HST images of a galaxy group at z = 2.81, and the sizes of damped Lyα galaxies

Abstract

We present Hubble Space Telescope (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 Lyα emission near the z = 2.8 quasar PKS 0528 − 250. One of the galaxies is a damped Lyα (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 subpixel 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 Lyα emission. The three galaxies are detected in all three bands, with average mB ∼ 26, mI ∼ 25. Two of the galaxies are compact with intrinsic (i.e. after correcting for the effect of the psf) half-light radii of ∼ 0.1 arcsec (0.4 h−1 kpc, q∘ = 0.5). The third galaxy comprises two similarly compact components separated by 0.3 arcsec. The HST images and a new ground-based Lyα image of the field provide evidence that the three galaxies are more extended in the light of Lyα than in the continuum. Combined with the evidence from the Lyα linewidths, previously measured, this suggests that we are measuring the size of the surface of last scattering of the escaping resonantly scattered Lyα 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 ∼ 3 of < 13 h−1 kpc, for q∘ = 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∘ 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∘ = 0.0 there is no evidence as yet that DLA clouds are more common than spiral galaxies locally. We summarize evidence that filamentary structures occur in the distribution of galaxies at high redshift.