New insights on the distant AGN population

TL;DR

This study identifies a significant population of heavily obscured quasars at redshift 1-3 using infrared SED fitting, revealing many are missed in X-ray surveys and highlighting their role in black hole growth and the X-ray background.

Contribution

Introduces a new infrared SED fitting technique to uncover obscured quasars, revealing a higher fraction of heavily obscured AGNs than previously detected in X-ray surveys.

Findings

Approximately 25% of IR quasars are unabsorbed.

Most sources show moderate-to-high absorption (N_H>10^23 cm^-2).

Around 25% are likely Compton thick (N_H>10^24 cm^-2).

Abstract

Current X-ray surveys have proved to be essential tools in order to identify and study AGNs across cosmic time. However, there is evidence that the most heavily obscured AGNs are largely missing even in the deepest surveys. The search for these obscured AGNs is one of the most outstanding issues of extragalactic astronomy, since they are expected to make a major contribution to the high energy peak of the X-ray background (XRB) and might constitute a particularly active and dusty phase of black hole and galaxy evolution. Using a newly developed SED fitting technique to decompose the AGN from the star-formation emission in the infrared band we identify a sample of 17 IR bright quasars at z=1-3. For the majority of these sources the X-ray spectrum is well characterised by an absorbed power law model, revealing that ~25% of the sources are unabsorbed; the remainder of these IR quasars have moderate-to-high absorption (N_H>10^23 cm^-2), and ~25% are not detected in the X-rays and are likely to be Compton thick (N_H>10^24 cm^-2). We therefore find a much higher fraction of obscured than unobscured quasars, indicating that a large fraction of the luminous black hole accretion was very heavily obscured at z~2.