Optical imaging of the host galaxies of X-ray selected BL Lacertae objects

We investigate the properties of the host galaxies of X–ray selected (high frequency peaked) BL Lac objects using a large and homogeneous data set of high spatial resolution R–band observations of 52 BL Lacs in the EMSS and Slew samples. The redshift distribution of the BL Lacs ranges from z = 0.04 to z>0.7, with average and median redshifts z = 0.26 and z = 0.24, respectively. Eight objects are at unknown redshift. We are able to resolve 45 objects out of the 52 BL Lacs. For all the well resolved sources, we find the host to be a luminous elliptical galaxy. In a few cases a disk is not ruled out but an elliptical model is still preferred. The average absolute magnitude of the host galaxies is < MR(host) > = –23.9±0.6, while the average scale length of the host is = 9±5 kpc. There is no difference in the host properties between the EMSS and Slew samples. We find a good agreement between the results derived by the surveys of Wurtz et al. (ground-based data) and Urry et al. (HST data), and by our new deeper imaging. The average luminosity of the BL Lac hosts is between those of F-R I and F-R II radio galaxies in Govoni et al., supporting the idea that both radio galaxy types could contribute to the parent population. The BL Lac hosts follow the F-P relation for giant ellipticals and exhibit a modest luminosity evolution with redshift. Finally, we find a slight correlation between the nuclear and host luminosity and a bimodal distribution in the nuclear/host luminosity ratio.