The Compton-thick AGN Population and the $N_{\mathrm{H}}$ 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 ($N_{\mathrm{H}}$) distribution of AGN in our cosmic backyard using a mid-infrared selected AGN sample within 15 Mpc. The column densities are measured from broadband X-ray spectral analysis, mainly using data from $\text{Chandra}$ and $\text{NuSTAR}$. Our sample probes AGN with intrinsic 2-10 keV luminosities of $L_{2-10,\mathrm{int}}=10^{37}$-$10^{43}$ erg s${}^{-1}$, reaching a parameter space inaccessible to more distant samples. We directly measure a 32${}_{-18}^{+30}\%$ CT AGN fraction and obtain an $N_{\mathrm{H}}$ distribution that agrees with that inferred by the $\text{Swift}$-BAT survey. Restricting the sample to the largely unexplored domain of low-luminosity AGN with $L_{2-10,\mathrm{int}}$ $\le$ $10^{42}$ erg s${}^{-1}$, we found a CT fraction of 19${}_{-14}^{+30}\%$, 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 $\approx 0.3$ dex). In contrast, the two samples have very different black hole mass ($M_{\mathrm{BH}}$) distributions, with our sample having $\approx$1.5 dex lower mean mass ($M_{\mathrm{BH}}$ $\sim$ 10${}^6$ $M_{\odot }$). Additionally, our sample contains a significantly higher number of LINERs and H${}_{\mathrm{II}}$-type nuclei. The Eddington ratio range probed by our sample, however, is the same as $\text{Swift}$-BAT, although the latter dominates at higher accretion rates, and our sample is more evenly distributed. The majority of our sample with $\hat{I}{»}_{\mathrm{Edd}}\ge$ 10${}^{-3}$ tend to be CT, while those with $\hat{I}{»}_{\mathrm{Edd}}<$ 10${}^{-3}$ are mostly unobscured or mildly obscured.

22 pages, 18 figures, 3 tables. Accepted for publication in MNRAS on 5 June 2025