Structural characteristics of faint galaxies serendipitously discovered with the Hubble Space Telescope WFPC2

Utilizing the F814W and F300W filters, Hubble Space Telescope Wide Field Pl anetary Camera-2 (WFPC2) images were taken of four low surface brightness g alaxies in the direction of the Virgo Cluster-V7L3, V2L8, V1L4, and Malin 1 . The high resolution of the WFPC2 combined with the extremely diffuse natu re of the four galaxies makes them essentially transparent, allowing for th e serendipitous discovery of 139 background galaxies visible through both t he disks and nuclei of the foreground galaxies. Surface photometry was done on the newly discovered galaxies through the F814W (I-band) filter. The de tected galaxies have both, r(1/4) and exponential-type profiles with radii (to the mu(F814W) = 25.0 mag arcsec(-2) limit) less than 5." 0. Their total magnitudes range from 18.9 through the survey cutoff at 25.0 in the F814W filter. The median central surface brightness of those galaxies with expone ntial profiles is approximately 1 mag brighter than the background F814W "s ky." Thus, with this data set we recover Freeman's law and hence know that we do not have a representative sample of distant galaxies (and neither doe s anyone else). When possible, the B, V, and I colors of these galaxies wer e determined using ground-based images, which show the galaxies to be fairl y red. Coupled with their small angular size, we estimate the redshifts to be 0.5 less than or equal to z less than or equal to 1.5. Classification of the galaxies was done strictly in structural terms, based only on the form of the derived luminosity profile. No morphological consi derations were made during the classification process. Twenty-three percent 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 addi tion, we have attempted to perform bulge/disk deconvolutions. While we find that most of the sample cannot be easily deconvolved into a classic bulgedisk, seven objects could be fitted in this way. For these seven objects we find (1) a large range in bulge-to-total luminosity and (2) some disks tha t have a large bulge-to-disk ratio. We also present one object, 283-10, whi ch is an excellent example of the structural ambiguity that exists in the l uminosity profiles of distant galaxies. In agreement with other studies, we also found a significant percentage of galaxies that have disturbed luminosity profiles indicative of probable gal axy-galaxy interactions or mergers. Indirect indicators suggest that the vo lume over which r(1/4) objects are selected is significantly larger than th e volume over which disk galaxies are selected. This implies a relatively l ow space density of r(1/4) at all redshifts out to z similar to 2.5 and is consistent with the general idea that r(1/4) galaxies are largely confined to galaxy clusters.