Directly imaging damped Ly-alpha galaxies at z>2. III: The star
  formation rates of neutral gas reservoirs at z~2.7
  We present results from a survey designed to probe the star formation
properties of 32 damped Ly-alpha systems (DLAs) at z~2.7. By using the
"double-DLA" technique that eliminates the glare of the bright background
quasars, we directly measure the rest-frame FUV flux from DLAs and their
neighbouring galaxies. At the position of the absorbing gas, we place stringent
constraints on the unobscured star formation rates (SFRs) of DLAs to 2-sigma
limits of <0.09-0.27 M/yr, corresponding to SFR surface densities
<10^(-2.6)-10^(-1.5) M/yr/kpc^2. The implications of these limits for the star
formation law, metal enrichment, and cooling rates of DLAs are examined. By
studying the distribution of impact parameters as a function of SFRs for all
the galaxies detected around these DLAs, we place new direct constraints on the
bright end of the UV luminosity function of DLA hosts. We find that <13% of the
hosts have SFR>2 M/yr at impact parameters b < (SFR/(M/yr))^(0.8)+6 kpc,
differently from current samples of confirmed DLA galaxies. Our observations
also disfavor a scenario in which the majority of DLAs arise from bright LBGs
at distances 20 < b < 100 kpc. These new findings corroborate a picture in
which DLAs do not originate from highly star forming systems that are
coincident with the absorbers, and instead suggest that DLAs are associated
with faint, possibly isolated, star-forming galaxies. Potential shortcomings of
this scenario and future strategies for further investigation are discussed.
