Structural Characteristics of Faint Galaxies Serendipitously Discovered with the HST WFPC2

Authors: Karen O'Neil, G. D. Bothun, & C. Impey

Published:The Astrophyical Journal Supplement, Vol 128, pg 99 (2000).


Utilizing the F814W and F300W filters, Hubble Space Telescope Wide Field Planetary Camera-2 (WFPC2) images were taken of four low surface brightness galaxies in the direction of the Virgo cluster -- V7L3, V2L8, V1L4, and Malin 1. 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. The detected galaxies have both r1/4 and exponential type profiles with radii (to the µF814W = 25.0 mag arcsec-2 limit) less than 5.0''. Their total magnitudes range from 18.9 through the survey cut-off at 25.0 in the F814W filter. 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). When possible, the B, V, and I colors of these galaxies were determined using ground-based images, which show the galaxies to be fairly red. Coupled with their small angular size, we estimate the redshifts to be 0.5 < z < 1.5.

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 r1/4 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 addition, we have attempted to perform bulge/disk deconvolutions. While we find that most of the sample cannot be easily deconvolved into a classic bulge+disk, 7 objects could be fit in this way. For these 7 objects we find a) a large range in bulge-to-total luminosity and b) some disks which have a large bulge-to-disk ratio. We also present one object, 283-10, which is an excellent example of the structural ambiguity that exists in the luminosity profiles of distant 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 r1/4 objects are selected is significantly larger than the volume over which disk galaxies are selected. This implies a relatively low space density of r1/4 at all redshifts out to z ~ 2.5 and is consistent with the general idea that r1/4 galaxies are largely confined to galaxy clusters.

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