Skip to main content

Research Repository

Advanced Search

Reliable identification of Compton-thick quasars at z~2 : Spitzer mid-infrared spectroscopy of HDF-oMD49

Alexander, D.M.; Chary, R.R.; Pope, A.; Bauer, F.E.; Brandt, W.N.; Daddi, E.; Dickinson, M.; Elbaz, D.; Reddy, N.A.

Reliable identification of Compton-thick quasars at z~2 : Spitzer mid-infrared spectroscopy of HDF-oMD49 Thumbnail


Authors

R.R. Chary

A. Pope

F.E. Bauer

W.N. Brandt

E. Daddi

M. Dickinson

D. Elbaz

N.A. Reddy



Abstract

Many models that seek to explain the origin of the unresolved X-ray background predict that Compton-thick active galactic nuclei (AGNs) are ubiquitous at high redshift. However, few distant Compton-thick AGNs have been reliably identified to date. Here we present Spitzer IRS spectroscopy and 3.6-70 μm photometry of a z=2.211 optically identified AGN (HDF-oMD49) that is formally undetected in the 2 Ms Chandra Deep Field-North (CDF-N) survey. The Spitzer IRS spectrum and spectral energy distribution of this object is AGN dominated, and a comparison of the energetics at X-ray wavelengths to those derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN is intrinsically luminous (L2-10keV~3×1044 ergs s-1) but heavily absorbed by Compton-thick material (NH>>1024 cm-2) i.e., this object is a Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49, we found a further six objects at z~2-2.5 in the literature that are also X-ray weak/undetected but have evidence for AGN activity from optical and/or mid-IR spectroscopy, and show that all of these sources are likely to be Compton-thick quasars with L2-10keV>1044 ergs s-1. On the basis of the definition of Daddi et al., these Compton-thick quasars would be classified as mid-IR excess galaxies, and our study provides the first spectroscopic confirmation of Compton-thick AGN activity in a subsample of these z~2 mid-IR-bright galaxies. Using the four objects that lie in the CDF-N field, we estimate the space density of reliably identified Compton-thick quasars [Φ~(0.7-2.5)×10-5 Mpc-3 for L2-10keV>1044 ergs s-1 objects at z~2-2.5] and show that Compton-thick accretion was probably as ubiquitous as unobscured accretion in the distant universe.

Citation

Alexander, D., Chary, R., Pope, A., Bauer, F., Brandt, W., Daddi, E., …Reddy, N. (2008). Reliable identification of Compton-thick quasars at z~2 : Spitzer mid-infrared spectroscopy of HDF-oMD49. Astrophysical Journal, 687(2), 835-847. https://doi.org/10.1086/591928

Journal Article Type Article
Publication Date Jan 1, 2008
Deposit Date Jan 30, 2012
Publicly Available Date Mar 28, 2024
Journal Astrophysical Journal
Print ISSN 0004-637X
Electronic ISSN 1538-4357
Publisher American Astronomical Society
Peer Reviewed Peer Reviewed
Volume 687
Issue 2
Pages 835-847
DOI https://doi.org/10.1086/591928
Keywords Galaxies, Active galaxies, High-redshift, Infrared, Ultraviolet, X-rays.

Files

Published Journal Article (618 Kb)
PDF

Copyright Statement
© 2008. The American Astronomical Society. All rights reserved. Printed in the U.S.A.





You might also like



Downloadable Citations