Structural Characteristics of Faint Galaxies Serendipitously Discovered with the HST WFPC2

Abstract

[Abridged] HST WFPC2 images were taken of four low surface brightness galaxies in the direction of the Virgo cluster. The high resolution of the WFPC2 combined with the extremely diffuse nature of the four galaxies makes them essentially transparent, allowing for the serendipitous discovery of 139 background galaxies visible through both the disks and nuclei of the foreground galaxies. Surface photometry was done on the newly discovered galaxies through the F814W (I-band) filter, showing the median central surface brightness of those galaxies with exponential profiles is approximately one magnitude brighter than the background F814W "sky". Thus, with this dataset we recover Freeman's law and hence know that we do not have a representative sample of distant galaxies (and neither does anyone else). Classification of the galaxies was done strictly in structural terms, based only on the form of the derived luminosity profile. No morphological considerations were made during the classification process. 23% of the galaxies we detected have the r^{1/4} profile typical of early type galaxies, matching most previous studies of both the Hubble Deep Field and the Medium Deep Survey which typically find 15% - 40% E/S0 galaxies. In agreement with other studies we also found a significant percentage of galaxies which have disturbed luminosity profiles indicative of probable galaxy-galaxy interactions or mergers. Indirect indicators suggest that the volume over which r^{1/4} objects are selected is significantly larger than the volume over which disk galaxies are selected. This implies a relatively low space density of r^{1/4} at all redshifts out to z ~ 2.5 and is consistent with the general idea that r^{1/4} galaxies are largely confined to galaxy clusters.