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Published October 21, 2018 | Published + Supplemental Material + Accepted Version
Journal Article Open

The Chandra COSMOS Legacy Survey: Compton thick AGN at high redshift

Abstract

The existence of a large population of Compton thick (CT; N_H > 10^(24) cm^(−2)) active galactic nuclei (AGN) is a key ingredient of most cosmic X-ray background synthesis models. However, direct identification of these sources, especially at high redshift, is difficult due to flux suppression and complex spectral shape produced by CT obscuration. We explored the Chandra Cosmological Evolution Survey (COSMOS) Legacy point source catalogue, comprising 1855 sources to select, via X-ray spectroscopy, a large sample of CT candidates at high redshift. Adopting a physical model to reproduce the toroidal absorber and a Monte-Carlo sampling method, we selected 67 individual sources with >5 per cent probability of being CT, in the redshift range 0.04 ≲ z ≲ 3.5. The sum of the probabilities above NH > 10^(24) cm^(−2) gives a total of 41.9 effective CT, corrected for classification bias. We derive number counts in the 2–10 keV band in three redshift bins. The observed log N–log S is consistent with an increase of the intrinsic CT fraction (fCT) from ∼0.30 to ∼0.55 from low to high redshift. When rescaled to a common luminosity (log(L_X/ erg s^(−1)) = 44.5), we find an increase from f_(CT) = 0.19^(+0.07)_(−0.06) to 0.30^(+0.10)_(−0.08) and f_(CT) = 0.49^(+0.12)_(−0.11) from low to high z⁠. This evolution can be parametrized as f_(CT) = 0.11^(+0.05)_(−0.04) (1+z)^(1.11±0.13)⁠. Thanks to Hubble Space Telescope-Advanced Camera for Surveys deep imaging, we find that the fraction of CT AGN in mergers/interacting systems increases with luminosity and redshift and is significantly higher than for non-CT AGN hosts.

Additional Information

© 2018 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model) Accepted 2018 July 25. Received 2018 July 25; in original form 2018 March 22. The authors thank the referee for the constructive comments and recommendations that helped to improve the readability and quality of the paper. This research has made use of data obtained from the Chandra Data Archive and software provided by the Chandra X-ray Center (CXC) in the CIAO application package. The authors acknowledge financial support from the CIG grant 'eEASY' no. 321913, from ASI-INAF grant no. 2014-045-R.0, and from PRIN-INAF-2014 ('Windy Black Holes Combing Galaxy Evolution').

Attached Files

Published - sty2025.pdf

Accepted Version - 1803.08547.pdf

Supplemental Material - sty2025_supplemental_figure_d1.pdf

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Created:
August 19, 2023
Modified:
October 19, 2023