BL Lac object

Host galaxies

Extensive surveys taken with the Hubble Space Telescope of 132 BL Lac objects comprising seven complete radio, X-ray, and optically selected samples in 2000 studied the morphologies of possible BL Lac host galaxies. The data concluded that in two-thirds of the BLL images taken, host galaxies are detected, including in nearly all with redshift z < 0.5. BL Lac objects are luminous enough that only one quarter (6/22) of the images taken with z > 0.5 were resolved because of relatively short exposure times. A de Vaucouleurs profile ${\displaystyle I(R)\propto R^{\frac {1}{4}}}$ looks to be a significantly preferred brightness profile for 58 of the 72 resolved host galaxies at over ~99% confidence. The results of this survey conclude that there is an 8% limit to the number of disk systems in BL Lac objects and is therefore consistent with the assumption that all BL Lac host galaxies could be elliptical. These ellipticals are very luminous with a median absolute K-corrected magnitude of ${\displaystyle M_{R}\thicksim -23.7\pm -0.6}$ mag (rms dispersion). This is comparable to the brightest cluster galaxies.