Mid-infrared luminous quasars in the GOODS–<i>Herschel</i>fields: a large population of heavily obscured, Compton-thick quasars at<i>z</i>≈ 2

Abstract

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR\nluminous quasars ($\\nu$L(6 micron)&gt;6x10$^{44}$ erg/s) at redshift z~1-3,\nidentified through detailed spectral energy distribution analyses of distant\nstar-forming galaxies, using the deepest IR data from Spitzer and Herschel in\nthe GOODS-Herschel fields. The aim is to constrain the fraction of obscured,\nand Compton-thick (CT, N$_H$&gt;1.5x10$^{24}$ cm$^{-2}$) quasars at the peak era\nof nuclear and star-formation activities. Despite being very bright in the\nmid-IR band, ~30% of these quasars are not detected in the extremely deep 2 Ms\nand 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis\nof the detected sources reveals that the majority (~67%) are obscured by column\ndensities N$_H$&gt;10$^{22}$ cm$^{-2}$; this fraction reaches ~80% when including\nthe X-ray undetected sources (9 out of 33), which are likely to be the most\nheavily-obscured, CT quasars. We constrain the fraction of CT quasars in our\nsample to be ~24-48%, and their space density to be\n$\\Phi$=(6.7$\\pm$2.2)x10$^{-6}$ Mpc$^{-3}$. From the investigation of the quasar\nhost galaxies in terms of star-formation rates (SFRs) and morphological\ndistortions, as a sign of galaxy mergers/interactions, we do not find any\ndirect relation between SFRs and quasar luminosity or X-ray obscuration. On the\nother hand, there is tentative evidence that the most heavily-obscured quasars\nhave, on average, more disturbed morphologies than the\nunobscured/moderately-obscured quasar hosts, which preferentially live in\nundisturbed systems. However, the fraction of quasars with disturbed morphology\namongst the whole sample is ~40%, suggesting that galaxy mergers are not the\nmain fuelling mechanism of quasars at z~2.\n