7.7. A Brief Comparison to Theories of Galaxy Formation
The Pritchet (1994)
review on primeval galaxies speculates that progenitors of galaxies like
our Milky Way should be very numerous, regardless of appearance. The
data, in the form of images and spectra of distant z
3 Lyman-break
systems and faint number counts, currently suggests that galaxies start
as many smaller subclumps and halos so that the local density of
L* galaxies (0.015
h503 Mpc-3) may substantially
underestimate the comoving space density of small objects at z = 5.
The dominant paradigm for understanding the Lyman-break population is the "dark halo" model: galaxies quiescently form stars at the bottom of the potential wells of massive dark matter halos. Assuming rest-frame UV luminosities correlate with galaxy mass, the brightest Lyman-break galaxies should form first in regions where the density is highest. Since these regions are expected to be strongly clustered spatially, the high-redshift, large-scale structures discussed in Section 7.6 are explained naturally. Over time, the halos merge, forming the massive galaxies we see locally.
Baugh et al. (1998)
present a semianalytic model of this hierarchical galaxy formation
scenario, focusing on the properties of Lyman-break galaxies at z
3. With a "suitable"
choice of parameters, they are able to reproduce the observed
Lyman-break galaxy properties for cold dark matter (CDM) cosmologies
with both
0 = 1
and 0
< 1. At high redshift, galaxies have very small bulges or no bulge at
all: typical half-light radii are ~ 1 h50-1
kpc, in good agreement with the z ~ 3 results of
Giavalisco et al. (1996)
and the HDF images at z > 4.
Baugh et al. (1998)
also reproduce the strongly biased spatial distribution, with b
4 and a comoving
correlation length r0
8
h50-1 Mpc at z
3. These models predict
that the average L* galaxy today was in
4 subunits at z
= 1 and 14 subunits
at z = 5.
However, the
Baugh et al. (1998)
models fair less well with respect to star formation rates. They predict
that at z 3,
most galaxies are only forming a few solar masses of stars per year and
only a very small fraction have star formation rates in excess of 40
h50-2
M
yr-1. This is at odds with more recent estimates of the
rest-frame UV extinction of the z
3 Lyman-break
population, e.g., the near-infrared spectroscopic results of
Pettini et al. (1998).
The hierarchical models also predict that galaxies form the bulk of
their stars at relatively low redshift (e.g.,
Baron & White 1987),
with 50% of the stars
formed since z
1. The
Baugh et al. (1998)
models predict that cosmic star formation history peaks around z
= 1-2, in rough concordance with the first measurements of the comoving
star formation history by
Madau et al. (1996).
More recent measurements, discussed in
Section 7.2, still show the rapid evolution
in comoving star formation rate from z = 0 to z ~ 1.5,
but, with larger samples of high-redshift objects less vulnerable to
cosmic variance and improved consideration of rest-frame UV extinction,
the revised plots show a plateau in the comoving star formation density
from z ~ 1.5 to z ~ 4
(Fig. 11).
An alternate view of the Lyman-break population maintains that these galaxies are primarily collision-induced galactic starbursts, triggered by small, gas-rich satellite galaxies (Lowenthal et al. 1997; Somerville, Primack, & Faber 1999; Kolatt et al. 1999). Using semianalytic models, Somerville et al. (1999) study the properties of individual galaxies in the "quiescent" dark halo scenario, similar to that addressed by Baugh et al. (1998), in comparison to the "collisional starburst" scenario. They argue that the high star formation rates, small nebular emission-line widths (~ 70 km s-1; Pettini et al. 1998), young ages (median age ~ 25 Myr; Sawicki & Yee 1998), and high surface densities are all better explained by the collisional starburst model. More recently, Kolatt et al. (1999) use high-resolution N-body simulations to address the clustering properties of Lyman-break sources in the collisional starburst model. They find that although most sources are relatively low mass in this scenario, they cluster around high-mass halos and therefore exhibit the observed strongly biased clustering.