The Evolution of Galaxy 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 astron- omers 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 for- mation 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.