Revealing X-ray obscured quasars in SWIRE sources with extreme mid-IR/optical flux ratios

TL;DR

This study identifies a large population of high-redshift, obscured quasars using mid-IR/optical flux ratios, revealing their X-ray properties and suggesting many are heavily obscured, including potential Compton-thick quasars.

Contribution

It demonstrates that mid-IR/optical flux ratio selection effectively finds obscured quasars at z>1, with detailed X-ray spectral analysis confirming their nature.

Findings

95% of detected sources are absorbed with NH>10^22 cm^-2

50% of detected sources are classified as Type 2 quasars

Many undetected sources likely host heavily obscured, possibly Compton-thick, quasars

Abstract

Recent works have suggested that selection criteria based on MIR colors can be used to reveal a population of dust-enshrouded, extremely luminous quasars at z>1. However the X-ray spectral properties of these intriguing sources still remain largely unexplored. We report on an X-ray spectroscopic study of a sample of 44 very bright mid-IR galaxies with extreme mid-IR to optical flux ratios (MIR/O>2000). The X-ray coverage of the sample is highly inhomogeneous (from snap-shot 5 ks Chandra observations to medium-deep XMM exposures of 70 ks) and, consequently, a sizable fraction of them (~43%) remains undetected in the 0.5-10 keV band. The vast majority (95%) of the detected sources (23) show an absorption column density NH>10e22 cm-2 and, remarkably, we also find that 50% of them can be classified as Type 2 quasars on the basis of their absorption properties and X-ray luminosity. Moreover, most of the X-ray undetected sources show extreme mid-IR colors, consistent with being luminous AGN-powered objects, suggesting they might host heavily obscured (possibly Compton-thick) quasars in X-rays. This demonstrates that our selection criteria applied to a wide area survey is very efficient in finding a large number of Type 2 quasars at z > 1. The existence of this class of very powerful, obscured quasars at high z could have important implications in the context of the formation and cosmological evolution of accreting supermassive black holes and their host galaxies.