Imaging Redshift Estimates for BL Lacertae Objects

We have obtained high dynamic range, good natural seeing i' images of BL Lacertae objects (BL Lacs) to search for the active galactic nucleus host and thus constrain the source redshift. These objects are drawn from a sample of bright flat-spectrum radio sources that are either known (via recent Fermi Large Area Telescope observations) gamma-ray emitters or similar sources that might be detected in continuing gamma-ray observations. All had spectroscopic confirmation as BL Lac sources, but no redshift solution. We detected hosts for 25/49 objects. As these galaxies have been argued to be standard candles, our measured host magnitudes provide redshift estimates (ranging from 0.2 to 1.0). Lower bounds are established on the redshifts of non-detections. The mean of the fit redshifts (and lower limits) is higher than those of spectroscopic solutions in the radio-loud and gamma-ray-loud parent samples, suggesting corrections may be needed for the luminosity function and evolution of these sources.