The Hubble Space Telescope survey of BL lacertae objects. I. Surface brightness profiles, magnitudes, and radii of host galaxies

We report on a large Hubble Space Telescope imaging survey of BL Lac object s, at spatial resolution greater than or similar to 10 times better than pr evious ground-based surveys. We focus on data reduction and analysis, descr ibing the procedures used to model the host galaxy surface brightness radia l profiles. A total of 69 host galaxies were resolved out of 110 objects ob served, including almost all sources at z less than or similar to 0.5. We c lassify them morphologically by fitting with either an exponential disk or a de Vaucouleurs profile; when one fit is preferred over the other, in 58 o f 69 cases, it is invariably the elliptical morphology. This is a very stro ng result given the large number of BL Lac objects, the unprecedented spati al resolution, and the homogeneity of the data set. With the present reclas sification of the host galaxy of 1418+546 as an elliptical, there remain no undisputed examples of a disk galaxy hosting a BL Lac nucleus. This implie s that, at 99% confidence, fewer than 7% of BL Lac objects can be in disk g alaxies. The apparent magnitude of the host galaxies varies with distance a s expected if the absolute magnitudes are approximately the same, with a sp read of +/-1 mag, out to redshift z similar to 0.5. At larger redshifts, on ly six of 23 BL Lac objects are resolved so the present data do not constra in possible luminosity evolution of the host galaxies. The collective Hubbl e diagram for BL Lac host galaxies and radio galaxies strongly supports the ir unification.