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Published July 1, 2014 | Published
Journal Article Open

The Angular Clustering of WISE-selected Active Galactic Nuclei: Different Halos for Obscured and Unobscured Active Galactic Nuclei


We calculate the angular correlation function for a sample of ~170,000 active galactic nuclei (AGNs) extracted from the Wide-field Infrared Survey Explorer (WISE) catalog, selected to have red mid-IR colors (W1 − W2 > 0.8) and 4.6 μm flux densities brighter than 0.14 mJy). The sample is expected to be >90% reliable at identifying AGNs and to have a mean redshift of 〈z〉 = 1.1. In total, the angular clustering of WISE AGNs is roughly similar to that of optical AGNs. We cross-match these objects with the photometric Sloan Digital Sky Survey catalog and distinguish obscured sources with r − W2 > 6 from bluer, unobscured AGNs. Obscured sources present a higher clustering signal than unobscured sources. Since the host galaxy morphologies of obscured AGNs are not typical red sequence elliptical galaxies and show disks in many cases, it is unlikely that the increased clustering strength of the obscured population is driven by a host galaxy segregation bias. By using relatively complete redshift distributions from the COSMOS survey, we find that obscured sources at 〈z〉 ~ 0.9 have a bias of b = 2.9 ± 0.6 and are hosted in dark matter halos with a typical mass of log (M/M_☉ h^(−1)) ~ 13.5. In contrast, unobscured AGNs at 〈z〉 ~ 1.1 have a bias of b = 1.6 ± 0.6 and inhabit halos of log (M/M_☉ h^(−1)) ~ 12.4. These findings suggest that obscured AGNs inhabit denser environments than unobscured AGNs, and they are difficult to reconcile with the simplest AGN unification models, where obscuration is driven solely by orientation.

Additional Information

© 2014 The American Astronomical Society. Received 2013 September 9; Accepted 2014 April 15; published 2014 June 13. We thank A. Myers for useful replies to questions and extend our gratitude to the WISE extragalactic science team for its continuous support and interesting discussions over the years. We also gratefully acknowledge the anonymous referee and numerous colleagues, including G. Hasinger and R. Hickox, who provided insightful comments that have improved our discussion. This publication makes use of data products from the Wide-field Infrared Survey Explorer, which is a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the National Aeronautics and Space Administration. Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the U.S. Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society, and the Higher Education Funding Council for England. The SDSS website is http://www.sdss.org/. R.J.A. was supported in part by an appointment to the NASA Postdoctoral Program at the Jet Propulsion Laboratory, administered by Oak Ridge Associated Universities through a contract with NASA. This research was partially supported by CONICET. R.J.A. was also supported in part by Gemini grant number 32120009. We also thank the NASA Astrophysics Data Analysis Program (ADAP) for its support.

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