The NuSTAR Local AGN NH Distribution Survey (NuLANDS) I: Towards a Truly Representative Column Density Distribution in the Local Universe

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 NH > 1024 cm−2. To tackle this issue, we present the NuSTAR Local AGN NH Distribution Survey (NuLANDS)—a legacy sample of 122 nearby (z < 0.044) AGN primarily selected to have warm infrared colours from IRAS between 25 – 60 μm. 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 a large library of X-ray spectroscopic models, we show that the inferred line-of-sight column density and intrinsic luminosity vary depending on the adopted obscurer geometry. To circumvent such issues we propagate the uncertainties per source into the parent NuLANDS column density distribution, finding a directly measured Compton-thick fraction of 35 ± 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 optimised sample for future follow-up with current and next-generation instruments aiming to study the local AGN population in an isotropic manner.