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

In an earlier paper, we reported the detection of Ly alpha emission fr om the damped Ly alpha absorber at z(abs) = 2.811 seen in the spectrum of the quasar PKS 0528 - 250. The source was the nearest to the line of sight to the quasar of three candidates detected in a narrow-band i mage. We present here a higher S/N spectrum of the first source, as we ll as spectra of the remaining two sources, and results from a deep br oadband image. We confirm Ly alpha emission from all three sources, an d provide redshifts and line widths for each line. Two of the sources display broad lines Delta upsilon > 600 km s(-1) (FWHM), and combined with the evidence of the shape of the sources this appears to rule out the possibility that the clouds are photoionised by the quasar. The l ine widths are plausibly explained as the result of resonant scatterin g of the Ly alpha photons escaping through a high column density of H I, and we conclude that we are probably seeing three regions of star f ormation (although the possibility that one or more of the sources con tains a low-luminosity AGN cannot be excluded). We infer a very low du st to gas ratio for the group, so the continuum magnitudes provide a u seful estimate of the SFR in each sourer. The total SFR for the group is estimated to be a few M(.)yr(-1). The three Ly alpha sources are ap proximately aligned, and we draw attention to the similarity between t his arrangement and the filamentary distribution of aggregations of co ld gas seen in simulations of galaxy formation. From the estimated gas and dynamical masses, and based on the results of these simulations, we judge that the three sources are likely to merge on a timescale of < 10(9) years. This suggests that at z similar to 3 at least some of t he damped Ly alpha systems are sub-components of galaxies in the proce ss of assembly, rather than fully-formed disks that turn into the stel lar disks of spiral galaxies. The total gas mass in the group is estim ated to be of the order of 10(10) M(.). The overall picture that emerg es is of star formation in a sub-L, galaxy, at an early stage in its e volution, during the process of assembly.