The Hubble Space Telescope survey of BL Lacertae objects.: III.: Morphological properties of low-redshift host galaxies

We report on the optical properties of a sample of 30 BL Lac host galaxies in the redshift range 0.03 < z < 0.2, as derived from Hubble Space Telescope observations. All galaxies are fully resolved in the WFPC2 (F702W filter) images, allowing a quantitative analysis in two dimensions. Most and possibly all of these galaxies have characteristics very similar to those of "normal" giant ellipticals. The luminosity, ellipticity, isophote twisting, and amount of disky or boxy isophotes are consistent with those found in nonactive ellipticals and in radio galaxies. In all cases the BL Lac nucleus is well centered in the main body of its host galaxy, a result that argues strongly against the microlensing hypothesis for any significant fraction of the population. A search for faint substructures in the host galaxies has not revealed notable signatures of tidal distortions or subcomponents (faint disks, bars, X features, etc.), and with only one exception, there are no prominent dusty features in the central regions. Instead, the BL Lac host galaxies are smooth and unperturbed, suggesting that strong external gravitational interactions are not important to ongoing activity. A careful examination of the environment around the nucleus, however, shows a high incidence of close companion objects, whose nature remains unclear pending spectroscopic observations.