EMISSION FROM A DAMPED LY-ALPHA ABSORBER AT Z = 2.81

It is widely held that the damped Lyalpha absorption lines seen in the spectra of many high-redshift quasars indicate the existence of large circular slabs of gas, that shrink to form the disks of todays spiral galaxies. TO attempt to detect Lyalpha emission from such a gas cloud, and measure its size, we have obtained a very deep image of the field of the quasar PKS0528-250 (z(abs) = 2.811, z(em) = 2.77) using a 23angstrom-wide filter centred on the damped line. From a comparison with B and V images we find three objects that are candidate sources of Lyalpha emission, detected at signal-to-noise ratios 13, 9, and 10. The nearest candidate lies at an angular separation of 1.6'' from the quasar, which corresponds to a projected distance 9(5)h-1 kpc, q(o) = 0.1(0.5). This object is confirmed by subsequent spectroscopy to be Lyalpha emission from the absorber itself, on the basis of the measured velocity difference between emitter and absorber of only 50 +/- 100 km s-1. The gas may be photoionised by the quasar, but we argue that hot stars are more likely the source of the ionising radiation. The line luminosity 2.5(1.1) x 10(42)h-2 ergs s-1 corresponds to a star formation rate of 2.2(1.0)h-2 M.yr-1 if there is no dust. However the true value could be higher by an order of magnitude or more if dust is present. The emission-line region is possibly extended, in which case the minimum size of the absorber is 27(18)h-1kpc. The two other Lyalpha candidates are of similar brightness and lie within only 21'' of the quasar. These two sources could be galaxies in a compact group. Alternatively, together with the first source, they could be regions of star formation in a single very large gas cloud, of minimum size 116(77)h-1kpc.