The stellar content of low redshift BL Lacertae host galaxies from multicolour imaging

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 ( mu(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/ near infrared colour ( with average B -H = 3.5 +/- 0.5) and colour gradient ( with average Delta (B-R)/Delta(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.