Infrared power-law galaxies in the chandra deep field-south: Active galactic nuclei and ultraluminous infrared galaxies

A. Alonso-Herrero, P. G. Pérez-González, D. M. Alexander, G. H. Rieke, D. Rigopoulou, E. Le Floc'h, P. Barmby, C. Papovich, J. R. Rigby, F. E. Bauer, W. N. Brandt, E. Egami, S. P. Willner, H. Dole, J. S. Huang

Research output: Contribution to journalArticle

174 Scopus citations

Abstract

We investigate the nature of a sample of 92 Spitzer MIPS 24 μm-selected galaxies in the CDF-S, showing power-law-like emission in the Spitzer IRAC 3.6-8 μm 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 μm) selected and optically bright (WDS IAB ≤ 24) star-forming galaxies whose SEDs show a very prominent stellar bump at 1.6 μm. 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 μm-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.

Original languageEnglish (US)
Pages (from-to)167-184
Number of pages18
JournalAstrophysical Journal
Volume640
Issue number1 I
DOIs
StatePublished - Mar 20 2006

Keywords

  • Galaxies: active
  • Galaxies: high-redshift
  • Infrared: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Alonso-Herrero, A., Pérez-González, P. G., Alexander, D. M., Rieke, G. H., Rigopoulou, D., Le Floc'h, E., Barmby, P., Papovich, C., Rigby, J. R., Bauer, F. E., Brandt, W. N., Egami, E., Willner, S. P., Dole, H., & Huang, J. S. (2006). Infrared power-law galaxies in the chandra deep field-south: Active galactic nuclei and ultraluminous infrared galaxies. Astrophysical Journal, 640(1 I), 167-184. https://doi.org/10.1086/499800