7.2. Star Formation Rates and the Cosmic Star Formation History
If the ionization is dominated by hot, young stars, the observed flux of
the Ly
emission line may
be used to estimate a lower bound to the star formation rate
in a galaxy. Using
the case B recombination
Ly /
H ratio of
10
(Osterbrock 1989),
and the Kennicutt (1983)
conversion from H
luminosity to ,
Madau et al. (1998)
find ~ 0.7 ×
10-42 h50-2
LLy
M
yr-1 where
LLy is measured in units of ergs s-1
(q0 = 0.5;
is
3.3 times larger for
q0 = 0.1). The star formation rate may also be
estimated from the observed UV continuum emission.
Madau et al. (1998)
calculate that a population older than 100 Myr will have
10-40L1500
M
yr-1 where L1500, the luminosity at 1500
Å, is measured in units of ergs s-1 Å-1.
Leitherer, Carmelle, &
Heckman (1995)
models yield similar results for a different initial mass functions
(IMFs) and ages less than 10 Myr. These are lower limits to
since no correction
to the UV flux for either internal absorption or dust extinction has
been made (see Section 7.5).
With sufficient numbers of well-observed distant systems, we may begin
studying the star formation history of the universe as a function of
comoving volume. The Canada-France Redshift Survey (CFRS;
Lilly et al. 1995)
demonstrated strong luminosity evolution in the blue field galaxy
population between z = 0 and z = 1.
Madau et al. (1996),
using the early results of photometric selection in the HDF, integrated
the results at z < 5 into a coherent picture of the star
formation history of the universe and suggested that the global star
formation peaks between z = 1 and z = 2, in remarkable
agreement with predictions based on the comoving H I density traced by
Ly absorption systems
(Pei & Fall 1995),
with hierarchical models in a cold dark matter dominated universe
(Baugh et al. 1998),
and with the quasar luminosity function
(Cavaliere & Vittorini
1998).
Substantial caveats temper this result, however: (1) the number of
spectroscopically measured redshifts between z = 1 and z =
2 is small.
Connolly et al. (1997)
use optical and near-infrared data to estimate photometric redshifts in
the HDF at 1 < z < 2 in order to span this redshift
"desert" for which few strong spectroscopic features are shifted into
the optical regime. Until a substantial number of confirmed redshifts
have been obtained, however, photometric redshifts in the 1 <
z < 2 range are not well constrained. Thus the epoch thought
to be the most productive in terms of star formation is also the least
well measured. (2) The small area
( 5
arcmin2) covered by the HDF makes global parameters inferred
from it vulnerable to perturbations from large-scale structure. (3) The
existence of the peak at z ~ 1.5 is contingent upon the
completeness of the estimates of global star formation at higher
redshift. The z ~ 4 point in the analysis of
Madau et al. (1996)
was based on a single object in the HDF at z = 4.02
(Dickinson 1998)
and should thus be treated as an uncertain lower limit. More recent
measurements, based on larger area surveys, show that the star formation
density is significantly higher at z
4
(Steidel et al. 1999).
Finally, (4) both the photometric and the Ly search techniques require
the objects to be UV bright. It is possible that a substantial fraction
of star-forming activity in dusty high-redshift systems has been
overlooked thus far. The recent discoveries of a relatively large number
of resolved sources in small area sub-mm surveys performed by SCUBA
suggest this to be the case (e.g.,
Hughes et al. 1998;
Barger et al. 1998;
Dey et al. 1999a).
Figure 11 illustrates a recent determination of
the star formation history of the universe. In sharp contrast to the
initial results of
Madau et al. (1996),
the UV luminosity density of the universe is approximately constant at
2 
z
4. Preliminary
results from the Berkeley long-slit searches suggest that it remains
constant to z 
5.5. The evolution of the star formation density of the universe, as
probed by apparently normal, star-forming systems, implies that their
evolution follows a significantly different trajectory than that of AGN,
especially quasars. Both radio-quiet and radio-loud quasars show a
considerable density decrease beyond z ~ 3 (cf.
Dunlop & Peacock 1990;
Hook & McMahon 1998;
Cavaliere & Vittorini
1998),
falling by a factor of ~ 3 from z = 3 to z = 4.
 |
Figure 11. The star formation history of
the universe. Points at z < 1 are from the Canada-France
Redshift Survey (CFRS;
Lilly et al. 1996);
points at 1 |
z, is 1/42 of the cosmic star formation
density,