S. Juneau
A New Diagnostic of Active Galactic Nuclei: Revealing Highly Absorbed Systems at Redshift >0.3
Juneau, S.; Dickinson, M.; Alexander, D.M.; Salim, S.
Abstract
We introduce the Mass-Excitation (MEx) diagnostic to identify active galactic nuclei (AGNs) in galaxies at intermediate redshift. In the absence of near-infrared spectroscopy, necessary for using traditional nebular line diagrams at z > 0.4, we demonstrate that combining [O III] λ5007/Hβ and stellar mass successfully distinguishes between star formation and AGN emission. The MEx classification scheme relies on a novel probabilistic approach splitting galaxies into sub-categories with more confidence than alternative high-z diagnostic diagrams. It recognizes that galaxies near empirical boundaries on traditional diagrams have an uncertain classification and thus a non-zero probability of belonging to more than one category. An outcome of this work is a system of statistical weights that can be used to compute global properties of galaxy samples. We apply the MEx diagram to 2812 galaxies at 0.3 < z < 1 in the Great Observatories Origins Deep Survey North and Extended Groth Strip fields, and compare it to an independent X-ray classification scheme. We identify Compton-thick AGN candidates with large X-ray absorption, which we infer from the luminosity ratio between hard X-ray emission and [O III] λ5007, a nearly isotropic tracer of AGNs. X-ray stacking of sources that were not detected individually supports the validity of the MEx diagram and yields a very flat spectral slope for the Compton-thick candidates (Γ ≈ 0.4 unambiguously indicating absorbed AGNs). We present evidence that composite galaxies, which are difficult to identify with alternative high-redshift diagrams, host the majority of the highly absorbed AGNs. Our findings suggest that the interstellar medium of the host galaxy provides significant absorption in addition to the torus invoked in AGN unified models.
Citation
Juneau, S., Dickinson, M., Alexander, D., & Salim, S. (2011). A New Diagnostic of Active Galactic Nuclei: Revealing Highly Absorbed Systems at Redshift >0.3. Astrophysical Journal, 736(2), Article 104. https://doi.org/10.1088/0004-637x/736/2/104
Journal Article Type | Article |
---|---|
Publication Date | Aug 1, 2011 |
Deposit Date | Apr 8, 2013 |
Publicly Available Date | Jan 3, 2014 |
Journal | Astrophysical Journal |
Print ISSN | 0004-637X |
Electronic ISSN | 1538-4357 |
Publisher | American Astronomical Society |
Peer Reviewed | Peer Reviewed |
Volume | 736 |
Issue | 2 |
Article Number | 104 |
DOI | https://doi.org/10.1088/0004-637x/736/2/104 |
Keywords | Galaxies, Active, Evolution, Fundamental parameters, ISM, X-rays. |
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Copyright Statement
© 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
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