Infrared Power-Law Galaxies in the Chandra Deep Field-South: Active Galactic Nuclei and Ultraluminous Infrared Galaxies Academic Article uri icon


  • We investigate the nature of a sample of 92 Spitzer MIPS 24 m-selected galaxies in the CDF-S, showing power-law-like emission in the Spitzer IRAC 3.6-8 m bands. The main goal is to determine whether the galaxies not detected in X-rays (47% of the sample) are part of the hypothetical population of obscured AGNs not detected even in deep X-ray surveys. The majority of the IR power-law galaxies are ULIRGs at z > 1, and those with LIRG-like IR luminosities are usually detected in X-rays. The optical-to-IR SEDs of the X-ray-detected galaxies are almost equally divided between a BLAGN SED class (similar to an optically selected QSO) and an NLAGN SED (similar to the BLAGN SED but with an obscured UV/optical continuum). A small fraction of SEDs resemble warm ULIRGs (e.g., Mrk 231). Most galaxies not detected in X-rays have SEDs in the NLAGN+ULIRG class as they tend to be optically fainter and possibly more obscured. Moreover, the IR power-law galaxies have SEDs significantly different from those of high-z (zsp > 1) IR (24 m) selected and optically bright (WDS IAB 24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 m. The galaxies detected in X-rays have 2-8 keV rest-frame luminosities typical of AGNs. The galaxies not detected in X-rays have global X-ray-to-mid-IR SED properties that make them good candidates to contain IR-bright X-ray-absorbed AGNs. If all these sources are actually obscured AGNs, we would observe a ratio of obscured to unobscured 24 m-detected AGNs of 2:1, whereas models predict a ratio of up to 3:1. Additional studies using Spitzer to detect X-ray-quiet AGNs are likely to find more such obscured sources. 2006. The American Astronomical Society. All rights reserved.

published proceedings

  • The Astrophysical Journal

altmetric score

  • 3

author list (cited authors)

  • Alonso-Herrero, A., Prez-Gonzlez, P. G., Alexander, D. M., Rieke, G. H., Rigopoulou, D., Le Floch, E., ... Huang, J.

citation count

  • 201

complete list of authors

  • Alonso-Herrero, A||Pérez-González, PG||Alexander, DM||Rieke, GH||Rigopoulou, D||Le Floc’h, E||Barmby, P||Papovich, C||Rigby, JR||Bauer, FE||Brandt, WN||Egami, E||Willner, SP||Dole, H||Huang, J-S

publication date

  • March 2006