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 absolute magnitude \langle M _ { H } \rangle = -26.2 \pm 0.45 mag and effective radius \langle r _ { e } \rangle = 10 \pm 5 kpc .
The rest-frame integrated color of the host galaxies is on average \langle R-H \rangle = 2.3 \pm 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 \mu _ { e } = 3.8 \log r _ { e } +14.8 , 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 .

Subject Headings : BL Lacertae objects — galaxies : structure — galaxies : elliptical