2004A&A...424..107K

We present H-band (1.65µm) imaging of 23 low redshift (z≤0.3) BL Lac objects that were previously investigated by us in the optical R-band. For all the observed objects, the host galaxy is clearly detected and well represented by an elliptical model, with average scale length R(e)=7.2±3.6kpc and average absolute magnitude M(H)=-25.9±0.6. BL Lac hosts are therefore luminous (massive) elliptical galaxies indistinguishable from those of low redshift radio galaxies and inactive ellipticals. The best-fit H-band Kormendy relation of µ_e=3.7 log r_e(kpc)+14.8mag/arcsec² is in 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 new observations are combined with literature data (Kotilainen et al., 1998A&A...336..479K; Scarpa et al., 2000ApJ...544..258S; Cheung et al., 2003ApJ...599..155C) of 18 objects in order to construct a sizeable dataset of 41 BL Lacs with determined host characteristics in both optical and NIR. This extended sample is used to investigate the optical-NIR colour and colour gradient properties of the host galaxies of BL Lacs and to perform a direct comparison with other elliptical galaxies with and without nuclear activity. The integrated optical/near-infrared colour (R-H=2.2±0.4) and colour gradient (Δ(R-H)/Δ(logr)=-0.38±0.60) distributions of the BL Lac hosts are much wider than those for normal ellipticals with old stellar populations, and many BL Lacs have bluer hosts and/or steeper colour gradients than those in normal ellipticals. The blue colours are most likely caused by a young stellar population, and indicates a link between star formation caused by an interaction/merging event and the onset of the nuclear activity. However, the lack of obvious signs of interaction may require a significant time delay between the event with associated SF episodes and the start of the nuclear activity.