Spectroscopy of the Largest Ever γ-Ray-selected BL Lac Sample

Creators: Shaw, Michael S.; Romani, Roger W.; Cotter, Garret; Healey, Stephen E.; Michelson, Peter F.; Readhead, Anthony C. S.; Richards, Joseph L.; Max-Moerbeck, Walter; King, Oliver G.; Potter, William J.

Abstract

We report on spectroscopic observations covering most of the 475 BL Lacs in the second Fermi Large Area Telescope (LAT) catalog of active galactic nuclei (AGNs). 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_• ~ 10^(8.5) – 10^9, substantially larger than the mean of optical AGNs and LAT Flat Spectrum Radio Quasar samples. A slow increase in M_• 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 γ-ray properties, suggesting that strong beaming is the primary cause of the range in continuum dominance.

Additional Information

© 2013 American Astronomical Society. Received 2012 October 29; accepted 2012 December 28; published 2013 January 31. We thank the referee for a careful reading and comments that helped us add materially to the paper. The Hobby–Eberly Telescope (HET) is a joint project of the University of Texas at Austin, the Pennsylvania State University, Stanford University, Ludwig-Maximilians-Universitaet Muenchen, and Georg-August-Universitaet Goettingen. The HET is named in honor of its principal benefactors, William P. Hobby and Robert E. Eberly. The Marcario Low Resolution Spectrograph is named for Mike Marcario of High Lonesome Optics, who fabricated several optics for the instrument but died before its completion. The LRS is a joint project of the Hobby–Eberly Telescope partnership and the Instituto de Astronoma de la Universidad Nacional Autonoma de Mexico. Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. The authors recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. This work also employs observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da Repύblica Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU). Additional observations were made with ESO Telescopes at the La Silla Paranal Observatory under programme 077.B-0056 078.B-0275 079.B-0831 083.B-0460 084.B-0711 087.A-0573. G.C. acknowledges support from STFC grant ST/H002456/1. We acknowledge support from NASA grants NNX09AW30G, NXX10AU09G, and NAS5-00147. A.C.S.R. is also supported under grant AST-0808050. Facilities: Fermi, Hale (DBSP), HET, Keck:I (LRIS), NTT, VLT:Antu (FORS2).

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