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 modelling of the data we find that five of the nine BL Lacs have contributions from ex-tended galaxy emission in their profiles. Tentative morphologies were derived for two of the BL Lacs, namely B2 0752 + 258, for which a disc morphology is slightly preferred, and S4 0954 + 65, for which an elliptical morphology is preferred. From our modelling we derive 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 average 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 magnitudes are similar to those found for the putative parent population of FRI radio galaxies, predicted by unified schemes. For those BL Lacs in which host galaxies have been previously detected at optical wavelengths we derive rest frame colours which are generally consistent with those of 'normal' galaxies. However, for TEX 0836 + 182 we find a rather blue colour which, together with the elongated morphology, may be indicative of interaction or spiral structure. Our results suggest that the host galaxies of BL Lacs are indeed similar to those of FRI radio galaxies, adding further weight to the unification scenario. In our present infrared frames of relatively modest resolution, we do not find that infrared observations are any more or less effective than optical observations at revealing the underlying host galaxy.