A. Alonso-Herrero et al. 2006 ApJ 640 167 doi:10.1086/499800
A. Alonso-Herrero1,2, P. G. Pérez-González2, D. M. Alexander3, G. H. Rieke2, D. Rigopoulou4, E. Le Floc'h2, P. Barmby5, C. Papovich2, J. R. Rigby2, F. E. Bauer6, W. N. Brandt7, E. Egami2, S. P. Willner5, H. Dole2,8 and J.-S. Huang5
Show affiliationsWe 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 (VVDS 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.
galaxies: active; galaxies: high-redshift; infrared: galaxies; X-rays: galaxies
Issue 1 (2006 March 20)
Received 2005 April 14, accepted for publication 2005 November 15
A. Alonso-Herrero et al. 2006 ApJ 640 167
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