C II* absorption in damped Lyα systems.: II.: A new window on the star formation history of the universe

Starting from the star formation rate (SFR) per unit area, (psi) over dot (*), determined for damped Lyalpha systems (DLAs) using the C II* method, we obtain the SFR per unit comoving volume, (rho) over dot (*) (z), at z approximate to 3. Pure warm neutral medium (WNM) solutions are ruled out since they generate more bolometric background radiation than observed, but the two-phase solutions dominated by the cold neutral medium (CNM) are consistent with the backgrounds. We find that (rho) over dot (*) (z) for DLAs agrees with the (rho) over dot (*) (z) for the Lyman break galaxies (LBGs). Although the mass of produced stars indicated by the SFRs is consistent with the current densities of known stellar populations, the mass of metals produced by z = 2.5 is 30 times larger than detected in absorption in DLAs. Of the three possible solutions to this "missing metals'' problem, the most likely appears to be that star formation occurs in compact bulge regions. We search for evidence of feedback and find no correlations between (psi) over dot (*) and N(H I), but possible correlations between (psi) over dot (*) and low-ion velocity width and (psi) over dot (*) and metal abundance. We show that (1) the correlation between cooling rate and dust-to-gas ratio is positive evidence for grain photoelectric heating, (2) the cosmic microwave background (CMB) does not significantly populate the C II excited fine-structure states, and (3) the ratio of C II* to resonance-line optical depths is a sensitive probe of the multiphase structure of the DLA gas. We address recent arguments that DLAs are comprised only of WNM gas and show them to be inconclusive. Despite the rough agreement between (rho) over dot (*) (z) for DLAs and LBGs, current evidence indicates that these are distinct populations.