EVOLUTION of GALAXIES

Content: Protogalaxies, Galaxies and disks evolution, Formation of galaxies, Signature of New Galaxy
Adapted from The Astronomy and Astophysics Encyclopedia and G.D. Bothun, J.E. Barnes, G. Lake , Annual Review of Astronomy and Astrophysics, K. Freeman­, J. Bland-Hawthorn et al.­

1. PROTOGALAXIES

Over the last 20 years, rather compelling evidence has been gathered which suggests that there is more mass contained in galaxies than can be accounted for by the amount of light that they emit. This blurs our concept of what actually constitutes a galaxy and suggests that the luminous stars that form from the gas in galaxies may simply be tracers of some large concentration of nonluminous matter, whose nature is unknown. This complicates our concept of galaxy formation because it is not clear whether it fundamentally refers to the problem of the formation of dark matter potentials or to the problem of the condensation of baryonic material within these dark potentials. The formation of the luminous component of galaxies, will be discussed here. In this context, the term protogalaxy refers to a galaxy that is experiencing its first generation of star formation, having consisted totally of hydrogen gas trapped in some dark matter potential prior to this point.

Although nearby galaxies presently exist in a wide variety of shapes and forms, there are three general features of the galaxy population that must be explained by any theory:

  1. Galaxies come in two basic kinds, namely, elliptical galaxies that are supported by the internal velocity dispersion of the stars (isotropic orbits) and spiral galaxies that are flattened disk systems in which the stellar orbits are circular.
  2. Most spiral galaxies contain a mini-elliptical galaxy at their center (i.e., a bulge).
  3. Galaxies are not randomly distributed in space but are highly clustered.

Galaxies may be conveniently parameterized by their bulge-to-disk (B/D) ratio, where the bulge component is a spheroidal distribution of stars with isotropic orbits and the disk component is a highly flattened distribution of stars and gas with circular orbits. Interestingly, the B/D ratio of a given galaxy seems to be dependent on its environment in that galaxies located in the cores of rich clusters are preferentially bulge dominated, whereas those located in the lower-density regions of the universe are preferentially disk dominated. This indicates that protogalaxies rarely form in isolation and hence interactions with other protogalaxies at the time of formation may greatly determine the evolutionary course of a given galaxy.

The formation of large-scale structure (i.e., clusters of galaxies, superclusters of clusters) is intimately related to the formation of individual galaxies. Currently, there are two competing scenarios for the formation of structure in the universe and current observations are incapable of distinguishing between them. Clearly, galaxies represent density enhancements in the universe which makes their existence somewhat difficult to understand in terms of the hot big bang theory for the origin of the universe. In particular, it is known from observations of the cosmic microwave background (CMB) that the early universe was very homogeneous on large scales. Furthermore, when the universe was less than approximately 300,000 years old, the energy density contained in this radiation field far exceeded that which was contained in matter. In this physical situation, gravity, in effect, was nonexistent and the distribution of matter was governed by the distribution of radiation, which we now measure to be quite homogeneous. Hence it is quite paradoxical that any inhomogeneities would form and so our understanding of the existence of galaxies is challenged at a very fundamental level.

Because of this paradox, it is common practice to use the present-day distribution of galaxies as a fossilized imprint of what the original spectrum of density perturbations must have been after matter and radiation decoupled and gravity became important. This leads to two possible scenarios for the formation of galaxies and clusters of galaxies. In the fragmentation picture, only very massive perturbations (mass up to 1016 Msun (solar masses); a typical galaxy has a mass of 2-5 × 1011 Msun) survived the radiation-dominated era. These large-mass perturbations are identified today as the largest known superclusters of galaxies. Subsequent cooling and fragmentation within the overall density perturbation then produces smaller-scale clusters of galaxies (of mass 1014-1015 Msun). Further fragmentation within those individual clusters then produced individual galaxies. In this scenario, virtually all galaxies that formed should be members of clusters and/or superclusters. To a large degree, this seems to be verified by current observations. However, this scenario also suggests that, because galaxies are forming via the process of fragmentation and collapse within a much larger cloud of gas, protogalaxies originally started out as objects that were about 10 times larger than galaxies today. Because protogalaxies are already clustered by this point, their environment fosters strong interactions between them. Moreover, because the disk component forms over a significantly longer time scale than the bulge component, these interactions do not favor the production of disk galaxies due to the tidal disruption of the gas destined to collapse into a disk. Hence, whereas this scenario does qualitatively predict the correlation between B/D and local density, it is not at all clear that any disk-dominated galaxies should have formed.

The alternative to the cooling and fragmentation picture is the idea of hierarchical clustering of small-mass units. In this picture, the first objects to form in the universe were 105-106 Msun. objects that gravitationally coalesced into larger-mass units (i.e., galaxies). The process of gravitational coalescence then continues with galaxies forming clusters of galaxies and then with clusters forming superclusters (and this process may still be continuing). Like the earlier scenario, the gravitational clustering idea also predicts that virtually all galaxies should be members of a cluster or a supercluster. However, in this picture, galaxies are allowed to originally form in isolation before becoming part of the cluster environment and this would tend to favor the production of disk galaxies. Moreover, this scenario also qualitatively predicts the correlation between B/D ratio and local density in the sense that bulge-dominated systems originally formed in denser areas than disk-dominated systems. Hence gravitational clustering around bulge-dominated galaxies will occur more quickly and disk-dominated systems will then infall at some later time into these newly formed clusters. This process of disk galaxy infall is something that is observed in the case of the Virgo cluster.

Although the two scenarios discussed previously can both give rise to galaxies that are distributed in a highly clustered manner, each makes different Predictions regarding the redshift at which galaxy and cluster formation should occur. In the cooling and fragmentation picture, it is possible that virialized clusters were present at redshifts as large as 2 (80% of the age of the universe). The best signature of a virialized cluster is the emission of x-rays from the hot intracluster medium. The strength of the x-ray emission is directly proportional to the mass of the cluster because the gas is heated by the cluster gravitational potential. Current x-ray surveys are only sensitive to x-ray emitting clusters out to a redshift of z = 0.7 (55% of the age of the universe) and hence can place no stringent limits on the epoch of cluster virialization. ROSAT and future x-ray satellites such as AXAF may prove invaluable in this regard.

One of the main difficulties with the hierarchical clustering idea is the amount of time it takes for the process to begin. Specifically, gravitational amalgamation of subunits can only occur efficiently when the subunits themselves are reasonably close to one another. Because the universe is expanding then, at a given mass density, there will be an epoch past which this process can never get started because the average spacing between subunits is too great. At present, there is at least an order of magnitude range in our estimate of the mass density of the universe. Various theoretical arguments favor a mass density that actually closes the universe. Observations of large-scale deviations from Hubble flow, however, indicate that the mass density is between 10 and 20% of the closure density. In such a low-density universe, the process of hierarchical clustering must start at redshifts of 50-100 (0.1% of the age of the universe) and the physical details regarding the merger of subunits at such an early time remain obscure.

Of course, speculation regarding the formation of protogalaxies would cease upon their discovery. In fact, one can make a simple argument that suggests that protogalaxies might easily be detectable. A typical elliptical galaxy has 1011 Msun of stars. Characteristics of the stellar population in present-day ellipticals suggest that the bulk of this star formation occurred very early on, perhaps in the phase of protogalactic collapse. The collapse or free-fall time of any cloud is given by sqrt[G rho]-1, which is 4 × 108 yr for a typical elliptical with an initial radius of 50 kpc. (G is the gravitational constant and p is the density.) In this case the predicted star formation rate would be approximately 250 Msun yr-1, which is a factor of 50 larger than the current star formation rate in nearby spiral galaxies. A star formation rate this high produces a great deal of luminosity in hot young stars as well as a supernova rate of about 1 per year! Most of this luminosity comes out at 1000 Å in the rest frame of the galaxy and so the redshift of this hypothesized protogalaxy would have to be 3 geq z leq 10 in order to be accessible to ground-based observations.

Before discussing possible reasons why this population of galaxies has yet to be detected, it is worth pointing out that protogalaxies with initial densities that are rather low will take a correspondingly longer time to form. In general, their formation process will be interrupted by the tidal shearing forces of neighboring galaxies. However, if these objects are in relative isolation, then some may be collapsing now, particularly in the case of flattened disk galaxies. Interestingly, a number of very low surface mass density disk galaxies have been discovered over the last two years. Although they are not protogalaxies by our adopted definition, their discovery in nevertheless important because these galaxies represent examples of galaxy formation that has been quiescent and that extended over a considerable period of time.

So the task of discussing protogalaxies now becomes one of suggesting reasons why they have escaped detection. We close with the following four possibilities. The most obvious possibility is that the first phase of vigorous star formation was essentially complete prior to z = 10 (which, coincidentally, is when the universe is approximately 1 galactic free-fall time old). However, in this case the radiation from the hot young stars is redshifted into the near-infrared part of the electromagnetic spectrum (e.g., 1-5 µm). Searches for this population with newly developed near-infrared imaging arrays are presently underway. Second, because the ultraviolet radiation from young stars is strongly absorbed by dust, it is possible that the early universe is not transparent to this radiation. This provides a rather effective screen that can potentially hide the process of galaxy formation from us. In this case, the absorbed ultraviolet photons should heat the dust to a temperature of 50-100 K. The radiation from this heated dust will be redshifted to the submillimeter portion of the electromagnetic spectrum. The energy density of these hypothetical sources is thought to be detectable with the SIRTF mission to be launched sometime around the year 2000. Third, it is well known that the peak in the redshift distribution of quasars is at z = 3. Because quasars are now known to be in the nuclei of galaxies and the energy source is thought to be the infall of gas onto a massive black hole, it is possible that this infall process is facilitated by protogalactic collapse. In that case the radiation from the quasar effectively overwhelms that from the rest of the galaxy, imposing a sort of cosmic censorship against observing galaxies in the act of formation. Finally, our expectation that protogalaxies should be observable stems from a relatively naive physical argument. Perhaps, galaxy formation is a far more gentle and quiescent process than we have assumed and there simply is no ultraluminous phase which marks the birth of a galaxy. Future advances in telescopes and instrumentation will hopefully yield firm detections of protogalaxies, from which we can finally study their formation in detail.


2. GALAXIES and DISKS EVOLUTION

Galaxies come in a bewildering variety of shapes and sites, but a large majority-perhaps 80%-possess a disk of some kind. Galactic disks are thin, basically circular distributions of stars, gas, and dust; this material moves on nearly circular orbits about a common center. Many disks exhibit beautiful spiral patterns as a result of this rotation, and some have pronounced bars crossing their centers. Other disks, however, are nearly featureless, and can only be identified by a characteristic falloff of brightness with radius.

The evolution of disk galaxies is inextricably bound up with the highly controversial problem of galaxy formation. This entry focuses on the history of disks in galaxies such as the Milky Way, as a way of distinguishing the present subject matter from larger issues. Progress in this field has come by combining detailed studies of the motions, compositions, and ages of stars in the solar neighborhood with less-detailed knowledge of the overall structure of the Milky Way and global observations of other galaxies. This approach, while productive, is fraught with uncertainties. Presently one cannot offer a definitive account of the evolution of disk galaxies.

The mere existence of a galactic disk has two basic implications. First, the gas from which the disk formed must have settled into circular orbits before the disk stars were born; once a star is formed its future path is determined entirely by gravitational forces, and gravity cannot circularize a random distribution of stellar orbits. Second, since the disk formed, the gravitational field has not undergone any sudden, dramatic changes, which would disrupt the circular pattern of stellar orbits. We cannot, however, rule out the possibility that the mass distribution, and hence the gravitational field, has evolved slowly.

The more massive a star, the brighter it shines, the bluer its color, and the shorter its lifetime. Thus it is relatively easy to tell the age of a system in which all stars formed at the same time by observing the colors of the brightest stars still on the main sequence. Stellar associations and open clusters in the disk of the Milky Way yield ages between 3 × 106 and 6 × 109 yr, and the oldest disk stars are at least 1010 years old. These widely ranging ages imply that star formation in the Milky Way started when the Universe was less than half its present age and continues up to the present time.

Observations of other galaxies show that the Milky Way is hardly unique in this respect. The broadband colors of a galaxy depend largely on the rate of star formation averaged over the last ~ 108 yr; higher rates of star formation yield bluer colors. Late-type galaxies (type Sc and Irr) indeed have rather blue colors, suggesting a constant rate of star formation. In very early-type disk galaxies (type SO), star formation has largely ceased, although it is hard to tell how long ago this occurred. Finally, the intermediate colors of galaxies like the Milky Way (type Sb) are consistent with a present star formation rate ~ 3 times lower than the average rate over the lifetime of the galaxy.

How long can galaxies continue to form stars at these rates? In all but the most extreme cases, the gas is ltapprox 15% of the total mass in stars. This suggests that late-type galaxies are literally about to run out of gas, ending their phase of star formation. It seems unlikely, however, that we find ourselves at such a unique moment in cosmic evolution. Alternatively, galaxies may accrete gas from their surroundings, their present gas content representing a rough balance between income and expenditure. This accretion hypothesis solves several problems in galactic evolution, but at present there is little direct evidence that galaxies such as the Milky Way are accreting significant amounts of gas.

Some of the gas invested in stars is returned to the galactic reservoir as the stars age, having been enriched in metals (elements heavier than H and He). Massive stars very rapidly become Type II supernovae, spewing a wide range of elements back into the clouds from which they form, whereas close, intermediate-mass binaries may evolve into Type I supernovae, favoring production of iron-group elements. Modeling galactic chemical evolution is in principle just a matter of bookkeeping, but in practice uncertainties in the physics of evolved stars make detailed predictions difficult. Some simple models, however, can at least be ruled out.

Simplest is the closed-box model, in which gas neither enters nor leaves the galaxy; as the metals must be built up over time, the closed-box model predicts large numbers of metal-poor, low-mass disk stars. In fact, only 2% of the low-mass stars in the solar neighborhood have less than a quarter of the solar fraction of metals, compared to the 44% predicted by the closed-box model. This is known as the G-dwarf problem; the scarcity of metal-poor disk stars indicates that a closed-box model is inappropriate for our galaxy.

One obvious solution to the G-dwarf problem is to supply metals from the outside. In addition to the disk, the Milky Way has a spheroidal component, which is older than the disk and massive enough to have contaminated the protodisk with a significant quantity of metals. This disk-spheroid model also explains why the metal fraction of a disk typically increases towards the center, because that is where the bulk of the mass in the spheroid component is found.

Paradoxically, another way to solve the G-dwarf problem is to slowly but steadily build the disk from metal-poor gas. In this case the metal fraction soon reaches roughly the present value, and then remains constant. By the present epoch most stars will have formed during the phase in which the metal fraction is constant, and metal-poor stars will be rare.

Which solution is preferred? Data for F stars show the fraction in metals increasing with time up until ~ 3 × 109 years ago, and then leveling off. If real, this leveling off indicates that our disk has only just reached the constant-metals phase, suggesting a compromise between the disk-spheroid and accretion solutions. These two hypotheses may be complementary sides of the same story; both challenge the assumption that galactic disks are closed systems.

The Sun and nearby disk stars share a common orbital motion about the galactic center, but in addition each has a small random velocity, reflecting the fact that their orbits are not perfect circles. On the average, older stars have larger random velocities; for stars less than 109 years old, rms velocities toward or away from the galactic center are sigmaR appeq 10 km s-1, while for the oldest disk start, we find sigmaR appeq 40 - 60 km s-1.

The most likely explanation for the trend of random velocity with age is that stars are born on nearly circular orbits, and are subsequently deflected onto more random orbits by fluctuations in the galactic gravitational field. This led Lyman Spitzer, Jr. and Martin Schwarzschild to postulate the existence of giant molecular clouds, long before such clouds were detected. Present calculations indicate that clouds of mass 106 Msun can produce random velocities of up to ~ 30 km s-1, but not the higher velocities seen in the oldest disk stars. In addition, the Spitzer-Schwarzschild mechanism predicts that random velocities grow rather slowly, roughly as sigmapropto t0.25, and the observations are better fit by sigmaproptot0.5. Another source of fluctuations is needed; transient spiral structure, to be discussed next, may fill the bill.

Many different kinds of spiral structure are seen in disk galaxies. Most photogenic are the grand-design two-armed spiral galaxies such as M51, but far more common are ragged or flocculent spirals made up of many short arms. The diversity of spiral galaxies is paralleled by the diversity of theories of spiral structure. Grand-design spirals are often discussed in terms of the Lin-Shu theory (after Chia-Chiao Lin and Frank H. Shu), which views the spirals as slowly turning wave patterns maintaining their form for many rotation periods. However, classic grand-design spirals like M51 often have close companions, and it is possible that such spirals are actually excited by tidal interactions. Flocculent spirals, on the other hand, are generally thought to evolve over time, with individual spiral arms constantly forming and dissolving.

Computer models of rotating disks can produce spiral patterns similar to those seen in real galaxies. In these models, thousands of particles represent the disk; each particle moves in the net gravitational field produced by all the others. If the particles start out in nearly circular orbits with small random velocities, striking multiarmed spiral patterns soon develop. These spirals result from the gravitational amplification of small fluctuations in a disk that rotates differentially (i.e., not like a solid body). As a result of these ever-changing spiral patterns, particles acquire increasingly large random velocities. After a few rotation periods the random velocities become large enough to shut off the gravitational amplifier, and the spiral-making activity dies away.

Transient spiral structure can in principle provide the fluctuating gravitational field needed to generate the random motions of old disk stars, and a theoretical analysis even predicts sigmaproptot0.5. However, the spiral activity seen in the simplest computer models lasts only a few rotation periods, whereas in real galaxies it persists more than 10 times as long. Computer experiments show that spiral structure can be maintained by adding stars to the disk on nearly circular orbits, consistent with the above discussion of random velocities. Moreover, the kind of spiral pattern produced depends on the rate at which stars are injected; high rates produce open, well-defined patterns typical of late-type spirals, and slower injection results in weaker, tightly wound spirals like those seen in early-type disk galaxies. These results support the view that accretion provides a disk galaxy with the shot in the arm needed to promote vigorous development of spiral structure, the type of the resulting spiral depending largely on the rate of accretion.

Complementing the mechanisms which build up galactic disks are those which destroy them. According to the accretion hypothesis, spiral galaxies are susceptible to starvation: If the inflow of raw material for new stars is cut off, the spiral soon fades, leaving a smooth disk resembling an SO galaxy. Indeed, SO galaxies are generally found in high-density regions where starvation is likely. A disk galaxy that has the misfortune to fall into a rich cluster may be swept clean of interstellar material by the ram pressure of the hot, low-density gas pervading such clusters. Alternatively, the overpressure of the cluster gas may compress molecular clouds within the galaxy, provoking a burst of rapid star formation. This process may account for some unusually blue galaxies observed in high-redshift clusters.

Finally, instead of accreting gas, a disk galaxy may ingest a companion. Computer simulations show that interactions between galaxies often result in mergers, the outcome depending on the mass ratio of the colliding systems. Large disk galaxies can swallow small companions, of less than ~ 10% their mass, with only minor damage: Random motions of disk stars increase, and the disks become thicker. A number of galaxies, including the Milky Way, are reported to have thick disks which may have been produced in this way. Mergers between disk galaxies of comparable mass have a very different outcome. So violent is the interaction that neither disk survives; such mergers may in fact produce elliptical galaxies.

Disk galaxies are thus rather fragile and delicate objects. The evidence, while fragmentary and largely circumstantial, suggests that these galaxies grow best in quiet, undisturbed locations where their disks can develop slowly without outside perturbations. When such galaxies become involved with others, they run the risk of violent transformation. But from the wreckage of such cosmic accidents a new disk galaxy may arise, given only time and a sufficient supply of raw materials.

3. FORMATION of GALAXIES

There are several different ways to define galaxy formation. We leave the problem of generating small fluctuations at a very early epoch or "seeding" at later times to the entries on cosmology. Here we will be concerned with the collapse of a gas cloud and the initial burst of star formation. This definition leads to some ambiguity. Galaxy formation can be the final assembly of the objects that we see today, or it can refer to the formation of the first stars or the formation of a fiducial fraction of the stars. It was once assumed that a large fraction of the stars formed in an initially rapid collapse. In this case, galaxy formation is defined by all of the above happening synchronously. This would produce extremely bright events at high redshift which would be unmistakable. By this criterion, we have not clearly seen any galaxies forming. However, if we adopt either half of the above definitions, we find that galaxy formation is an ongoing process and that it is relatively easy to find nearby examples of galaxies in formation.
The formation of galaxies was a gradual process beginning at a redshift of ~ 4 and continuing today. There are numerous observations of galaxies forming. Current and average rates of star formation are known in disk galaxies. Extended emission around radio galaxies has been seen to a redshift of 3.4. Protogalaxies have been "discovered" in the form of large numbers of blue objects in deep surveys, flat-red extended sources, fuzz around high-redshift radio galaxies and nearby gas-rich, low-surface-brightness objects. Relating all these observations to systematic properties of the distribution of dark and luminous matter as well as the cosmological conditions proposed for galaxy formation is the hard work for the coming decade.

From direct observations, it is not clear when galaxies formed. The local disk of the Milky Way is an example of continuous galaxy formation. We find that the present rate of star formation is not much smaller than the lifetime average. Generally, the current star formation rate in late-type spiral galaxies multiplied by a Hubble time is approximately the observed disk mass. Similarly, in giant elliptical galaxies with cooling flows, the cooling flow rate multiplied by the Hubble time is roughly the mass of the galaxy. Some nearby objects with high ratios of gas mass to stellar luminosity (gtapprox 5 in solar units) have been labeled protogalaxies. Extreme examples are Malin-1 and the Giovanelli-Haynes object, but there are several others (for example, DDO 170 and DDO 154). These objects are fully assembled but have had little star formation.

On the other hand, we find elliptical galaxies at relatively high redshifts (z ltapprox 0.8) where the 4000-Å break in the spectrum shows that the bulk of the stars in these systems were formed much earlier. These galaxies must have formed before z ~ 2. There are objects with z ltapprox 3.5 seen around radio sources. One of these, at z = 3.4, is observed to have a high ratio of 2 µM to optical flux, which is interpreted as indicating an old stellar population. These unusual objects, which are found in surveys of radio galaxies or in searches for very red objects, may not be representative of the formation of an average galaxy. Nonetheless, it is striking that old stellar populations exist at high redshift.

The epoch of galaxy "assembly" can be calculated if we assume that the event was isolated in both space and time. Consider the evolution of a "top hat" fluctuation, a spherical region of space with an excess density embedded in a flat Universe (one with exactly the critical density). Such a system behaves like a miniature closed Universe and turns around when its density is 9pi2 / 16 times the critical density, rhocrit = 3 H2 / (8piG), where H is the expansion rate or Hubble constant. If pressure is unimportant, it will collapse by a factor of 2 to virialize. Thus its final density is 4.5pi2 rhocrit@turnaround. Hence, the density of an object that formed without dissipation uniquely specifies its redshift of formation. For a typical disk galaxy, the redshift of turnaround for dissipationless formation is (1 + zturn)no dissipation ~ 40. Once we know the additional collapse factor owing to dissipation, C, we can correct this number to find the true epoch of formation: (1 + zturn) = C-1(1 + zturn)no dissipation. There are two ways to find C. If galaxies formed as part of a dissipationless clustering hierarchy, then their surface brightnesses would match a smooth extrapolation of the properties of groups and clusters. In reality, their surface brightnesses are roughly 100 times the extrapolated value, which implies that C ~ 10. (A more rigorous determination matches the luminosity density within galaxies to that derived from the two-point correlation function of galaxies.)

The second method of finding C uses the angular momentum of galaxies. The dissipational collapse factor needed for a tidally torqued protogalaxy to produce a centrifugally supported disk again tells us that C ~ 10. Therefore, we find zturn ~ 3. The maximum half-mass radius of an average disk galaxy was ~ 100 kpc and was achieved at z ~ 4.

The preceding arguments assumed that the dimensionless cosmological density parameter Omega = 1 where Omega = rho / rhocrit. If Omega = 0.1, this does not change the evaluation of the collapse factor owing to dissipation, but it does change the formation redshifts. In such a model, galaxy formation occurred at a redshift of ~ 6. For the sake of simplicity, we will continue to assume that the Universe is flat.

We calculated the above numbers for disk galaxies. We run into difficulties if we instead consider elliptical galaxies. Using the density argument, one concludes that elliptical galaxies have dissipated by a factor of 20. However, the angular momentum of ellipticals is exactly what is expected from tidal torques without any subsequent dissipation. For this reason, understanding the origin of the Hubble sequence of galaxy shapes has been a stumbling block for theories of galaxy formation.

Ten years ago, one popular picture was that ellipticals were products of dissipationless collapse at high redshift, whereas spirals formed later with considerable dissipation. Here, ellipticals and bulges are formed in an early epoch of Compton cooling, whereas disks result from radiative cooling at a later time. For some time, this was the standard model of dissipational galaxy formation. In this picture, the problem was split into two halves, where the morphological types are cooled by different physical processes from perturbations of vastly different amplitudes and owe their luminosity functions to two independent processes.

It would be preferable to discover a control parameter that causes a bifurcation leading to disks and spheroids. Over 50 years ago, Sir James Jeans noted that the flattest spheroids had the maximum flattening of the Maclaurin sequence. He proposed that angular momentum was the control parameter and the dynamical instability of rapidly rotating systems was the physical mechanism that separated disk galaxies and spheroids. A variant of this scheme links initial angular momentum to "overdensity," and hence the clustering environment. Unfortunately, the range of spins measured in N-body experiments is not sufficiently broad to explain the full Hubble sequence. These simulations and other calculations also show that the spin has a relatively weak overdensity dependence. Nevertheless, the Hubble sequence is a sequence of angular momentum and, until recently, most schemes focused on stretching the range of initial angular momentum to produce the final range.

The key to the shift away from this picture has been the realization that, to first order, the Hubble sequence is a velocity or mass sequence. The characteristic velocity dispersion of an elliptical galaxy is ~ 250 km s-1, which implies a circular velocity of ~ 425 km s-1, whereas a typical spiral galaxy has a rotation velocity of only ~ 180 km s-1. Recent N-body simulations have shown how the final virial velocity of a system determines its morphology. If the dark matter has a characteristic velocity dispersion at the epoch of galaxy formation, then more-massive objects underwent "cold collapses," and the less-massive collapses were "warm." Gas settles gently into circular orbits in warm collapses, leading to a disk galaxy. In cold collapses, the gas undergoes violent relaxation leading to strong density gradients and the outward transport of angular momentum. The dense inner region quickly makes stars, whereas the gas in the outer parts takes a long time to cool. After 10 dynamical times, roughly half of the gas (with approximately a third of the specific angular momentum) forms a dense slowly rotating bulge.

The final outcome of these simulations is that the more-massive elliptical galaxies have a half-light radius that is half that of a spiral galaxy and a specific angular momentum that is down by a factor of 3 from that of a spiral. This is an excellent match to the observations and shows that it is possible for disks and ellipticals to form from a continuous fluctuation spectrum.

Disks form in these experiments with an angular momentum distribution similar to that in the initial state. This validates our early calculation for the redshift of formation, zturn. The observed differences between spirals and ellipticals also fix the velocity dispersion of the dark matter at turnaround, sigmaturn. Numerous simulations have shown that zturn, sigmaturn, and the mass of the protogalaxy uniquely determine the density profile and core radius of the dark matter. This prediction of the density distributions is qualitatively borne out by current observations and may prove to be a stringent test of the theory.

The characteristic velocity dispersion needed to separate disks from spheroids results in a pressure that is more important for the perturbations that become disks. As a result, we expect that disk formation will be delayed by comparison to that of the spheroids. Indeed, the simulations show a rapid early formation of spheroids (during the violent relaxation phase which occurs on a collapse time scale), whereas disks are formed more slowly by continual infall.

Alternative approaches to galaxy formation emphasize environmental factors. These are the ongoing addition and removal of mass: accretion, cannibalism, merging, and stripping. The merger hypothesis proposes to make disks and collide them to make ellipticals. Accretion advocates envision forming spheroids early and slowly depositing disk material around them. There are composite schemes where systems that undergo a lot of merging become ellipticals, and relatively undisturbed coherent collapses turn into disks. These schemes require that star formation is carefully timed to take advantage of the merger dynamics. There is no doubt that all of these effects have varying degrees of importance.

Evidence is rapidly emerging that supports the notion that galaxies formed at a z ~ 3. In deep surveys, a population of blue galaxies emerges at B-magnitudes greater than 22. These objects are so numerous that it is difficult to explain them as anything other than a cosmologically distant population of star-forming galaxies. They must have a redshift z > 0.8, as an irregular galaxy at lower redshift would be too red. They cannot have z > 3.5, as this would place the Lyman continuum break in the B-band and they would again become too red. More recently, a large population of spatially extended objects with "flat red" spectra have been discovered. If we attribute the red population to the Lyman break and note that no "ultrared" objects have been found, we conclude that the process of galaxy formation started at z ~ 4.

A recent study of z > 4 quasars indicates that the illusive Gunn-Peterson trough has been found. (The trough is caused by the Lyman absorption of intervening hydrogen, which lies at all redshifts up to that of the quasar.) At lower redshifts, absorption-line systems with low neutral fractions are seen. Since it is now believed that significant ionization sources other than quasars are needed to ionize these clouds, the output from young galaxies staring at z ltapprox 4 is an appealing option.

The absorption-line systems at lower redshifts may also probe galaxy formation. If the clouds with high optical depths (neutral column densities gtapprox 1018 cm-2) are normally associated with galaxies, it would imply a covering fraction of order unity to a radius of 50 kpc. This is in good agreement with our estimated size of protogalaxies.

Also seen are Lyman-alpha clouds, which have lower column densities. These clouds may be a part of the galaxy-formation process, either small-scale gravitational collapse in the hierarchical growth of galaxies or condensations from thermal instabilities in the protogalaxy. An alternative proposal is that they are intergalactic, confined by an explosively heated medium. Searches for He+(lambda304) and He I(lambda584) absorption features are the key to determining where these clouds fit in the formation process.

4. THE NEW GALAXY: Signatures of Its Formation

The formation and evolution of galaxies is one of the great outstanding problems of astrophysics. Within the broad context of hierachical structure formation, we have only a crude picture of how galaxies like our own came into existence. A detailed physical picture where individual stellar populations can be associated with (tagged to) elements of the protocloud is far beyond our current understanding. Important clues have begun to emerge from both the Galaxy (near-field cosmology) and the high redshift universe (far-field cosmology). Here we focus on the fossil evidence provided by the Galaxy. Detailed studies of the Galaxy lie at the core of understanding the complex processes involved in baryon dissipation. This is a necessary first step toward achieving a successful theory of galaxy formation.

Compiled by G.T.Petrov, 2004