Hubble Space Telescope 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 (MS 1221.8+2452, MS 1407.9+5954, and MS 2143.4+0704) reveal that each of these BL Lac objects is well centered in an extended nebulosity that is consistent in brightness and morphology with light from an elliptical galaxy at their previously reported redshifts. Each of the detected host galaxies have radial surface-brightness profiles that are well fitted by a de Vaucouleurs law with effective radii of between 3 and 12 kpc (H0 = 50 km s-1 Mpc-1, q0 = 0). The absolute magnitudes of the host galaxies fall in the range of luminosities determined for other BL Lacertae object host galaxies, -24.7 < MI < -23.5. 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 for 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 (MS 2143.4+0704). The apparent magnitudes and image properties of the companions of the BL Lac objects are cataloged as part of this work. The three BL Lac objects appear to occur in diverse environments, from relative isolation (MS 1221.8+2452) to, possibly, a rich group of galaxies (MS 1407.9+5954).