Astrophys. J., 818, 88 (2016/February-2)
On R-W1 as a diagnostic to discover obscured active galactic nuclei in wide-area X-ray surveys.
LAMASSA S.M., CIVANO F., BRUSA M., STERN D., GLIKMAN E., GALLAGHER S., URRY C.M., CALES S., CAPPELLUTI N., CARDAMONE C., COMASTRI A., FARRAH D., GREENE J.E., KOMOSSA S., MERLONI A., MROCZKOWSKI T., NATARAJAN P., RICHARDS G., SALVATO M., SCHAWINSKI K. and TREISTER E.
Abstract (from CDS):
Capitalizing on the all-sky coverage of WISE and the 35% and 50% sky coverage from Sloan Digital Sky Survey and Pan-STARRS, respectively, we explore the efficacy of mR(optical) - m3.4µm (mid-infrared), hereafter R - W1, as a color diagnostic to identify obscured supermassive black hole accretion in wide-area X-ray surveys. We use the ∼16.5 deg2 Stripe 82 X-ray survey data as a test bed to compare R - W1 with R - K, an oft-used obscured active galactic nucleus (AGN) selection criterion, and examine where different classes of objects lie in this parameter space. Most stars follow a well-defined path in R - K versus R - W1 space. We demonstrate that optically normal galaxies hosting X-ray AGNs at redshifts 0.5 < z < 1 can be recovered with an R - W1 > 4 color cut, while they typically are not selected as AGNs based on their W1 - W2 colors. Additionally, different observed X-ray luminosity bins favor different regions in R - W1 parameter space: moderate-luminosity AGNs (1043 erg erg/s < L0.5–10kev < 1044 erg/s) tend to have red colors, while the highest-luminosity AGNs ( L0.5–10kev > 1045 erg/s) have bluer colors; higher spectroscopic completeness of the Stripe 82X sample is needed to determine whether this is a selection effect or an intrinsic property. Finally, we parameterize X-ray obscuration of Stripe 82X AGNs by calculating their hardness ratios (HRs) and find no clear trends between HR and optical reddening. Our results will help inform best-effort practices in following up obscured AGN candidates in current and future wide-area, shallow X-ray surveys, including the all-sky eROSITA mission.
galaxies: active - infrared: galaxies - quasars: general - quasars: supermassive black holes - X-rays: galaxies - X-rays: general
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