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The Multiwavelength AGN Population and the X-ray Background

Published online by Cambridge University Press:  25 July 2014

Ezequiel Treister ,
Claudia M. Urry ,
Kevin Schawinski ,
Brooke D. Simmons ,
Priyamvada Natarajan  and
Marta Volonteri
Ezequiel Treister
Affiliation:
Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile email: etreiste@astro-udec.cl
Claudia M. Urry
Affiliation:
YCAA & Dept. of Physics, Yale University, P.O. Box 208121, New Haven, CT 06520
Kevin Schawinski
Affiliation:
Institute for Astronomy, Department of Physics, ETH Zurich, Wolfgang-Pauli-Strasse 16, CH-8093 Zurich, Switzerland
Brooke D. Simmons
Affiliation:
Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH, UK.
Priyamvada Natarajan
Affiliation:
YCAA, Dept. of Astronomy & Dept. of Physics, Yale University, P.O. Box 208121, New Haven, CT 06520
Marta Volonteri
Affiliation:
Institut d' Astrophysique de Paris, 98bis Bd. Arago, 75014 Paris, France
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Abstract

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In order to fully understand galaxy formation we need to know when in the cosmic history are supermassive black holes (SMBHs) growing more intensively, in what type of galaxies this growth is happening and what fraction of these sources are invisible at most wavelengths due to obscuration. Active Galactic Nuclei (AGN) population synthesis models that can explain the spectral shape and intensity of the cosmic X-ray background (CXRB) indicate that most of the SMBH growth occurs in moderate-luminosity (LX~ 1044 erg/s) sources (Seyfert-type AGN), at z~ 0.5−1 and in heavily obscured but Compton-thin, NH~ 1023cm−2, systems. However, this is not the complete history, as a large fraction of black hole growth does not emit significantly in X-rays either due to obscuration, intrinsic low luminosities or large distances. The integrated intensity at high energies indicates that a significant fraction of the total black hole growth, 22%, occurs in heavily-obscured systems that are not individually detected in even the deepest X-ray observations. We further investigate the AGN triggering mechanism as a function of bolometric luminosity, finding evidence for a strong connection between significant black hole growth events and major galaxy mergers from z~ 0 to z~ 3, while less spectacular but longer accretion episodes are most likely due to other (stochastic) processes. AGN activity triggered by major galaxies is responsible for ~60% of the total black hole growth. Finally, we constrain the total accreted mass density in supermassive black holes at z > 6, inferred via the upper limit derived from the integrated X-ray emission from a sample of photometrically selected galaxy candidates. We estimate an accreted mass density <1000 M⊙Mpc−3 at z~ 6, significantly lower than the previous predictions from some existing models of early black hole growth and earlier prior observations.

Keywords

galaxies: activegalaxies: high-redshiftquasars: generalX-rays: diffuse background
Type
Contributed Papers
Information
Proceedings of the International Astronomical Union , Volume 9 , Symposium S304: Multiwavelength AGN Surveys and Studies , October 2013 , pp. 188 - 194
Copyright
Copyright © International Astronomical Union 2014 

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