Structure and photometry of an I < 20.5 galaxy sample from the Hubble Space Telescope medium deep survey

A set of 100 faint galaxies from nine Hubble Space Telescope Wide Field Camera I-band images have been analyzed as part of the Medium Deep Survey (MDS) Key Project. This sample reaches a depth of I ≲ 20.5 (corresponding to B ∼ 22-23) and complements the first set of fainter galaxies analyzed by the MDS team. Images were deconvolved using the Lucy-Richardson algorithm and a newly developed procedure designed to yield a more reliable determination of structure in the low-S/N regime. These deconvolved images were used to characterize the structure of the galaxies through quantitative measurements of total magnitudes, half-light radii, exponential disk scale lengths, and disk-to-total ratios. Extensive testing was done to establish the validity of the procedures used and to characterize the degree of systematic errors present in the analysis techniques. The observed size-magnitude distributon appears consistent with a scenario in which luminous galaxies have evolved little in intrinsic luminosity, size, or structure over recent epochs in a "normal" cosmology (0 < q₀ < 0.5 and Λ₀ = 0). The predicted nonevolving distributions were based on models designed to fit existing counts, colors, and redshifts of faint galaxies and on the observed correlations between metric rest-frame size and luminosity found in a nearby galaxy sample studied by Kent (1984, 1985). The typical galaxy in our sample is expected to be at z ≃ 0.3, and to have a luminosity ∼0.5 mag fainter than L* and a half-light radius of ∼ 1″ or ∼6 kpc (H₀ = 50 km s^-1 Mpc^-1). The observed distribution of disk-to-total ratios, while uncertain, is in agreement with that of Kent's sample and thus supports the view that substantial evolution has not occurred over the look-back times characteristic of our sample.