Gregory D. Bothun
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:
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.
Most spiral galaxies contain a mini-elliptical galaxy at their center (i.e., a bulge).
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 106 M
(solar masses); a typical
galaxy has
a mass of 2-5 x 1011 M) 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 M). 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
M. 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 M 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
)]-1, which is 4 x 108 yr for a
typical elliptical with an
initial radius of 50 kpc. (G is the gravitational constant and
is
the density.) In this case the predicted star formation rate would be
approximately 250 M 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 
z 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.