Title: CII^{*} Absorption in Damped Lyman Alpha Systems: A New Window on
       the Star Formation History and Multi-phase Structure of
       Protogalaxies

Art Wolfe
University of California at San Diego


Abstract:
 
I describe a technique that for the first time measures star formation
rates (SFRs) in damped lyman alpha systems (DLAs). The idea is that stars
forming in DLAs emit FUV radiation that heats the gas by ejecting
photoelectrons from grains known to be present in the gas. We infer the
heating rate by equating it to the cooling rate detected by measuring the
strength of CII^{*} 1335.7 absorption. Since the heating rate is
proportional to the dust-to-gas ratio times the grain photoelectric
heating efficiency times the SFR per unit area, \psi_{*}, we infer
\psi_{*} by  measuring cooling rates and dust-to-gas ratios in a sample
of 27 DLAs. We describe two-phase models used to infer \ps_{*} and show
that two solutions are possible: one in which CII^{*} absorption areises
in cold neutral medium (CNM} gas and another in warm neutral medium (WNM)
gas. We show that the inferred \psi_{*} corresponds to an average over
the entire DLA rather than to gas along  the line of sight. We combine
these results with DLA statistics to obtain the SFR per unit comoving
volume, \rho_{*}. We show that WNM solutions are ruled out since they
generate more background radiation than observed. By contrast, the CNM
solutions are consistent with the backgrounds. Interestingly, the
\rho_{*} we derive are consistent with SFRs deduced for the Lyman Break
Galaxies (LBGs). We speculate that while the R < 25.5 sample of LBGs
cannot explain the DLA SFRs, the fainter R > 25.5 HDF sample of LBGs could turn
out to be DLAs. Other implications of this work are as follows:
First, star formation in DLAs may occur in a compact bulge population
located at the center of the H I responsible for DLA absorption. This
would help to explain why the mass of metals produced by the forming stars
is not seen in absorption and would also explain the slow build up of
metals seen in DLAs. Second, we found no statistically significant
evidence for feedback in the DLAs. Third, we describe possible evidence
of a WNM in two, possibly three DLAs. Fourth we show that the large
CII/CI ratios in DLAs is consistent with the presence of CNM gas. Fifth,
we argue that 21 cm absorption detected in high DLAs is weak not because
the spin temperature of the gas is high, but rather because the column
densities of individual CNM clouds covering the background radio source
are low. Finally, we show that the CMB is not a significant source of
excitation of the excited fine-structure states in C^{+}.