Constraining the Population of Compton-Thick AGN and N_H Distribution in the Local Universe

Abstract

Compton-thick active galactic nuclei (CTAGN) are defined as AGN that are obscured along our line-of-sight by absorbing gas with a column density of N_H ≳ 10^(24) cm^(-2). This is what makes it difficult to identify them at X-ray energies E <; 10 keV, where the direct emission from the AGN is severely absorbed by the obscuring gas. Because of this, a large fraction of the CTAGN population is believed to be missed by current low X-ray energy observations. In this paper, we present updated results from an ongoing project to form the most unbiased census of CTAGN population in the local universe, using a sample of mid-infrared (mid-IR) selected AGN within D <; 15 Mpc. We find that 20% of the AGN in the sample has been confirmed to be CT from current hard X-ray studies (E ≳ 10 keV). More candidates are then selected using multiwavelength techniques, i.e. optical [OIII]λ5007:X-ray and mid-IR:X-ray flux ratios. Based on these analyses, we initially find a further 25% of potential candidates. NuSTAR (Nuclear Spectroscopic Telescope Array) recently observed two of these candidates at high X-ray energies and is able to confirm one of them, NGC 5643, as a CTAGN and rule out the other one as an obscured AGN. This constrains the total CTAGN population in the sample to 25-40%, though it could also be as high as 65% accounting for those that we still lack data. Finally, we use these results to estimate the intrinsic Nu distribution of the local AGN population.

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