Astronomy and Astrophysics – Astrophysics – Cosmology and Extragalactic Astrophysics
Scientific paper
2009-11-12
Astronomy and Astrophysics
Astrophysics
Cosmology and Extragalactic Astrophysics
Accepted to ApJ; 20 pages, 6 figures, 6 tables
Scientific paper
We compare the relative merits of AGN selection at X-ray and mid-infrared wavelengths using data from moderately deep fields observed by both Chandra and Spitzer. The X-ray-selected AGN sample and associated optical follow-up are drawn from the SEXSI program. Mid-infrared data in these fields are derived from Spitzer imaging, and mid-infrared AGN selection is accomplished primarily through application of the IRAC color-color AGN `wedge' selection technique. Nearly all X-ray sources in these fields which exhibit clear spectroscopic signatures of AGN activity have mid-infrared colors consistent with IRAC AGN selection. These are predominantly the most luminous X-ray sources. X-ray sources that lack high-ionization and/or broad lines in their optical spectra are far less likely to be selected as AGN by mid-infrared color selection techniques. The fraction of X-ray sources identified as AGN in the mid-infrared increases monotonically as the X-ray luminosity increases. Conversely, only 22% of mid-infrared-selected AGN are detected at X-ray energies in the moderately deep (~100 ks) Chandra data. We hypothesize that the IRAC AGN that lack X-ray detections are predominantly high-luminosity AGN that are obscured and/or lie at high redshift. A stacking analysis of X-ray-undetected sources shows that objects in the mid-infrared AGN selection wedge have average X-ray fluxes in the 2-8 keV band three times higher than sources that fall outside the wedge. Their X-ray spectra are also harder. It is evident from this comparative study that in order to create a complete, unbiased census of supermassive black hole growth and evolution, a combination of sensitive infrared, X-ray and hard X-ray selection is required. We conclude by discussing what samples will be provided by upcoming survey missions such as WISE, eROSITA, and NuSTAR.
Eckart Megan E.
Harrison Fiona A.
Helfand David J.
McGreer Ian D.
Stern Daniel
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