Journal
ASTROPHYSICAL JOURNAL
Volume 728, Issue 1, Pages -Publisher
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/728/1/38
Keywords
galaxies: active; galaxies: fundamental parameters; galaxies: nuclei; galaxies: Seyfert; galaxies: statistics
Categories
Funding
- NSF [AST00-71198, AST00-71048, AST05-07483, AST05-07428, AST08-07630, AST08-08133]
- NASA [G05-6141A, GO8-9129A]
- STFC
- Alfred P. Sloan Foundation
- National Aeronautics and Space Administration
- National Science Foundation
- U.S. Department of Energy
- Japanese Monbukagakusho
- Max Planck Society
- University of Chicago
- Fermilab
- Institute for Advanced Study
- Japan Participation Group
- Johns Hopkins University
- Los Alamos National Laboratory
- Max-Planck-Institute for Astronomy (MPIA)
- Max-Planck-Institute for Astrophysics (MPA)
- New Mexico State University
- University of Pittsburgh
- Princeton University
- United States Naval Observatory
- University of Washington
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [0806732, 808133] Funding Source: National Science Foundation
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We develop a new diagnostic method to classify galaxies into active galactic nucleus (AGN) hosts, star-forming galaxies, and absorption-dominated galaxies by combining the [OIII]/H beta ratio with rest-frame U-B color. This can be used to robustly select AGNs in galaxy samples at intermediate redshifts (z < 1). We compare the result of this optical AGN selection with X-ray selection using a sample of 3150 galaxies with 0.3 < z < 0.8 and I-AB < 22, selected from the DEEP2 Galaxy Redshift Survey and the All-wavelength Extended Groth Strip International Survey. Among the 146 X-ray sources in this sample, 58% are classified optically as emission-line AGNs, the rest as star-forming galaxies or absorption-dominated galaxies. The latter are also known as X-ray bright, optically normal galaxies (XBONGs). Analysis of the relationship between optical emission lines and X-ray properties shows that the completeness of optical AGN selection suffers from dependence on the star formation rate and the quality of observed spectra. It also shows that XBONGs do not appear to be a physically distinct population from other X-ray detected, emission-line AGNs. On the other hand, X-ray AGN selection also has strong bias. About 2/3 of all emission-line AGNs at L-bol > 10(44) erg s(-1) in our sample are not detected in our 200 ks Chandra images, most likely due to moderate or heavy absorption by gas near the AGN. The 2-7 keV detection rate of Seyfert 2s at z similar to 0.6 suggests that their column density distribution and Compton-thick fraction are similar to that of local Seyferts. Multiple sample selection techniques are needed to obtain as complete a sample as possible.
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