The Compton-thick AGN population and the NH distribution of low-mass AGN in our cosmic backyard

Abstract

We present a census of the Compton-thick (CT) active galactic nucleus (AGN) population and the column density (N_H)
distribution of AGN in our cosmic backyard using a mid-infrared selected AGN sample within 15 Mpc. The column densities
are measured from broad-band X-ray spectral analysis, mainly using data from Chandra and NuSTAR. Our sample probes AGN
with intrinsic 2–10 keV luminosities of L_2−10,int = 10³⁷-10⁴³ erg s⁻¹, reaching a parameter space inaccessible to more distant
samples. We directly measure a 32 +30−18 per cent CT AGN fraction and obtain an N_H distribution that agrees with that inferred by the Swift-BAT survey. Restricting the sample to the largely unexplored domain of low-luminosity AGN with L_2−10,int ≤ 10⁴² erg s⁻¹, we found a CT fraction of 19+30−14 per cent, consistent with those observed at higher luminosities. Comparing the host-galaxy properties between the two samples, we find consistent star formation rates, though the majority of our galaxy have lower stellar masses (by ≈ 0.3 dex). In contrast, the two samples have very different black hole mass (M_BH) distributions, with our sample having ≈1.5 dex lower mean mass (M_BH ∼ 10⁶ M_®). Additionally, our sample contains a significantly higher number of LINERs and HII-type nuclei. The Eddington ratio range probed by our sample, however, is the same as Swift-BAT, although the latter dominates at higher accretion rates, and our sample is more evenly distributed. The majority of our sample with λ_Edd ≥ 10⁻³ tend to be CT, while those with λ_Edd < 10⁻³ are mostly unobscured or mildly obscured.

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Identifiers

ORCID for P. Gandhi: orcid.org/0000-0003-3105-2615

ORCID for P.G. Boorman: orcid.org/0000-0001-9379-4716

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