We present HST WFPC2 observations in three bands ( 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 emission 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 relation between the DLA clouds and the Lyman-break galaxies and the stage in the evolution of galaxies they represent .
We describe a procedure for combining the undersampled WFPC2 images pointed on a sub-pixel grid , which largely recovers the full sampling of the WFPC2 point spread function ( psf ) .
These three galaxies have similar properties to the Lyman-break galaxies except that they have strong Ly \alpha emission .
The three galaxies are detected in all three bands , with average m _ { B } \sim 26 , m _ { I } \sim 25 .
Two of the galaxies are compact with intrinsic ( i.e .
after correcting for the effect of the psf ) half-light radii of \sim 0.1 arcsec ( 0.4 h ^ { -1 } kpc , q _ { \circ } = 0.5 ) .
The third galaxy comprises two similarly compact components separated by 0.3 arcsec .
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 light of Ly \alpha than in the continuum .
Combined with the evidence from the Ly \alpha line widths , previously measured , this suggests that we are 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-section-weighted mean radius of the DLA gas clouds at z \sim 3 of < 13 h ^ { -1 } kpc , for q _ { \circ } = 0.5 .
The true value is likely substantially smaller than this limit as DLA clouds at small impact parameter are harder to detect .
Given the observed sky covering factor of the absorbers this implies that for q _ { \circ } = 0.5 the space density of DLA clouds at these redshifts is more than five times the space density of spiral galaxies locally , with the actual ratio probably considerably greater .
For q _ { \circ } = 0.0 there is no evidence as yet that DLA clouds are more common than spiral galaxies locally .
We summarise evidence that filamentary structures occur in the distribution of galaxies at high redshift .