Astronomy and Astrophysics – Astronomy
Compton-thick Active Galactic Nuclei (AGN), accreting supermassive black holes at the centers of galaxies surrounded by absorbing material with a column density (NH > 1024 cm-2), are common in the local universe. To determine an unbiased estimate of the fraction of heavily obscured AGN and to study their properties, we have analyzed two samples of Seyfert 2 galaxies (local AGN where the line of sight is through the obscuring "torus") selected based on intrinsic AGN flux proxies, which are to first-order unaffected by obscuration: an [OIII]-flux limited sample from the Sloan Digital Sky Survey and a mid-infrared selected sample from the original IRAS 12-micron survey. Subsequent 2-10 keV X-ray observations can then indicate the amount of obscuration present as this emission is subject to photoelectric absorption and possibly Compton scattering (for NH > 1024 cm-2) from the putative torus. Signatures of heavy absorption include X-ray attenuation when normalized by intrinsic AGN flux, including infrared parameters derived from detailed Spitzer spectral analysis, and large equivalent width (EW) of the neutral Fe K-alpha line at 6.4 keV. Our results indicate that the majority of these samples evidence heavy absorption, with a continuum of values rather than a segregation into Compton-thin and Compton-thick sub-populations, and that the fitted column density from X-ray modeling can severely under-represent the attenuation implied by these obscuration diagnostics. We find that no statistically significant trends exists between obscuration and host galaxy properties calculated from optical and Spitzer data, suggesting that Compton-thick AGN do not inhabit unique host galaxies from less obscured type 2 Seyferts. Using a sample of 300 star-forming galaxies, we are also exploring the relationship between AGN, star-forming galaxies and composite systems using infrared and optical diagnostics.
LaMassa Stephanie M.
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