1.1. Initial Conditions:
CDM Cosmology
The initial conditions for the formation of galaxies are provided
by the now-standard
CDM cosmological
model. The combined results of the WMAP satellite study of
Cosmic Microwave Background anisotropies, large-scale
structure, and Type Ia supernovae observations yield
best-fit values for the cosmological parameters of roughly
=
0.7,
m = 0.3,
b = 0.04,
and H0 = 70h70 km s-1
Mpc-1
(Bennett et
al. 2003).
1
The original model of galaxy formation was Monolithic Collapse
(Eggen et
al. 1962),
where gravitational collapse of a cloud of primordial gas
very early in the lifetime of the Universe formed all parts of
each galaxy at the same time. Modern evidence rules out this
model on two fronts; the widely varying ages of different
components of the Galaxy provide a counter-example, and the
CDM
cosmology predicts "bottom-up" i.e. hierarchical rather
than "top-down" structure formation.
Hierarchical structure formation is a generic feature of Cold
Dark Matter (CDM) models. Small overdensities are able to
overcome the cosmological expansion and collapse first,
and the resulting dark matter "halos" merge together to form
larger halos which serve as sites of galaxy formation.
This process continues until the present day,
making galaxy formation an ongoing process.
The nearly-scale-invariant primordial power spectrum inferred
from combining WMAP with large-scale structure observations provides
power on all scales in the distribution of CDM. The
baryons fall into the CDM potential wells after decoupling, leaving
only trace evidence of their previous acoustic oscillations as a series of
low-amplitude peaks in the matter power spectrum.
The non-linear collapse of dark matter overdensities
occurs on larger and larger scales, so the typical collapsed halo
mass grows with time, but no preferred scale is introduced.
CDM therefore
provides a distribution of halos where galaxies
can form, with the details of the process up to baryonic physics.
Despite the lack of preferred galaxy scales in the distribution of
dark matter halos, baryonic physics causes galaxies to
have minimum and maximum masses. The maximum mass is that of CD galaxies
in cluster centers with baryonic masses ~ 1012
M and
virial masses ~ 1013
M
; there are
~ 1014
M
of baryons
available in a rich cluster but
virialization of galaxies and heating of gas to the high virial
temperature prevent most of this mass
from finding its way to the central galaxy. The minimum mass observed
today is that of dwarf galaxies, ~ 108
M
, but galaxies
may initially have formed as small as 106
M
(the baryonic
Jean's mass after recombination i.e. the minimum mass for which gravity
overwhelmed pressure support). Explaining the lack of observed galaxies
with circular velocities below 30 km/s is a major goal; it is suspected
that feedback from supernovae explosions may have quenched star
formation in such low-mass objects immediately after a single burst of
star formation
(Dekel & Silk
1986).
The growth of cosmological structure and collapse of dark matter halos
is a feature of the matter-dominated epoch. During radiation-domination,
perturbations on scales smaller than the sound horizon were unable to
grow due to acoustic oscillations in the photon-baryon fluid that gave
rise to the famous peaks in the CMB angular power spectrum and the
lower-amplitude peaks in the matter power spectrum. Now that we have
entered a phase of dark energy domination, structure growth is slowing
and will cease entirely as the universe enters a new phase of
inflation. This cosmological "freeze-out" in structure formation is
recent, since equality between the dark energy and matter densities
occurred at zeq = 0.4. The slowing of structure
formation occurs gradually, so the growth of cosmological structure
continued nearly unabated until zeq, even though we
see strong observational evidence for "downsizing" at z < 1
where high-mass
galaxies grow far more slowly than lower-mass galaxies (e.g.
Treu et al. 2005,
Smith 2005).
Another term being used by some is "anti-hierarchical",
which is basically a synonym for "downsizing" but seems to
imply inconsistency with hierarchical cosmology. However, the observed
freeze-out in galaxy (and possibly supermassive black hole) formation in
massive galaxies is not inconsistent with CDM models; rather,
it appears to be caused by baryonic feedback which is not well
understood at present (see Section 6.).
The slowing of cosmological structure growth since
z 0.4 may,
however, play a role in the recent decline
of the cosmic star formation rate density discussed by
Bell et
al. (2005).
1.2. Final Conditions: Low-redshift Galaxies
The study of galaxy formation is made easier by having full boundary conditions. The final conditions are the Hubble sequence of mature galaxies we see in the nearby universe at redshift zero. Indeed, much has been learned about galaxy formation from "archaeological" evidence in the ages and chemical abundances of various Galactic stellar populations, and expanding these studies to the rest of the Local Group and beyond is quite useful. Nonetheless, there are great advantages to observing galaxies in the act of formation, which motivates the study of high-redshift galaxies. At z > 2, galaxy-mass halos are rare so the majority of galaxies we observe reside in dark matter halos that have only recently collapsed i.e. at high-redshift most galaxies are young. In this sense, z > 2 can be considered the epoch of galaxy formation.
1 We include
h70, analogous to the traditional parameter
h
h100, even though its value appears quite close to 1.
Back.