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

TL;DR

This study identifies a significant population of heavily-obscured, Compton-thick quasars at z~2 using deep IR and X-ray data, revealing their properties, obscuration levels, and host galaxy characteristics, and suggesting mergers are not the primary fueling mechanism.

Contribution

It provides the first robust estimate of the fraction and space density of Compton-thick quasars at z~2 using combined IR and X-ray data, and analyzes their host galaxy morphologies and star-formation activity.

Findings

Approximately 24-48% of the quasars are Compton-thick.

Most detected quasars are obscured by N$_H$>10$^{22}$ cm$^{-2}$.

Heavily-obscured quasars tend to have more disturbed host galaxy morphologies.

Abstract

We present the infrared (IR) and X-ray properties of a sample of 33 mid-IR luminous quasars ($\nu$L(6 micron)>6x10$^{44}$ erg/s) 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, N$_H$>1.5x10$^{24}$ cm$^{-2}$) quasars at the peak era of nuclear and star-formation activities. Despite being very bright in the mid-IR band, ~30% of these quasars are not detected in the extremely deep 2 Ms and 4 Ms Chandra X-ray data available in these fields. X-ray spectral analysis of the detected sources reveals that the majority (~67%) are obscured by column densities N$_H$>10$^{22}$ cm$^{-2}$; this fraction reaches ~80% 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%, and their space density to be $\Phi$=(6.7$\pm$2.2)x10$^{-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%, suggesting that galaxy mergers are not the main fuelling mechanism of quasars at z~2.