TheHubble Space TelescopeSurvey of BL Lacertae Objects. I. Surface Brightness Profiles, Magnitudes, and Radii of Host Galaxies

We report on a large HST imaging survey of BL Lac objects, at spatialresolution ~10 times better than previous ground-based surveys. We focus ondata reduction and analysis, describing the procedures used to model the hostgalaxy surface brightness radial profiles. A total of 69 host galaxies wereresolved out of 110 objects observed, including almost all sources at z < 0.5.We classify them morphologically by fitting with either an exponential disk ora de~Vaucouleurs profile; when one fit is preferred over the other, in 58 of 69cases, it is invariably the elliptical morphology. This is a very strong resultgiven the large number of BL Lac objects, the unprecedented spatial resolution,and the homogeneity of the data set. With the present reclassification of thehost galaxy of 1418+546 as an elliptical, there remain no undisputed examplesof a disk galaxy hosting a BL Lac nucleus. This implies that, at 99%confidence, fewer than 7% of BL Lacs can be in disk galaxies. The apparentmagnitude of the host galaxies varies with distance as expected if the absolutemagnitudes are approximately the same, with a spread of +-1 mag, out toredshift z < 0.5. At larger redshifts, only 6 of 23 BL Lacs are resolved so thepresent data do not constrain possible luminosity evolution of the hostgalaxies. The collective Hubble diagram for BL Lac host galaxies and radiogalaxies strongly supports their unification.