Deep Hubble Space Telescope imaging of 53W044: An S0 radio galaxy at z=0.311

We present images with the Wide-Field Camera (WFC) and Faint-Object Camera (FOC) of the radio galaxy 53W044, as part of a morphological study of the faint radio-source population. The WFC images are used to examine the structure of the galaxy, and show evidence for a significant disk (contributing 18%-20% of the light in the observed V and I bands), which leads us to classify 53W044 as an S0. This radio galaxy is near the maximum radio power associated with sources in S0 host galaxies. The WFC images also reveal a close companion at a projected distance of 8 kpc for H₀=75 km s^-1 Mpc^-1 and q₀=0.5 The V-I colors are consistent with a common redshift and so with the possibility of physical interaction. There is excess light asymmetrically distributed around the companion possibly stars tidally stripped from one pair member. The FOC image (at 2750 Å) is combined with ground-based spectroscopy to study 53W044's stellar population, which appears normal for an E/S0 galaxy of modest luminosity (M_V=-21.7). We find no evidence for a significant contribution from a nuclear blue-continuum source (at the level of less than 7% of the flux emitted at 2100 Å), and the stellar population is old with a continuum level at 2100 Å consistent with what is seen in nearby radio galaxies. We see no evidence for spectral evolution in this galaxy compared to similar objects at the present epoch We can use this UV flux to place a rather soft limit on the formation redshift (when the last major star formation occurred) z_f≥0.8, rising to z_f≥1.15 for a Hubble constant of 100 km s^-1 Mpc^-1. We present a hybrid deconvolution scheme, combining either multiple passes of σ-CLEAN plus spatial filtering, or σ-CLEAN and Lucy-Richardson iterations, at savings in computation speed, and potentially improved accuracy in regions of high signal and fine structure. We also discuss measurement of quantitative morphological parameters for galaxies using modeling and convolution with the HST point-spread function. A faint blue galaxy appears in all three passbands projected 16 arcsec from the radio galaxy. The WFC images show elongation with most of the flux inside a 0.4×0.8 arcsec area and some evidence for internal clumps, and combined with the FOC data show a flat continuum similar to that seen in many nearby starburst systems. At apparent magnitude V∼23, recent redshift surveys suggest typical redshifts of less than z=0.7 for objects of this kind.