We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars (νL6 μm ≥ 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 4 Ms 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.

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Published in Monthly Notices of the Royal Astronomical Society, v. 456, no. 2, p. 2105-2125.

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

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We gratefully acknowledge financial support from the Science and Technology Facilities Council (STFC, ST/L00075X/1; ADM, DMA and CMH) and the Leverhulme Trust (DMA). FEB acknowledges support from CONICYT-Chile (Basal-CATA PFB-06/2007, FONDECYT 1141218, Gemini-CONICYT 32120003, ‘EMBIGGEN’ Anillo ACT1101), and the Ministry of Economy, Development, and Tourism’s Millennium Science Initiative through grant IC120009, awarded to The Millennium Institute of Astrophysics, MAS. WNB acknowledges the Chandra X-ray Center grant G04-15130A and NASA ADP grant NNX10AC99G. YQX acknowledges support of the Thousand Young Talents program (KJ2030220004), the 973 Program (2015CB857004), the USTC startup funding (ZC9850290195), the NSFC-11473026, NSFC-11421303, the Strategic Priority Research Program ‘The Emergence of Cosmological Structures’ of the Chinese Academy of Sciences (XDB09000000) and the Fundamental Research Funds for the Central Universities (WK3440000001).