Pushing the Limit: A New & Reliable High-Redshift Emission Line Diagnostic for Separating AGNs from Star Formers

Type Conference Paper
Names Laura Trouille, A. Barger, C. Tremonti
Proceedings Title Bulletin of the American Astronomical Society
Conference Name American Astronomical Society, AAS Meeting #217, #326.03
Volume 43
Date January 1, 2011
Short Title Pushing the Limit
URL http://adsabs.harvard.edu/abs/2011AAS...21732603T
Library Catalog NASA ADS
Abstract Accurate identification of active galaxies is essential for properly assessing their impact on galaxy evolution. Classic optical emission line diagnostics for separating AGN-dominated sources from those dominated by star formation are limited to z<0.5 (e.g., in the BPT diagram, [OIII]/Hb versus [NII]/Ha, [NII] leaves the optical spectral window at z=0.5). In this talk, we present our new diagnostic based on [NeIII], [OII], and g-z color, which cleanly separates AGNs from star formers (SFs) out to z<1.4. We test our diagnostic on the SDSS DR7 galaxies. Using their BPT classification as AGNs or SFs as reference, we find that our new diagnostic reproduces the classification for 98% of the SDSS-AGNs and 96% of the SDSS-SFs. We also apply our new diagnostic to the OPTX (CDF-N, CLANS, and CLASXS) sample, a highly spectroscopically complete sample of Chandra X-ray selected AGNs. We find that all but one of our 457 X-ray selected AGNs lies in the AGN-dominated regime of our new diagnostic, while 20% would be classified as SFs using the BPT diagram (see also Winter2010 Swift-BAT results). Our new optical diagnostic thus provides a highly reliable method for distinguishing AGNs from SFs out to z<1.4. We use this new diagnostic in combination with X-ray stacking to probe sources that lie in the ambiguous region of the BPT diagram, between the AGN and SF regimes (as designated by the Kewley01 and Kauffman03 divisions). Previous optical studies suggest that these sources are SF dominated. However, we find that the stacked signal for these sources is X-ray hard, and furthermore the majority are identified as AGNs by our new diagnostic. These findings suggest that their radiation field is dominated by AGN activity, and therefore these sources should be included in AGN samples.
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