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Mid-infrared luminous quasars in the GOODS-Herschel fields: A large population of heavily obscured, Compton-thick quasars at z ≈ 2

Lookup NU author(s): Professor Chris HarrisonORCiD

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Abstract

© 2015 The Authors. Published by Oxford University Press. We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL6μm ≥ 6 × 1044 erg s-1) at redshift z ≈ 1-3, identified through detailed spectral energy distribution analyses of distant star-forming galaxies, using the deepest IR data from Spitzer and Herschel in the GOODS-Herschel fields. The aim is to constrain the fraction of obscured, and Compton-thick (CT, NH >1.5×1024 cm-2) quasars at the peak era of nuclear and star formation activities. Despite being very bright in the mid-IR band, ≈30 per cent of these quasars are not detected in the extremely deep 2 and 4Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (≈67 per cent) are obscured by column densities NH >1022 cm-2; this fraction reaches≈80 per cent when including the X-ray-undetected sources (9 out of 33), which are likely to be the most heavily obscured, CT quasars. We constrain the fraction of CT quasars in our sample to be ≈24-48 per cent, and their space density to be Φ = (6.7 ± 2.2) × 10-6 Mpc-3. From the investigation of the quasar host galaxies in terms of star formation rates (SFRs) and morphological distortions, as a sign of galaxy mergers/interactions, we do not find any direct relation between SFRs and quasar luminosity or X-ray obscuration. On the other hand, there is tentative evidence that the most heavily obscured quasars have, on average, more disturbed morphologies than the unobscured/ moderately obscured quasar hosts, which preferentially live in undisturbed systems. However, the fraction of quasars with disturbed morphology amongst the whole sample is ≈40 per cent, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z ≈ 2.

Publication metadata

Author(s): Del Moro A, Alexander DM, Bauer FE, Daddi E, Kocevski DD, McIntosh DH, Stanley F, Brandt WN, Elbaz D, Harrison CM, Luo B, Mullaney JR, Xue YQ

Publication type: Article

Publication status: Published

Journal: Monthly Notices of the Royal Astronomical Society

Year: 2016

Volume: 456

Issue: 2

Pages: 2105-2125

Print publication date: 21/02/2016

Online publication date: 30/12/2015

Acceptance date: 19/11/2015

ISSN (print): 0035-8711

ISSN (electronic): 1365-2966

Publisher: Oxford University Press

URL: https://doi.org/10.1093/mnras/stv2748

DOI: 10.1093/mnras/stv2748

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