Prof. Dimitra Rigopoulou

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. © 2006. The American Astronomical Society. All rights reserved.

Abstract:

A deep mid- and far-infrared survey in the extended Groth strip (EGS) area gives 3.6 to 8 μm flux densities or upper limits for 253 Lyman break galaxies (LBGs). The LBGs are a diverse population but with properties correlated with luminosity. The LBGs show a factor of 30 range in indicated stellar mass and a factor of 10 range in apparent dust content relative to stellar mass. About 5% of LBGs are luminous at all wavelengths, with powerful emission at rest 6 μm. In the rest 0.9 to 2 μm spectral range these galaxies have stellar spectral slopes with no sign of an AGN power-law component, suggesting that their emission is mainly powered by intensive star formation. Galaxies in this luminous population share the infrared properties of cold Submillimeter Common-User Bolometric Array (SCUBA) sources: both are massive and dusty starburst galaxies at 2 < z < 3; their stellar mass is larger than 10 11 M⊙. We suggest that these galaxies are the progenitors of present-day giant elliptical galaxies, with a substantial fraction of their stars already formed at z ≈ 3. © 2005. The American Astronomical Society. All rights reserved.