GALAXIES, ELLIPTICAL, ORGIN AND EVOLUTION PETER J. QUINN During the 1920s, astronomers began to realize that many of the diffuse nebulae that they sketched and photographed through their telescopes actually were distant galaxies similar to our own Milky Way. These distant galaxies were grouped into two broad categories: the spirals and the ellipticals. Both of these categories can be further divided into subtypes, giving a classification scheme known as the Hubble sequence. Spirals are disk-like systems consisting of stars, gas, and dust. They possess global features such as spiral arms as well as your active, star-forming regions. Ellipticals, on the other hand, appear to be ellipsoidal distributions of mainly old stars, lacking in cold gas and spectacular features like spiral arms. To the two main galaxy families we can add the SO galaxies, which are similar to spirals in the sense that they have a flattened disk of stars, but which are also similar to ellipticals in that they lack a young stellar population. There are also irregular galaxies which cannot easily be classified as either spiral or elliptical. For bright, isolated galaxies, the relative abundance of the various galaxy types is Elliptical:SO:Spiral:Irregular = 13:22:61:4. The exact mix of galaxy types depends on the environment in which we choose to sample them. In the densest parts of rich clusters of galaxies, for instance, spiral and irregular galaxies are almost completely absent. In this entry we will examine the origin and evolution of elliptical galaxies. In asking why ellipticals are the way they are, we will contrast them to the observed structural and kinematic makeup of spirals and ask what mechanisms operating during galaxy formation and evolution could have produced these two very different types of galaxies. The following ideas are a synthesis of the work of many researchers, based on clues from the observations of ellipticals and insights gained from computer models of cosmological galaxy formation and mergers of galaxies. WHEN DID ELLIPTICALS FORM? In the mid-1960s, researchers discovered that the Earth is being bombarded with microwave radiation from deep space. This radiation is very uniform in intensity over the whole sky and is now generally believed to be the relic radiation from the Big Bang. We can only set an upper limit on the order of 1 part in 10,000 for the fluctuations in the microwave radiation temperature on scales less than an arcminute. This means that the perturbations in the matter density that eventually resulted in galaxies had to grow from dimples of 0.01% to their current density (about 1000 times the mean density of the Universe) over the time in which the Universe had expanded by a factor of 10,000. Just exactly when galaxies appeared as dense systems of gas and stars in this period between the epoch corresponding to a redshift of 10,000 and now is still an open question. One constraint on the epoch of galaxy formation comes from the time it would take a large galaxy to collapse. In other words, a galaxy must be at least one collapse time old or else it would not be a recognizable galaxy today. For a galaxy like the Milky Way, the collapse time is approximately twice the free-fall time from the outer edge of its dark halo (50-100 kpc), which is about 2x10**yr. This corresponds to a collapse redshift of less than 4. This seems reasonable, as the most distant quasars we have yet detected have redshifts of about 4. So, sometime between a time when the Universe was about one quarter of its current size and now, most large galaxies began to form. However, there is evidence that some galaxies are undergoing major changes at even smaller redshifts. The types of galaxies that are prevalent in large clusters of galaxies (which are now mostly ellipticals) seem to change at a redshift of about 0.5(6x10**years ago for a Hubble constant of 50 km s** Mpc**). Beyond (earlier than) redshift 0.5, there appear to be more blue, star-forming galaxies then at lower redshifts. So the general population of galaxies in clusters seems to have not been in any kind of equilibrium until fairly recently in the Universe. Indeed, we see galaxies very close to us that are undergoing major changes either due to massive bursts of star formation or collisions with other galaxies. Hence we should not necessarily think of galaxy building as happening at a given time and then turning off. Rather, galaxies may be continually modified during their history. If the processes that formed the first systems of stars and gas are the same as those we see operating at smaller redshifts (e.g., mergers and tidally induced star formation), then both formation and evolution can be considered as part of the same ongoing process. Because the stars in ellipticals are at least as old as the stars in the spheroidal bulges and old disks of spiral galaxies, we can assume that the components of ellipticals were formed at a similar time to the oldest components of spirals. WHERE DID ELLIPTICALS FORM? One of the most important observations relevant to the formation of galaxies of various morphological types is that the properties of galaxies reflect their environments. The mix of spirals and ellipticals depends on the local density of galaxies, in that denser regions have a higher proportion of ellipticals. Hence the mechanisms that were responsible for forming different types of galaxies could not have been purely internal. There had to be some external influence that chose the preferred type of galaxy in a given environment. There are two important questions raised by the discovery of a "morphology-density" connection. Where did the morphology-density relationship develop? Because a similar morphology-density relationship exists for compact groups as well as for clusters, and because clusters have a longer collapse time than small groups, we are led to conclude that clusters of galaxies probably inherited their morphological mix from smaller-scale environments that are now incorporated into clusters. When did the morphology-density relationship develop? We know from the previous section that galaxies take a few times 10**yr to collapse. Compact groups of galaxies have a mean density lower than that of a galaxy, so their collapse time is longer. This means that the initial galaxy had already formed before the environment around it had collapsed. If galaxies do not change much after they are formed, then the morphology-density relationship is a consequence of the initial, pre-group-collapse environment. In that case the mean mass density of a region in which a galaxy forms must drive the resultant galaxy morphology. However, if the collapse and evolution of the group environment has an influence on the morphology of the galaxies it contains, then the morphology-density relationship evolves as the group does. We occasionally find ellipticals in the field, outside rich groups and clusters. This could be an example of a group environment that has evolved to complete collapse, that is, where all the constituent members have been accumulated into one large elliptical galaxy. Indeed, close encounters between galaxies in groups very often result in mergers, because the internal motions in the galaxies and their orbital motions are similar, making tidal coupling between the motions of the galaxies and the motions of their constituent stars very efficient. We are now left in a situation where we are not sure whether ellipticals were formed as a consequence of the same process that formed the region that is now a compact group of galaxies (that is, gravitational collapse of a region that is more dense than the mean of the Universe around it) or as a consequence of the evolution of the group environment after collapse. In order to sort out the relative importance of initial conditions and evolution for the current appearance of ellipticals, we need to examine some dynamical properties of ellipticals that may contain clues. HOW DID ELLIPTICALS FORM? A great deal of progress has been made in the area of galaxy formation from the study of dynamical evolution in cosmology using supercomputers and N-body models. These models allow us to ask questions about the evolution of the structure of objects that form through purely dissipationless collapse, that is, where gravity is the only force responsible for moving matter around. In reality, galaxies contain neutral gas and plasma that can interact electromagnetically as well as gravitationally. However, we believe that the dark matter that dominates the size and mass of a galaxy evolved in a dissipationless manner, as it is probably not made of normal matter. One important property of a galaxy is the total amount of angular momentum it contains. This is predicted by N-body models and probably does not depend too much on what type of matter is in the model (i.e., dissipative or nondissipative), because the angular momentum is produced by tidal interactions between protogalaxies before they collapse. If the dissipative material in a protogalaxy loses energy and shrinks, it will eventually form a disk that is held up by its angular momentum. If we measure the amount of angular momentum in a galaxy like the Milky Way we find that it agrees with the angular momentum content in a protogalaxy as determined from the N-body models. So it appears that spiral galaxies have retained most of the angular momentum that they acquired as protogalaxies. Elliptical galaxies are similar in size to spiral galaxies of comparable luminosity. Hence the two types of galaxies presumably collapsed by about the same amount due to the protogalactic gas cooling by radiation. However, whereas spirals are today rapidly rotating because of all their angular momentum, ellipticals hardly rotate at all. So at some point elliptical galaxies lost more than 90% of their angular momentum, whereas spirals did not. This loss of angular momentum has to be related to the process that made ellipticals different from spirals and hence to some property of the local environment. How can we remove angular momentum from a proto-elliptical? Angular momentum is removed by applying a torque; that is, something has to pull on the matter that formed the elliptical and hence slow down its orbital motion. Such a process is very common in dynamical collapse. It is called dynamical friction. Consider a cloud of matter which is not completely homogeneous; that is, it contains some lumps. If we start this cloud of matter off being a little more dense than the surrounding Universe, then it will eventually stop expanding with the Hubble flow and begin collapsing. As the lumps of matter move through the cloud they create a wake of matter behind them similar to the wake caused by a boat passing through water. This wake is formed by particles that are deflected by the gravitational attraction of a lump and eventually converge behind it. The wake slows the lump down and hence removes some of its kinetic energy and angular momentum. Eventually the lumps will settle to the center of the cloud and form a low-angular-momentum, tightly bound system. Our protogalaxy consists of both dissipationless dark matter and normal matter that can potentially form stars. If the normal dissipative matter in proto-ellipticals was in the form of lumps, then we would be in a good position to produce a low-angular-momentum galaxy. The dark matter could do the job of absorbing the unwanted angular momentum. By implication this means that the normal matter in the disks of spirals would have to remain quite smoothly distributed during the collapse process. Now all we need is a process that preferentially makes lumpy protogalaxies in dense environments and we would have the basis of a theory for the formation of ellipticals. Again, the N-body models have been useful in pointing out an effect that was already known from analytic studies. Consider two identical regions of the early Universe that are each destined to become a galaxy. These regions are higher in density than the Universe at large and contain lumps of matter of various sizes (consisting of gas, dark matter, and possibly some stars). We will call these lumpy, overdense regions protogalaxies. One protogalaxy is located in a region of the Universe that has a lower-than-average density and the other in a region that has above-average density, that is, one protogalaxy is on top of a "hill" of the density field and the other is in a "valley". The protogalaxy in the low-density region will evolve into a galaxy in a low-density environment, and the other will evolve into a galaxy within a group of galaxies. It can be shown that the final density of a system after gravitational collapse scales as the third power of its initial density with respect to the Universe at large. If the mean density of the regions within which our two protogalaxies are collapsing varies by a factor of only 2 (the height of the density hill over that of the density valley), then the final protogalaxies will have densities that differ by a factor of 8. So protogalactic systems in regions that will become groups of galaxies will generally have higher densities than those in the field. At some point the lumps in the protogalaxies will begin to form stars. We know from studies of external galaxies that the ability of gas to turn into stars is related to the density of the gas. So it would be natural to expect that many of the lumps in protogalaxies in dense regions will consist of stars as well as gas. Lumps of stars are efficiently decelerated by dynamical friction and hence the final stellar system formed by the accumulation of the lumps will have a low specific angular momentum. Therefore, via the action of lumps of stars forming in dense protogalaxies, we can account for both the low-angular-momentum content of ellipticals and their preference for high-density environments. As noted before, small dense groups of galaxies are also excellent places for galaxies to tidally interact and eventually merge into a single system. The merging process is dynamically similar to the formation of a galaxy from lumpy initial conditions. Two galaxies are brought together because of an exchange of energy and angular momentum between orbital and internal motions. This exchange takes place between either the luminous parts of the galaxies and their dark halos or, in the final stages of the merger, between the luminous components. N-body models have again showed that disks are rather fragile and tend not to survive mergers between galaxies of comparable mass. When disk galaxies collide, the final result is an elliptical galaxy, where again the stars have lost angular momentum during the merger. Merging between galaxies in small groups thus adds to the morphology-density relationship, as it removes disk-like systems from high-density environments. In this way we can think of mergers going on today in small groups as being a continuation of the elliptical-making process that began at higher redshifts. A COMPARISON WITH BASIC PROPERTIES If ellipticals form through a process of agglomeration of smaller systems, then based on what we know of dissipationless merging, we can compare the theory with some basic properties of ellipticals. SHAPE Whether the proto-elliptical lumps are themselves small regular galaxies or some type of irregular systems of stars and gas, the final distribution of stars after multiple mergers will be very similar to a triaxial system which will, in projection, look similar to an elliptical. If two roughly spherical lumps of stars collide on a nearly radial orbit, then the final system will be roughly prolate, with the long axis pointing in the direction of the initial orbital motion. This shape will be supported by anisotropic velocities, not rotation. Some of the initial orbital motion will be maintained along the collision axis. Hence triaxial aggregations of stars are a natural consequence of a sequence of dissipationless mergers. SIZE AND MASS Ellipticals span a range of luminosities from 10*-10** solar luminosities. Some ellipticals could have been formed from lumpy protogalaxies and then have remained untouched, whereas others could have suffered major mergers late in their history. It is possible that large ellipticals may have experienced more late merging than small ellipticals. The progenitor lumps of small ellipticals may no longer exist in their original form today, but we know that systems as small as 10* solar luminosities exist (globular clusters and dwarf spheroidals) and are commonly found around large galaxies. Spiral galaxies like the Milky Way have luminosities of several times 10** solar luminosities. Because these disk galaxies cannot have suffered major mergers (as these would have destroyed the disks), we know that individual protogalaxies could be at least as large as the Milky Way. Although we have potential lumps that span the range of luminosity of current ellipticals, the distribution of lump sizes that made up a given elliptical is still unclear. Numerical simulations are currently not capable of following the evolution of normal and dark matter over a range of 10* in mass that would be necessary to follow the buildup of a large galaxy from very small lumps. If low-mass ellipticals need to have their angular momentum lowered as much as large ellipticals seem to, then the proto-elliptical lumps must have been smaller than 10* solar luminosities. DENSITIES The mean and peak luminosity density of ellipticals is higher than that of spirals of a comparable luminosity. Two possible processes arise from the above theory to contribute to this higher density. First, we believe that ellipticals arise in environments where the density of collapsed objects is higher than in the regions where spirals generally form. These deeper potential wells will lead to denser stellar systems after gaseous dissipation and dynamical friction have done their work. Second, since the proto-elliptical lumps have lost kinetic energy to the dark matter through dynamical friction, then the binding energy per unit mass of the final amalgamated system may be higher than that of the system of progenitor lumps. STELLAR MOTIONS Because mergers naturally give rise to systems with shapes similar to ellipticals, the permitted equilibrium orbits are also those that we suspect are present in ellipticals. We know that large ellipticals have very small net rotational motion. So the dynamical support provided by the mean motion of the stars is insignificant for the system. This is clearly not the case in spirals, where the whole disk maintains its shape due to the centripetal acceleration provided by organized rotation of disk stars. Yet, despite the inconsequential size of the mean motion in ellipticals, this motion is generally aligned with the structural axes and is organized throughout the elliptical. In other words, the shape of the elliptical is aligned with the small mean motion in the sense that the long axis of the galaxy is usually perpendicular to the mean rotation axis. Also, the magnitude of the mean motion varies with radius in a manner similar to the way that circular orbit velocities vary with radius for the elliptical. This organization of mean motion and correlation with structural axes has been seen in N-body models of mergers. When small dense systems are dropped into initially spherical, nonrotating systems, the final system is flattened and brought into rotation by the matter and kinetic energy deposited by the small system during the merger. In this way, lumps falling into galaxies can organize them in a way that appears consistent with ellipticals. This would strongly suggest that ellipticals have had a long history of lumpy accretions. ELLIPTICAL EVOLUTION Many of the kinematic properties (e.g., mean motion alignment and distribution) and peculiarities (shells, counterrotating cores, and peculiar isophotes) suggest that many ellipticals have had a long history of bombardment by small systems of stars and gas. This rain of material is probably the tail end of a process that was more intense at earlier times. As shells, counterrotation, and other features of ellipticals have been shown to result from mergers, it is reasonable to suppose that ellipticals evolve primarily by merger processes. The fraction of ellipticals with shells suggests that all large ellipticals went through at least one shell-formig event in their history. The dynamical processes occurring in shell and counterrotating-core formation are similar to those seen in models of lumpy protogalaxy collapse, and so the evolution of ellipticals via mergers is nothing more than a continuation of the galaxy-building process that began at higher redshifts. CONCLUSIONS Although mergers of mostly stellar lumps can account for most of the structural and kinematic properties of large ellipticals, we are still unable to explore this process across the entire range of masses appropriate for ellipticals; This is basically due to the limitations of current computer hardware. Our ability to form a complete theory of elliptical formation is limited by our poor knowledge of the star formation process and the history of chemical enrichment in a galaxy that it implies. Although we can incorporate gas as well as dissipationless matter into our models, we lack a basic understanding of how the transition from the dissipative gas to the dissipationless stellar system occurs. The motivation for further N-body studies of dissipationless formation is that we may be able to place useful constraints on star formation physics by completely understanding the dissipationless processes. For example, certain spectra of lump sizes and densities may be required to fit the kinematics and morphology of ellipticals across the range of elliptical masses. These spectra, when compared to the input cosmological spectra of density perturbations, may reveal some density constraints on star formation. Similarly, the point in the mass range of ellipticals where dissipationless merging of purely stellar systems no longer accounts for the gross kinematic properties of ellipticals (such as specific-angular-momentum content) may mark the transition between systems that are formed purely by primordial processes (dissipation and dynamical friction) and those that have been affected by mostly dissipationless mergers late in their evolution. A guiding light for future theoretical research on the formation and evolution of ellipticals will surely be observations of galaxies at high redshifts, presumably in the process of formation. Recently, new techniques based on low-frequency radio observations have been extremely successful at finding galaxies at high redshifts (z*3-4). These galaxies are indeed very complex and lumpy, as our theoretical picture would predict. A detailed study of these lumpy, high-redshift galaxies using instruments like the Hubble Space Telescope will provide the essential physical input for future developments in our theory of elliptical galaxy formation. Additional Reading Faber, S.M., ed.(1986). Nearly Normal Galaxies, from the Planck Time to the resent.(Proceedings of the Eighth Santa Cruz Summer Workshop in Astronomy and Astrophysics). SpringerVerlag, Berlin. Fall, S.M. and Lynden-Bell, D., eds.(1981). The Structure and Evolution of Normal Galaxies. Cambridge University Press, Cambridge. Kormendy, J. and Djorgovski, S.(1989). Surface photometry and the structure of elliptical galaxies. Ann. Rev. Astron. Ap. 27 Zurek, W.H., Quinn, P.J., and Salmon, J.K.(1988). Rotation of halos in open and closed universes: Differentiated merging and natural selection of galaxy types. Ap. J. 330 519.