Imaging of the Host Galaxies of Three X-Ray Selected BL Lacertae Objects

Hubble Space Telescope (HST) WFPC-2 I-band (F814W) images of three X-ray selected BL Lacertae objects (MS1221.8+2452, MS1407.9+5954, & MS2143.4+0704) reveal that each of these BL Lac objects is well-centered in an extended nebulosity that is consistent in brightness and morphology with being light from an elliptical galaxy at the previously reported redshifts of these BL Lac objects. Each of the detected host galaxies have radial surface brightness profiles that are well fit by a DeVaucouleurs’ law with effective radii of between 3 to 12 kpc (H_0=50 km s^{-1} Mpc^{-1}, q_0 = 0). The absolute magnitudes of the host galaxies fall in the range -24.7 < M_I < -23.5, in the range of luminosities determined for other BL Lacertae object host galaxies. In addition to allowing the measurement of the host galaxy magnitudes and radial surface brightness profiles, the HST images allow a search for substructure in the host galaxies and the presence of close companion galaxies at spatial resolutions not yet achievable from the ground. While no evidence was found for any bars'' or spiral arms, boxy’’ isophotes are present in the host galaxy of at least one of the three objects observed as part of this study (MS2143.4+0704). The apparent magnitudes and image properties of the companions of the BL Lac objects are catalogued as part of this work. The three BL Lacs appear to occur in diverse environments, from being fairly isolated (MS1221.8+2452) to possibly being a member of a rich group of galaxies (MS1407.9+5954).