Context. We present B-band imaging of 18 low redshift (z ≤ 0.3) BL Lac objects for which their host galaxies were previously resolved in the R-band and the near-infrared H-band. For a subset of the objects, U-and V-band imaging also is presented. Aims. These multiwavelength data are used to investigate the blue-red-near-infrared colours and the colour gradients of the host galaxies of BL Lacs in comparison with other elliptical galaxies with and without nuclear activity. Methods. For all the BL Lacs observed in the B-and V-bands, and all objects at z < 0.15 in the U-band, the host galaxy is clearly resolved. In all cases galaxies are well represented by an elliptical model, with average absolute magnitude M B = −21.6 ± 0.7 and average scale length R e = 7.6 ± 3.2 kpc. BL Lac host galaxies are therefore luminous (massive) elliptical galaxies, in agreement with previous studies in other bands. Results. The best-fit B-band Kormendy relation of (µ e = 3.3 log R e (kpc) + 18.4 mag arcsec −2 ) is in reasonable agreement with that obtained for normal ellipticals and radio galaxies. This structural and dynamical similarity indicates that all massive elliptical galaxies can experience nuclear activity without significant perturbation of their global structure. The distributions of the integrated blue/nearinfrared colour (with average B − H = 3.5 ± 0.5) and colour gradient (with average ∆(B − R)/∆(log r) = −0.14 ± 0.75) of the BL Lac hosts are much wider than those for normal ellipticals, and most BL Lac objects have bluer hosts and/or steeper colour gradients than those in normal ellipticals. Conclusions. The blue colours are likely caused by a young stellar population component, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. This result is corroborated by stellar population modelling, indicating a presence of young/intermediate age populations in the majority of the sample, in agreement with low redshift quasar hosts. The lack of strong signs of interaction may require a significant time delay between the event with associated star formation episodes and the start of the nuclear activity.

No abstract

Abstract. We present K band adaptive optics observations of three high-redshift (z ∼ 2.2) high-luminosity quasars, all of which were studied for the first time. We also observed several point spread function (PSF) calibrators, non-simultaneously because of the small field of view. The significant temporal PSF variations on timescales of minutes inhibited a straightforward scaled PSF removal from the quasar images. Characterising the degree of PSF concentration by the radii encircling 20% and 80% of the total flux, respectively, we found that even under very different observing conditions the r 20 vs. r 80 relation varied coherently between individual short exposure images, delineating a well-defined relation for point sources. Placing the quasar images on this relation, we see indications that all three objects were resolved. We designed a procedure to estimate the significance of this result, and to estimate host galaxy parameters, by reproducing the statistical distribution of the individual short exposure images. We find in all three cases evidence for a luminous host galaxy, with a mean absolute magnitude of M R = −27.0 and scale lengths around ∼4-12 kpc. Together with a rough estimate of the central black hole masses obtained from C line widths, the location of the objects on the bulge luminosity vs. black hole mass relation is not significantly different from the low-redshift regime, assuming only passive evolution of the host galaxy. Corresponding Eddington luminosities are L nuc /L Edd ∼ 0.1-0.6.