Welcome to the new version of CaltechAUTHORS. Login is currently restricted to library staff. If you notice any issues, please email coda@library.caltech.edu
Published May 1, 2009 | Published
Journal Article Open

Host Galaxies, Clustering, Eddington Ratios, and Evolution of Radio, X-Ray, and Infrared-Selected AGNs


We explore the connection between different classes of active galactic nuclei (AGNs) and the evolution of their host galaxies, by deriving host galaxy properties, clustering, and Eddington ratios of AGNs selected in the radio, X-ray, and infrared (IR) wavebands. We study a sample of 585 AGNs at 0.25 < z < 0.8 using redshifts from the AGN and Galaxy Evolution Survey (AGES). We select AGNs with observations in the radio at 1.4 GHz from the Westerbork Synthesis Radio Telescope, X-rays from the Chandra XBoötes Survey, and mid-IR from the Spitzer IRAC Shallow Survey. The radio, X-ray, and IR AGN samples show only modest overlap, indicating that to the flux limits of the survey, they represent largely distinct classes of AGNs. We derive host galaxy colors and luminosities, as well as Eddington ratios, for obscured or optically faint AGNs. We also measure the two-point cross-correlation between AGNs and galaxies on scales of 0.3-10 h^(–1) Mpc, and derive typical dark matter halo masses. We find that: (1) radio AGNs are mainly found in luminous red sequence galaxies, are strongly clustered (with M_(halo) ~ 3 × 10^(13) h^(–1) M_☉ ), and have very low Eddington ratios λ10^(–3); (2) X-ray-selected AGNs are preferentially found in galaxies that lie in the "green valley" of color-magnitude space and are clustered similar to the typical AGES galaxies (M_(halo) ~ 10^(13) h^(–1) M_☉), with ≾10^(–3)λ≾ 1; (3) IR AGNs reside in slightly bluer, slightly less luminous galaxies than X-ray AGNs, are weakly clustered (M -(halo) ≾ 10^(12) h^(–1)M_☉), and have λ>10^(–2). We interpret these results in terms of a simple model of AGN and galaxy evolution, whereby a "quasar" phase and the growth of the stellar bulge occurs when a galaxy's dark matter halo reaches a critical mass between ~10^(12) and 10^(13)M_☉ . After this event, star formation ceases and AGN accretion shifts from radiatively efficient (optical- and IR-bright) to radiatively inefficient (optically faint, radio-bright) modes.

Additional Information

© 2009 The American Astronomical Society. Received 2008 September 19; accepted 2009 February 2; published 2009 April 20. We thank our colleagues on the NDWFS, AGES, IRAC Shallow Survey, and XBoötes teams. We thank Matthew Ashby, Anca Constantin, Alexey Vikhlinin, and Steven Willner for fruitful discussions, and thank the referee, Philip Hopkins, for helpful suggestions. We also thank Martin White for sharing dark matter simulation results. The NOAO Deep Wide-Field Survey, and the research of A.D. and B.T.J. are supported by NOAO, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under a cooperative agreement with the National Science Foundation. This paper would not have been possible without the efforts of the Chandra, Spitzer, KPNO, and MMT support staff. Optical spectroscopy discussed in this paper was obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona. R.C.H. was supported by an SAO Postdoctoral Fellowship, NASA GSRP Fellowship, and Harvard Merit Fellowship, and by Chandra grants GO5-6130A and GO5-6121A.

Attached Files

Published - Hickox2009p2256Astrophys_J.pdf


Files (3.1 MB)
Name Size Download all
3.1 MB Preview Download

Additional details

August 21, 2023
October 19, 2023