The HST Survey of BL~Lacertae Objects. IV. Infrared Imaging of Host Galaxies

Abstract

The HST NICMOS Camera 2 was used for H-band imaging of 12 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band (Urry et al. 2000; Scarpa et al. 2000). Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, bright ellipticals with average H-band absolute magnitude M=-26.2+-0.45 mag and effective radius 10+-5 kpc. The rest-frame integrated color of the host galaxies is on average R-H=2.3+-0.3, consistent with the value for both radio galaxies and normal, non-active elliptical galaxies, and indicating the dominant stellar population is old. The host galaxies tend to be bluer in their outer regions than in their cores, with average color gradient Delta(R-H)/Delta(log r)=-0.2 mag, again consistent with results for normal non-active elliptical galaxies. The infrared Kormendy relation, derived for the first time for BL Lac host galaxies, is m(e) = 3.8*log(R)+14.8 (where m(e) is the surface brightness at the effective radius R), fully in agreement with the relation for normal ellipticals. The close similarity between BL Lac host galaxies and normal ellipticals suggests the active nucleus has surprisingly little effect on the host galaxy. This supports a picture in which all elliptical galaxies harbor black holes which can be actively accreting for some fraction of their lifetime.