Damped Lyman alpha absorption systems (DLAs) are 
a population of objects that act as neutral-gas reservoirs
for star formation at high redshift. If the star formation efficiency
in DLAs is the same as in current galaxies, a significant fraction
of the sky would be covered by emission from low surface-brightness
objects. I describe results of a recent survey for such emission
using deep images from the Hubble Ultra Deep Field. The low
rate of detection implies a low efficiency for in situ star formation
throughout the neutral gas. But evidence that the gas
emits cooling radiation suggests it is being heated. I discuss
why in DLAs with cooling rates below a critical value
the gas is heated by X-ray background radiation, and in DLAs with cooling
rates above the critical value local sources of FUV radiation are required.
The local sources are likely to be compact Lyman Break galaxies (LBGs)
embedded in the DLA gas.
I suggest that the  bimodality detected in the cooling rates results 
from different modes of mass accretion from the intergalactic medium. 
DLAs with high cooling rates are massive objects (M>10^{12}M_{sun}) that 
undergo `hot-mode' accretion, while DLAs with low cooling rates are less 
massive and undergo `cold-mode' accretion.