HST IMAGES OF A GALAXY GROUP AT Z=2.81, AND THE SIZES OF DAMPED LY-ALPHA GALAXIES

We present Hubble Space Telescope (HST) WFPC2 observations in three ba nds (F450W = B, F467M and F814W = I) of a group of three galaxies at z = 2.8 discovered in a ground-based narrow-band search for Ly alpha em ission near the z = 2.8 quasar PKS 0528 - 250. One of the galaxies is a damped Ly alpha (DLA) absorber and these observations bear on the re lation between the DLA clouds and the Lyman-break galaxies and the sta ge in the evolution of galaxies they represent. We describe a procedur e for combining the undersampled WFPC2 images pointed on a subpixel gr id, which largely recovers the full sampling of the WFPC2 point spread function (psf), These three galaxies have similar properties to the L yman-break galaxies except that they have strong Ly alpha emission. Th e three galaxies are detected in all three bands, with average m(B) si milar to 6, m(I) similar to 25, Two of the galaxies are compact with i ntrinsic (i,e, after correcting for the effect of the psf) half-light radii of similar to 0.1 arcsec (0.4 h(-1) kpc, q(o) = 0.5). The third galaxy comprises two similarly compact components separated by 0.3 arc sec, The HST images and a new ground-based Ly alpha image of the field provide evidence that the three galaxies are more extended in the lig ht of Ly alpha than in the continuum. Combined with the evidence from the Ly alpha linewidths, previously measured, this suggests that we ar e measuring the size of the surface of last scattering of the escaping resonantly scattered Ly alpha photons, The measured impact parameters for this DLA galaxy (1.17 arcsec), for a second confirmed system, and for several candidates, provide a preliminary estimate of the cross-s ection-weighted mean radius of the DLA gas clouds at z similar to 3 of < 13 h(-1) kpc, for q(o) = 0.5. The true value is likely substantiall y smaller than this limit as DLA clouds at small impact parameter are harder to detect. Given the observed sky covering factor of the absorb ers this implies that for q(o) = 0.5 the space density of DLA clouds a t these redshifts is more than five times the space density of spiral galaxies locally, with the actual ratio probably considerably greater. For q(o) = 0.0 there is no evidence as yet that DLA clouds are more c ommon than spiral galaxies locally. We summarize evidence that filamen tary structures occur in the distribution of galaxies at high redshift .