The host galaxies of BL Lac objects in the near--infrared

We present the results of near-infrared H band (1.65 microns) imaging of 11 BL Lac objects with redshifts ranging from z = 0.05 to 0.9. We are able to clearly detect the host galaxy in seven low redshift (z<=0.24) BL Lacs, while the four unresolved BL Lacs have either high or unknown redshift. The galaxies hosting the low redshift BL Lacs are large (average bulge scale length R(e) = 8.8+-9.9 kpc) and luminous (average M(H) = -25.8+-0.5), i.e. slightly brighter than the typical galaxy luminosity L* (M*(H) = -25.0+-0.2), and of similar luminosity to or slightly fainter than brightest cluster galaxies (M(H) = -26.3+-0.3). The average optical/near-infrared colour and colour gradient of the BL Lac hosts (R-H = 2.2+-0.5; d(R-H)/d(log r) = -0.09$+-0.04) are consistent with the hosts being normal ellipticals, indicating that the nuclear activity has only a marginal effect on the star formation history and other properties of the hosts. The BL Lac hosts appear slightly less luminous than those of higher redshift flat spectrum radio quasars. The nucleus-to-galaxy luminosity ratio of the BL Lacs is similar to that of low redshift radio galaxies and consistent with what found in previous optical studies of BL Lacs. However, it is smaller that that found for flat spectrum radio quasars, suggesting there is a difference in the intrinsic brightness of the nuclear source or in the Doppler beaming factor between the two types of blazars.