Instead of studying galaxies as discrete objects residing in
dark matter halos, one can track the cosmological quantities that
comprise the baryon budget. Galaxy formation and evolution plays
the fundamental role in the processing of baryons from neutral
hydrogen to molecular gas to stars to metals.
Star formation is inextricably linked with galaxy formation; whether
you choose to define a galaxy as a large conglomeration of stars or an
overdensity of baryons inside a collapsed dark matter halo, the galaxies
in our universe form great numbers of stars.
The cosmological quantities of interest provide integral constraints
on star formation. The cosmic star formation rate density
(SFRD) is an integral constraint averaged over the volume of the
universe observable at a given redshift. The cosmic density of neutral gas,
gas, the
cosmic density of metals,
Z, and
the cosmic stellar mass density all provide integral constraints on the
SFRD over time, as will be discussed below.
The sum of the cosmic infrared background
(CIB) and cosmic far-infrared background (FIRB) radiation provides an
integral constraint on the SFRD from the Big Bang all the way to
z = 0 by tracing the energy generated by nuclear reactions in stars.
2.1. Cosmic Density of Neutral Gas
The Damped Lyman 
Absorption systems (DLAs,
Wolfe et
al. 1986)
are quasar absorption line systems with HI column densities
2 × 1020
cm-2,
sufficient to self-shield against the high-redshift ionizing background.
Studying quasar absorption-line spectra provides a
(nearly) unbiased sample of lines-of-sight through the cosmos
ideal for measuring cosmological quantities. The DLAs have been found to
contain the majority of neutral hydrogen atoms at high redshift
(see the recent review by
Wolfe, Gawiser
& Prochaska 2005).
Moreover, DLAs contain the vast majority of neutral gas, by which
we mean neutral hydrogen and helium in regions that are
sufficiently neutral to cool and participate in star
formation, as lower column density systems
are predominantly ionized. Hence the DLAs provide the reservoir of
neutral gas that is available for star formation.
In a simple closed box model,
d
gas / dt = - d
/ dt, and
the net decrease in the cosmic density
of neutral gas from z = 3 to z = 0
is assumed to have all been turned into stars
(see Fig. 5 of
Wolfe et
al. 2005).
In that case, the DLAs appear to have formed about half of the stars
seen in galaxies today. The truth is more complicated in
hierarchical cosmology, where an open box model must be used;
 |
(1) |
Cosmological models for infall of gas from the intergalactic medium (IGM), merging of lower column-density systems, and gas loss due to galactic winds are still quite uncertain, but the star formation rates actually measured for DLAs (Wolfe, Gawiser, & Prochaska 2003a; Wolfe, Prochaska, & Gawiser 2003b, Wolfe et al. 2004) imply that DLAs could have formed all present-day stars. Unfortunately, large uncertainties in the source and sink terms prevent us from using changes in the cosmic density of neutral gas as an integral constraint on the cosmic SFRD at the present time.
2.2. Star Formation Rate Density
The cosmic star formation rate density has now been measured out
to z 
6
(Giavalisco
et al. 2004).
The high-redshift points are taken from only the Lyman break galaxies, and
it is unclear how severe the resulting incompleteness is
since we are not sure if all star-forming galaxy populations at these
redshifts are known. The plot is traditionally shown in
misleading units of
M
Mpc-3 yr-1 versus redshift;
in order to integrate-by-eye, one should plot
this quantity versus time, and this
has the effect of greatly increasing the apparent amount of star formation
at low redshifts. Despite significant uncertainties in the SFRD
at z > 3 due to incompleteness and large dust corrections,
it appears that most stars in the present-day
universe formed at z < 2 (see Fig. 33 of
Pettini 2004).
The cosmic stellar mass density provides an integral
constraint on the SFRD, *(t) =
0t
d* / dt. See
Dickinson et
al. (2003)
for a recent compilation, and
Niv Drory's contribution to this volume for an update.
Note that the stellar masses of galaxies are not direct observables
but are inferred from rest-frame optical (and near-infrared) photometry
by modelling each object's star formation history using an assumed
initial mass function (IMF).
2.4. Cosmic Metal Enrichment History
The cosmic metal density is really a history of
cosmic metal enrichment due to star formation,
*(t)
= 1/42 0t
d* / dt
(Pettini 2004).
Wolfe et al.
(2005,
see their Fig. 7) show that the
cosmic metallicity traced by DLAs rises gradually from
a mean value of [M/H] = -1.5 at z
4
to a mean value of -0.7 at z
1.
The range of observed DLA metallicities is somewhat higher than
that of halo stars but overlaps, and is somewhat lower than that of
thick disk
stars and far lower than the near-solar values seen for thin disk
stars in the Milky Way. The DLAs uniformly show
greater metal enrichment than the Lyman
forest
but less than values inferred for Lyman break galaxies or quasars
at the same epoch (see Figs. 8, 32 of
Pettini 2004,
and see
Leitherer 2005
for a review).
The values given above are the cosmic mean metallicity of the neutral gas
traced by DLAs,
but they do not represent a full census of metals, which
can also be found in heavily star-forming regions that have already
used up their neutral gas or can be expelled by galactic winds
into the IGM, which is predominantly ionized.
It is therefore useful to compare the observed DLA metallicities
with those expected from the DLA star formation rates; this leads
to a factor of ten deficit in the observed metallicities called the
"Missing Metals Problem"
(Wolfe et
al. 2005,
Hopkins et
al. 2005,
Pettini 1999).
The most likely explanation is that
the star-forming regions of the galaxies seen as DLAs have superwinds
sufficiently strong to move most of the metals produced into the IGM.