The luminous host galaxies of high redshift BL Lac objects

We present the first near-infrared Ks-band ( 2.1 mu m) imaging study of a sizeable sample of 13 high redshift ( 0.6 < z < 1.3) BL Lac objects in order to characterize the properties of their host galaxies. We are able to clearly detect the surrounding nebulosity in eight objects, and marginally in three others. In all the well resolved objects, we find that the host galaxy is well represented by a de Vaucouleurs r(1/4) surface brightness law. In only two cases the object remains unresolved. These new observations represent in most cases the first detection of the host galaxy and taken together with previous optical studies of z > 0.5 BL Lacs substantially increase the number of detected hosts ( from similar to 20 to similar to 30). This dataset allows us to explore the evolution of BL Lac hosts from z similar to 1 to the present epoch. We find that the host galaxies of high redshift BL Lacs are large ( average bulge scale length [R(e)] similar to 7 kpc) and similar to those hosting low redshift BL Lacs, indicating that there is no evolution in the host galaxy size. On the other hand, these host galaxies are very luminous (average [M(K)] = - 27.9 +/- 0.7). They are similar to 3 mag brighter than the typical galaxy luminosity L*, and similar to 1-1.5 mag more luminous than brightest cluster galaxies at low redshift. They are also similar to 1 mag brighter than radio galaxies at low redshift and they appear to deviate from the K - z relationship of radio galaxies. On the other hand, these high luminosities agree with the few optical studies of high redshift BL Lacs and are similar to those of flat spectrum radio quasars studied by us in the near-infrared. The nuclear luminosity and the nucleus-galaxy luminosity ratio of the high redshift BL Lacs are much larger than those found for low redshift BL Lacs and similar to those observed in flat spectrum radio quasars at similar redshift. This mainly reflects the selection effects in the surveys and may be due to either an higher intrinsic nuclear luminosity, or due to enanched luminosity because of strong beaming. Contrary to what is observed in low redshift BL Lacs, the luminosities of the host galaxy and of the nucleus appear fairly well correlated, as expected from the black hole mass - bulge luminosity relationship found in nearby spheroids, if the nuclear emission works at the same regime. Our observations indicate that high redshift BL Lacs radiate with a wide range of power with respect to their Eddington luminosity, and this power is intermediate between the low level observed in nearby BL Lacs and the higher level occurring in luminous radio-loud quasars. The comparison with BL Lac host galaxies at lower redshift suggests that there is a similar to 2 mag brightening of the hosts. We argue that the large luminosity of the hosts is due to a strong selection effect in the surveys of BL Lacs that makes observable only the most luminous sources at z > 0.5 and produces a correlation between the nuclear and the host luminosity that emerges at high redshift. However, this may also suggest a strong luminosity evolution which is inconsistent with a simple passive evolution of the stars in the host galaxies, and requires a contribution from recent star formation episodes that takes place at z > 0.5.