Faint COSMOS AGN at z ~ 3.3 - I. Black Hole Properties and Constraints on Early Black Hole Growth

Creators: Trakhtenbrot, B.; Civano, F.; Urry, C. Megan; Schawinski, K.; Marchesi, S.; Elvis, M.; Rosario, D. J.; Suh, H.; Mejía-Restrepo, J.; Simmons, B. D.; Faisst, A. L.; Onodera, M.

Abstract

We present new Keck/MOSFIRE K-band spectroscopy for a sample of 14 faint, X-ray-selected active galactic nuclei (AGNs) in the COSMOS field. The data cover the spectral region surrounding the broad Balmer emission lines, which enables the estimation of black hole masses (M_(BH) and accretion rates (in terms of L/L_(Edd)). We focus on 10 AGNs at z ≃ 3.3, where we observe the Hβ spectral region, while for the other four z ≃ 2.4 sources we use the Hɑ broad emission line. Compared with previous detailed studies of unobscured AGNs at these high redshifts, our sources are fainter by an order of magnitude, corresponding to number densities of order ~10^(−6)^–10^(−5) Mpc^(-3). The lower AGN luminosities also allow for a robust identification of the host galaxy emission, necessary to obtain reliable intrinsic AGN luminosities, BH masses and accretion rates. We find the AGNs in our sample to be powered by supermassive black holes (SMBHs) with a typical mass of M_(BH) ≃ 5 x 10^8 M_⊙ —significantly lower than the higher-luminosity, rarer quasars reported in earlier studies. The accretion rates are in the range L/L_(Edd) ~ 0.1–0.4, with an evident lack of sources with lower L/L_(Edd) (and higher M_BH), as found in several studies of faint AGNs at intermediate redshifts. Based on the early growth expected for the SMBHs in our sample, we argue that a significant population of faint z ~ 5−6 AGNs, with M_(BH) ~ sim 10^6M_⊙, should be detectable in the deepest X-ray surveys available, but this is not observed. We discuss several possible explanations for the apparent absence of such a population, concluding that the most probable scenario involves an evolution in source obscuration and/or radiative efficiencies.

Additional Information

© 2016 American Astronomical Society. Received 2015 December 4; accepted 2016 April 16; published 2016 June 24. We thank the anonymous referee, whose numerous suggestions helped improve the paper. The new MOSFIRE data presented here 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. We are grateful for the support from Yale University that allows access to the Keck telescopes. We thank M. Kassis, L. Rizzi, and the rest of the staff at the W. M. Keck observatories at Waimea, HI, for their support during the observing runs. We 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. Some of the analysis presented here is based on data products from observations made with European Southern Observatory (ESO) Telescopes at the La Silla Paranal Observatory under ESO program ID 179.A-2005 and on data products produced by TERAPIX and the Cambridge Astronomy Survey Unit on behalf of the UltraVISTA consortium. This work made use of the MATLAB package for astronomy and astrophysics (Ofek 2014). We thank A. Weigel and N. Caplar for beneficial discussions. F.C. and C.M.U. gratefully thank Debra Fine for her support of women in science. This work was supported in part by NASA Chandra grant numbers GO3-14150C and GO3-14150B (F.C., S.M., H.S., M.E.). K.S. gratefully acknowledges support from Swiss National Science Foundation Grant PP00P2_138979/1. J.M. acknowledges support for his PhD by CONICYT-PCHA/doctorado Nacional para extranjeros, scholarship 2013-63130316. A.F. acknowledges support from the Swiss National Science Foundation.

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