We present the results of near–infrared H band ( 1.65 \mu m ) imaging of 11 BL Lac objects with redshifts ranging from z = 0.05 to 0.9 .
We are able to clearly detect the host galaxy in seven low redshift ( z \leq 0.24 ) BL Lacs , while the four unresolved BL Lacs have either high or unknown redshift .
The galaxies hosting the low redshift BL Lacs are large ( average bulge scale length R ( e ) = 8.8 \pm 9.9 kpc ) and luminous ( average M ( H ) = –25.8 \pm 0.5 ) , i.e . slightly brighter than the typical galaxy luminosity L* ( M* ( H ) = –25.0 \pm 0.2 ) , and of similar luminosity to or slightly fainter than brightest cluster galaxies ( M ( H ) = –26.3 \pm 0.3 ) .
The average optical/near–infrared colour and colour gradient of the BL Lac hosts ( R–H = 2.2 \pm 0.5 ; \Delta ( R–H ) / \Delta ( log r ) = –0.09 \pm 0.04 ) are consistent with the hosts being normal ellipticals , indicating that the nuclear activity has only a marginal effect on the star formation history and other properties of the hosts .
The BL Lac hosts appear slightly less luminous than those of higher redshift flat spectrum radio quasars .
The nucleus–to–galaxy luminosity ratio of the BL Lacs is similar to that of low redshift radio galaxies and consistent with what found in previous optical studies of BL Lacs .
However , it is smaller that that found for flat spectrum radio quasars , suggesting there is a difference in the intrinsic brightness of the nuclear source or in the Doppler beaming factor between the two types of blazars .