The Cosmic Evolution of Fermi BL Lacertae Objects
Fermi has provided the largest sample of γ-ray-selected blazars to date. In this work we use a uniformly selected set of 211 BL Lacertae (BL Lac) objects detected by Fermi during its first year of operation. We obtained redshift constraints for 206 out of the 211 BL Lac objects in our sample, making it the largest and most complete sample of BL Lac objects available in the literature. We use this sample to determine the luminosity function of BL Lac objects and its evolution with cosmic time. We find that for most BL Lac classes the evolution is positive, with a space density peaking at modest redshift (z ≈ 1.2). Low-luminosity, high-synchrotron-peaked (HSP) BL Lac objects are an exception, showing strong negative evolution, with number density increasing for z ≾ 0.5. Since this rise corresponds to a drop-off in the density of flat-spectrum radio quasars (FSRQs), a possible interpretation is that these HSPs represent an accretion-starved end state of an earlier merger-driven gas-rich phase. We additionally find that the known BL Lac correlation between luminosity and photon spectral index persists after correction for the substantial observational selection effects with implications for the so-called "blazar sequence." Finally, by estimating the beaming corrections to the luminosity function, we find that BL Lac objects have an average Lorentz factor of y = 6.1^(+1.1)_(-0.8), and that most are seen within 10° of the jet axis.
Additional Information© 2014 American Astronomical Society. Received 2013 June 3; accepted 2013 September 26; published 2013 December 13. The comments from an expert referee are gratefully acknowledged. M.A. acknowledges generous support from NASA grant NNH09ZDA001N for the study of the origin of the extragalactic gamma-ray background, and hospitality at Goddard and NRL while writing part of this work. R.W.R acknowledges NASA grants NNX08AW30G and NNX11A044G and extensive consultation with the OVRO Fermi group. D.G. acknowledges financial contribution from the agreement ASI-INAF I/009/10/0. M.A. is indebted to J. Wall for originally sharing the details of the ML method used in this and other works. The Fermi-LAT Collaboration acknowledges generous ongoing support from a number of agencies and institutes that have supported both the development and the operation of the LAT as well as scientific data analysis. These include the National Aeronautics and Space Administration and the Department of Energy in the United States; the Commissariat à l'Energie Atomique and the Centre National de la Recherche Scientifique/Institut National de Physique Nucléaire et de Physique des Particules in France; the Agenzia Spaziale Italiana and the Istituto Nazionale di Fisica Nucleare in Italy; the Ministry of Education, Culture, Sports, Science and Technology (MEXT), High Energy Accelerator Research Organization (KEK), and Japan Aerospace Exploration Agency (JAXA) in Japan; and the K. A. Wallenberg Foundation, the Swedish Research Council, and the Swedish National Space Board in Sweden. Additional support for science analysis during the operations phase is gratefully acknowledged from the Istituto Nazionale di Astrofisica in Italy and the Centre National d'Études Spatiales in France. Facility: Fermi (LAT)
Published - 0004-637X_780_1_73.pdf
Submitted - 1310.0006v1.pdf