 Scientific American, Inc. and digesting nearby matter is in stark contrast to the static view that astronomers held just a few years ago. OGY

VICE Taking Their Temperature

AL SER ver since astronomers obtained the CHIV Eﬁrst good x-ray images in the early 1980s, they have wanted to measure ASE AND AR ALIFORNIA INSTITUTE OF TECHNOL AB C

T the variation of gas temperature across clusters. But making these measurements is substantially more difficult than mak- 1993–1995 © LEICESTER DA ing images, because it requires an anal- COMA CLUSTER looks different in visible light (left) and in x-rays (right). In visible ysis of the x-ray spectrum for each point light, it appears to be just an assemblage of galaxies. But in x-rays, it is a gargantuan in the cluster. Only in 1994 did the first ball of hot gas some five million light-years across. temperature maps appear. The maps have proved that the formation of clusters is a violent process. [see “Rich Clusters of Galaxies,” by One lump to the southwest is moving Images of the cluster Abell 2256, for Paul Gorenstein and Wallace Tucker; into the main body of the cluster, where example, show that x-ray emission has Scientific American, November 1978]. other lumps already reside. Virgo, by not one but rather two peaks. The Since the early 1970s, the x-ray emis- comparison, has an amorphous shape. western peak is slightly flattened, sug- sion has been scrutinized by other satel- Although it has regions of extra x-ray gesting that a group slamming into the lites, such as the Einstein X-Ray Obser- emission, these bright spots are coming main cluster has swept up material just vatory, the Roentgen Satellite (ROSAT) from some of the Messier galaxies rath- as a snowplow does. A temperature and the Advanced Satellite for Cosmol- er than from clumps of gas [see right il- map supports this interpretation [see il- ogy and Astrophysics (ASCA). Our own lustration on page 56]. Only the core lustration on opposite page]. The west- research mainly uses ROSAT. The first region in the northern part of Virgo has ern peak, it turns out, is comparatively x-ray telescope to record images of the a nearly symmetrical structure. cool; its temperature is characteristic of entire sky, ROSAT is well suited for ob- Such x-ray images have led astrono- the gas in a group of galaxies. Because servations of large diffuse objects such mers to conclude that clusters form from groups are smaller than clusters, the as clusters and is now engaged in mak- the merger of groups. The lumps in the gravitational forces within them are ing detailed images of these regions. main body of the Coma cluster presum- weaker; therefore, the speed of the gas With this new technology, astronomers ably represent groups that have already molecules within them—that is, their have extended the discoveries of Mes- been drawn in but have not yet been temperature—is lower. A typical group sier, Zwicky and the other pioneers. fully assimilated. Virgo seems to be in is 50 trillion times as massive as the sun When viewed in x-rays, the Coma an even earlier stage of formation. It is and has a temperature of 10 million de- cluster has a mostly regular shape with still pulling in surrounding material and, grees C. By comparison, a typical clus- a few lumps [see left illustration on page at the current rate of progress, will look ter weighs 1,000 trillion suns and regis- 56]. These lumps appear to be groups like Coma after a few billion years. This ters a temperature of 75 million degrees of galaxies—that is, miniature clusters. dynamic view of clusters gobbling up C; the heaviest known cluster is five

GAS GALAXIES GALAXIES “SNOWPLOWED” GAS GAS

GROUP

CLUSTER FILMS

SLIM BEFORE MERGER DURING MERGER ABSORPTION OF GALAXY GROUP allows a cluster to grow to cluster, their progress unimpeded by the tenuous gas. Eventually colossal size. Pulled in by gravity, the group slams into the cluster, the galaxies and gas mix together, forming a uniﬁed cluster that pushing gas out the sides. The galaxies themselves pass through the continues to draw in other groups until no more are to be found.

54 Scientific American December 1998 The Evolution of Galaxy Clusters Copyright 1998 Scientific American, Inc. times as massive and nearly three times as hot. CLUSTER 2256 GALAXY Two hot regions in Abell 2256 ap- GROUP pear along a line perpendicular to the presumed motion of the group. The heat seems to be generated as snowplowed material squirts out the sides and smash- es into the gas of the main cluster. In fact, these observations match comput- er simulations of merging groups. The group should penetrate to the center of the cluster in several hundred million years. Thus, Abell 2256 is still in the early stages of the merger. The late stages of a merger are appar- ent in another cluster, Abell 754. This SQUIRTED MATERIAL cluster has two distinguishing features. THREE GALAXY CLUSTERS are at different stages in their evolution, as shown in these First, optical photographs show that its x-ray images (left column) and temperature maps (right column). The first cluster, Abell galaxies reside in two clumps. Second, x- 2256, is busily swallowing a small group of galaxies, which is identified by its relatively low ray observations reveal a bar-shaped fea- temperature. On the map red is comparatively cool, orange intermediate and yellow hot. ture from which the hot cluster gas fans out. One of the galaxy clumps is in the bar region, and the other is at the edge of CLUSTER 754 CENTER OF CLUSTER GALAXY GROUP the high-temperature region to the west. Theorists can explain this structure with an analogy. Imagine throwing a water balloon, which also contains some pebbles, into a swimming pool. The balloon represents the merging group: the water is gas, and the pebbles are galaxies. The swimming pool is the main cluster. When the balloon hits the water in the pool, it ruptures. Its own water stays at the surface and mixes very slow- TION ly, but the pebbles can travel to the oth- DIRECTION OF OUP MO er side of the pool. A similar process ap- GR parently took place in Abell 754. The gas from the merging group was sud- denly stopped by the gas of the cluster, The second cluster, Abell 754, is several hundred million years further along in its di- while the group galaxies passed right gestion of a galaxy group. The hapless group probably entered from the southeast, be- through the cluster to its far edge. cause the cluster is elongated in that direction. The galaxies of the group have separat- A third cluster, Abell 1795, shows ed from their gas and passed through the cluster. what a cluster looks like billions of years after a merger. The outline of this cluster is perfectly smooth, and its tempera- CLUSTER 1795 ture is nearly uniform, indicating that the cluster has assimilated all its groups and settled into equilibrium. The excep- tion is the cool region at the very center. The lower temperatures occur because HRINGER gas at the center is dense, and dense gas Ö emits x-rays more efficiently than tenuous gas. If left undisturbed for two or three billion years, dense gas can radi- ate away much of its original energy, thereby cooling down. , ULRICH G. BRIEL AND HANS B As the gas cools, substantial amounts Y COOLING FLOW of lukewarm material build up—enough

for a whole new galaxy. So where has TRICK HENR A all this material gone? Despite exhaus- J. P tive searches, astronomers have yet to The third cluster, Abell 1795, has gone several billion years since its last meal. Both its locate conclusively any pockets of tepid x-ray brightness and gas temperature are symmetrical. At the core of the cluster is a gas. That the cluster gas is now losing cool spot, a region of dense gas that has radiated away much of its heat.

ver since the big bang, the universe has been expanding. ago—and new clusters should still be forming and growing to- EAll objects not bound to one another by gravity or some day. But if the universe has only one quarter of the matter other force are being pulled apart. But will the cosmic expan- needed to stop its expansion, then all the massive clusters sion continue forever, or will the gravity of all the matter in the were in place four billion years ago—and no further growth universe be sufﬁcient to halt it? Traditional attempts to answer has taken place since then. the question have foundered because they require a careful The observed cluster evolution rate favors the latter scenario: census of the total amount of matter in the universe—and that because galaxy clusters have essentially stopped growing, there is difﬁcult, because most of it is invisible dark matter. must be comparatively little matter in the universe. Therefore, Now there is a new ap- the cosmos will expand for- proach made possible by 100 ever (unless there exists ma- studying the evolution of terial with exotic physical 90 FILMS galaxy clusters. Over time, properties, such as a gravita- SLIM clusters grow as they ac- 80 tional repulsion that varies crete matter, until the mat- 70 with time). Other recent ter within their gravitational 60 RANGE measurements of cosmic CONSISTENT WITH reach is exhausted. The 50 expansion, using distant su- more matter there is, the OBSERVATIONS pernovae and other mark- faster and bigger they can 40 ers, agree. Although the grow (right). If the universe 30 case is not closed, several has enough matter to come 20 independent pieces of evi- to a halt, then fewer than 10 10 dence now make it more PERCENT OF CLUSTERS FORMED

percent of the massive clus- DURING PAST FOUR BILLION YEARS likely that astronomers do 0 ters that exist today were 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 know the ultimate fate of in place four billion years AVERAGE DENSITY OF UNIVERSE (COSMOLOGICAL UNITS) the cosmos. —J.P.H.

heat is obvious from the temperature nearby objects. Astronomers’ efforts to containing only people of a certain age— maps. Perhaps the heat loss started only understand their growth are analogous for example, class pictures from grade fairly recently, or perhaps the collision to understanding human growth from school, high school and college. Similarly, of galaxy groups prevents cool gas from a single photograph of a crowd of peo- astronomers can observe clusters at ever collecting in one spot. These so-called ple. With a little care, you could sort the increasing distances, which correspond cooling ﬂows remain yet another un- people in the picture into the proper age to ever earlier times. On average, the solved mystery. sequence. You could then deduce that clusters in a more distant sample are as people age, they generally get taller, younger than those in a nearby one. Bottoms Up among other visible changes. Therefore, researchers can piece together You could also study human growth “class photos” of clusters of different he sequence represented by these by examining a set of photographs, each ages. The advantage of this approach is Tthree Abell clusters is probably un- dergone by every cluster as it grows. Galaxy groups occasionally join the cluster; with each, the cluster gains hot gas, bright galaxies and dark matter. The extra mass creates stronger gravitational forces, which heat the gas and accelerate the galaxies. Most astronomers believe that almost all cosmic structures ag- HRINGER glomerated in this bottom-up way. Star Ö clusters merged to form galaxies, which in turn merged to form groups of galaxies, which are now merging to form clusters of galaxies. In the future it will be the clusters’ turn to merge to form still , ULRICH G. BRIEL AND HANS B larger structures. There is, however, a Y limit set by the expansion of the uni-

verse. Eventually, clusters will be too far TRICK HENR A apart to merge. Indeed, the cosmos may J. P be approaching this point already. X-RAY IMAGES of Coma (left) and Virgo (right) clusters show the hot intergalactic By cosmological standards, all the gas that dominates the luminous part of these structures. The gas in Coma has a more above-mentioned clusters (Coma, Vir- regular shape than that in Virgo, suggesting that the cluster has reached a more ad- go, and Abell 2256, 754 and 1795) are vanced stage of formation. Both clusters are surrounded by infalling material.

56 Scientific American December 1998 The Evolution of Galaxy Clusters Copyright 1998 Scientific American, Inc. that it lets astronomers work with a versity of Alabama at Huntsville, C. seeing another component of the clusters’ whole sample of clusters, rather than just Stuart Bowyer of the University of Cali- dark matter for the first time. The up- a few individual clusters. The disadvan- fornia at Berkeley and their colleagues coming x-ray facilities may identify this tage is that the younger objects are too studied five clusters using the sensitive new component. far away to study in detail; only their Extreme Ultraviolet Explorer satellite. Those of us involved in this work feel a average properties can be discerned. These clusters, they discovered, shine special bond with Charles Messier as he One of us (Henry) applied this meth- brightly in the extreme ultraviolet. In strained to glimpse those faint patches of od to observations from the ASCA x- some ways, this discovery was as unex- light in Virgo, not knowing their true sig- ray satellite. He found that distant, younger clusters are cooler than nearby, older APPROXIMATE DIAMETER IN LIGHT-YEARS (LOG SCALE) ones. Such a temperature 0 10 102 103 104 105 106 107 108 109 1010 change shows that clusters become hotter and hence more massive over time—fur- GALAXY SUPER- OBSERVABLE ther proof of the bottom-up GROUP CLUSTER UNIVERSE 1013 M 1016 M 1022 M model. From these observa- STAR AND PLANETARY -1 2 tions researchers have esti- SYSTEM 10 TO 10 M mated the average rate of STAR CLUSTER GALAXY GALAXY WALLS AND cluster evolution in the uni- 102 TO 106 M 1011 M CLUSTER VOIDS 1017 M verse. The rate, which is re- 1015 M lated to the overall evolution of the universe and to the nature of the dark matter, im- plies that the universe will expand forever [see box on opposite page]. New x-ray observations may shed light on the remain- ing dark matter in clusters. By the end of 2000 there will be three advanced x-ray ob- SLIM FILMS servatories in orbit: the Ad- vanced X-Ray Astrophysics Facility from the U.S., the X- HIERARCHY OF COSMIC STRUCTURES ranges from stars and planets to the universe itself. The ray Multi-mirror Mission largest objects held together by gravity are galaxy clusters with masses up to 1015 times that of the sun from Europe and ASTRO-E (denoted as M.). Although there is a higher level of organization consisting of superclusters and great from Japan. walls, these patterns are not bound gravitationally. On even larger scales, the universe is featureless. In the meantime, observa- Astronomers think most of these structures form from the progressive agglomeration of smaller units. tions of another form of radiation, known as extreme ultraviolet light, are yielding mysteries pected as the first detection of x-rays nificance. As advanced as our technology of their own. The extreme ultraviolet from clusters in the early 1970s. Al- has become, we still strain to understand has an energy that is only slightly lower though some of the radiation comes these clusters. We feel a bond with future than that of x-rays. It is heavily ab- from the same gas that generates the x- observers as well, for science advances in sorbed by material in our galaxy, so as- rays, there appears to be an additional a continuous process of small incre- tronomers assumed that most clusters source in at least some of the clusters. ments. We have been helped by those are not visible in this wavelength band. This finding is very new and has not yet who preceded us; we share our new un- But recently Richard Lieu of the Uni- been explained. Perhaps astronomers are derstanding with those who follow. SA

The Authors Further Reading

J. PATRICK HENRY, ULRICH G. BRIEL and HANS BÖHRINGER are x-ray X-ray Emission from Clusters of Galaxies. astronomers who study clusters of galaxies. The ﬁrst two met in the late 1970s Craig L. Sarazin. Cambridge University Press, 1988. while working at the Smithsonian Astrophysical Observatory on one of the instru- Clusters and Superclusters of Galaxies. Edited ments on the Einstein X-ray Observatory satellite. Henry is now an astronomy pro- by A. C. Fabian. Kluwer Academic Publishers, 1992. fessor at the University of Hawaii. He says he enjoys sitting on his lanai and think- Stormy Weather in Galaxy Clusters. Jack O. ing about large-scale structure while watching the sailboats off Diamond Head. Burns in Science, Vol. 280, pages 400–404; April Briel and Böhringer are staff members of the Max Planck Institute for Extraterres- 1998. trial Physics in Garching. Briel is an observer who tested and calibrated the ROSAT An X-Rated View of the Sky. Joshua N. Winn in instrument that made the temperature maps discussed in this article. Böhringer is a Mercury, Vol. 27, No. 1, pages 12–16; January/Feb- theorist who studies galaxy clusters, cosmology and the interstellar medium. ruary 1998.