NEAR-INFRARED IMAGING OF THE HOST GALAXIES OF BL LACERTAE OBJECTS

Near-infrared images have been obtained of nine BL Lacertae objects in order to investigate their host galaxy properties. From numerical mod elling of the data we find that five of the nine BL Lacs have contribu tions from ex-tended galaxy emission in their profiles. Tentative morp hologies were derived for two of the BL Lacs, namely B2 0752 + 258, fo r which a disc morphology is slightly preferred, and S4 0954 + 65, for which an elliptical morphology is preferred. From our modelling we de rive host galaxy absolute magnitudes of M-H = -25.6 + 0.5 for B2 0752 + 258, and M-H = -26.3 +/- 0.8 for S4 0954 + 65. We also find an avera ge K-band absolute K-corrected host galaxy magnitude, for the BL Lacs, of [M-K]= -26.3 +/- 0.6 assuming an elliptical galaxy model, and [M-K ]= -26.1 +/- 0.9 assuming a disc galaxy model. The derived absolute ma gnitudes are similar to those found for the putative parent population of FRI radio galaxies, predicted by unified schemes. For those BL Lac s in which host galaxies have been previously detected at optical wave lengths we derive rest frame colours which are generally consistent wi th those of 'normal' galaxies. However, for TEX 0836 + 182 we find a r ather blue colour which, together with the elongated morphology, may b e indicative of interaction or spiral structure. Our results suggest t hat the host galaxies of BL Lacs are indeed similar to those of FRI ra dio galaxies, adding further weight to the unification scenario. In ou r present infrared frames of relatively modest resolution, we do not f ind that infrared observations are any more or less effective than opt ical observations at revealing the underlying host galaxy.