Kavli Affiliate: Claudio Ricci
| First 5 Authors: Peter G. Boorman, Poshak Gandhi, Johannes Buchner, Daniel Stern, Claudio Ricci
| Summary:
Hard X-ray-selected samples of Active Galactic Nuclei (AGN) provide one of
the cleanest views of supermassive black hole accretion, but are biased against
objects obscured by Compton-thick gas column densities of $N_{rm H}$ $>$
10$^{24}$ cm$^{-2}$. To tackle this issue, we present the NuSTAR Local AGN
$N_{rm H}$ Distribution Survey (NuLANDS)$-$a legacy sample of 122 nearby ($z$
$<$ 0.044) AGN primarily selected to have warm infrared colors from IRAS
between 25$-$60 $mu$m. We show that optically classified type 1 and 2 AGN in
NuLANDS are indistinguishable in terms of optical [OIII] line flux and
mid-to-far infrared AGN continuum bolometric indicators, as expected from an
isotropically selected AGN sample, while type 2 AGN are deficient in terms of
their observed hard X-ray flux. By testing many X-ray spectroscopic models, we
show the measured line-of-sight column density varies on average by $sim$ 1.4
orders of magnitude depending on the obscurer geometry. To circumvent such
issues we propagate the uncertainties per source into the parent column density
distribution, finding a directly measured Compton-thick fraction of 35 $pm$
9%. By construction, our sample will miss sources affected by severe
narrow-line reddening, and thus segregates sources dominated by small-scale
nuclear obscuration from large-scale host-galaxy obscuration. This bias implies
an even higher intrinsic obscured AGN fraction may be possible, although tests
for additional biases arising from our infrared selection find no strong
effects on the measured column-density distribution. NuLANDS thus holds
potential as an optimized sample for future follow-up with current and
next-generation instruments aiming to study the local AGN population in an
isotropic manner.
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