The Hubble Space Telescope survey of BL Lacertae objects. IV. Infrared imaging of host galaxies

The Hubble Space Telescope NICMOS Camera 2 was used for H-band imaging of 1 2 BL Lacertae objects taken from the larger sample observed with the WFPC2 in the R band by Urry and coworkers and Scarpa and coworkers. Ten of the 12 BL Lacs are clearly resolved, and the detected host galaxies are large, br ight ellipticals with average absolute magnitude <M-H> = -26.2 +/- 0.45 mag and effective radius <r(e)>= 10 +/- 5 kpc. The rest-frame integrated color of the host galaxies is on average <R-H >= 2.3 +/- 0.3, consistent with th e value for both radio galaxies and normal, nonactive elliptical galaxies a nd indicating that the dominant stellar population is old. The host galaxie s 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 r esults for normal nonactive elliptical galaxies. The infrared Kormendy rela tion, derived for the first time for BL Lac host galaxies, is mu (e) = 3.8 log r(e) + 14.8, fully in agreement with the relation for normal elliptical s. The close similarity between BL Lac host galaxies and normal ellipticals suggests that the active nucleus has surprisingly little effect on the hos t galaxy. This supports a picture in which all elliptical galaxies harbor b lack holes that can be actively accreting for some fraction of their lifeti me.