Searching for low surface brightness galaxies in the Hubble Ultra Deep Field:: Implications for the star formation efficiency in neutral gas at z∼3

Applying the Kennicutt-Schmidt law to damped Ly alpha absorption systems (DLAs), we predict that 3% of the sky should be covered with extended objects brighter than mu(V) approximate to 28.4 mag arcsec(-2), if DLAs at redshift z = [2.5; 3.5] undergo in situ star formation. We test this hypothesis by searching the Hubble Ultra Deep Field (UDF) F606W image for low surface brightness features of angular sizes, ranging between theta(DLA) = 0.25 (d(DLA) = 2 kpc) and 4.0 (d(DLA) = 31 kpc). After convolving the F606W image with smoothing kernels of angular diameters theta(kern) = theta(DLA), we find the number of detected objects to decrease rapidly to zero at theta(kern) > 1. Our search yields upper limits on the comoving SFR densities that are between factors of 30 and 100 lower than predictions, suggesting a reduction by more than a factor of 10 in star formation efficiency at z similar to 3. The lower star formation efficiencies may be due to the reduced molecular content of the DLA gas. We also find that the cosmological increase with redshift of the critical surface density for the Toomre instability may be sufficient to suppress star formation to the levels implied by the UDF observations. The upper limits on in situ star formation reduce the predicted metallicities at z similar to 3 to be significantly lower than observed, and reduce the heat input in the gas to be substantially lower than the inferred cooling rates. In contrast, the radiative output from compact Lyman break galaxies (LBGs) with R < 27 is sufficient to balance the comoving cooling rate. Thus, many DLAs may host more compact regions of active star formation, which may chemically enrich these DLAs. Such regions are likely to be LBGs.