Spectroscopy of The Largest Ever Gamma-ray Selected BL Lac Sample

TL;DR

This study provides extensive spectroscopic redshifts for a large sample of BL Lac objects, revealing their distribution, host galaxy properties, and implications for their evolution and gamma-ray emission characteristics.

Contribution

It offers the largest spectroscopic redshift catalog for gamma-ray selected BL Lacs, improving redshift completeness and analyzing host galaxy and black hole mass properties.

Findings

Redshifts now known for 44% of the sample

Median redshift increased from 0.23 to 0.33

Detected hosts are bright ellipticals with large black hole masses

Abstract

We report on spectroscopic observations covering most of the 475 BL Lacs in the 2nd Fermi LAT catalog of AGN. Including archival measurements (correcting several erroneous literature values) we now have spectroscopic redshifts for 44% of the BL Lacs. We establish firm lower redshift limits via intervening absorption systems and statistical lower limits via searches for host galaxies for an additional 51% of the sample leaving only 5% of the BL Lacs unconstrained. The new redshifts raise the median spectroscopic z from 0.23 to 0.33 and include redshifts as large as z=2.471. Spectroscopic redshift minima from intervening absorbers have ~ z= 0.70, showing a substantial fraction at large z and arguing against strong negative evolution. We find that detected BL Lac hosts are bright ellipticals with black hole masses M_\bullet ~ 10^{8.5-9}, substantially larger than the mean of optical AGN and LAT Flat Spectrum Radio Quasar samples. A slow increase in M_\bullet with z may be due to selection bias. We find that the power-law dominance of the optical spectrum extends to extreme values, but this does not strongly correlate with the gamma-ray properties, suggesting that strong beaming is the primary cause of the range in continuum dominance.