  We investigate the nature of a sample of 92 Spitzer/MIPS 24 micron selected
galaxies in the CDFS, showing power law-like emission in the Spitzer/IRAC 3.6-8
micron 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 AGN 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 spectral energy distributions
(SEDs) of the X-ray detected galaxies are almost equally divided between a
BLAGN SED class (similar to an optically selected QSO) and a NLAGN SED (similar
to the BLAGN SED but with an obscured UV/optical continuum). A small fraction
of SEDs resemble warm ULIRG galaxies (e.g., Mrk231). 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 (z_sp>1) IR (24 micron)
selected and optically bright (VVDS I_AB<=24) star-forming galaxies whose SEDs
show a very prominent stellar bump at 1.6 micron. The galaxies detected in
X-rays have 2-8 keV rest-frame luminosities typical of AGN. 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 AGN. If all these sources
are actually obscured AGN, we would observe a ratio of obscured to unobscured
24 micron detected AGN of 2:1, whereas models predict a ratio of up to 3:1.
Additional studies using Spitzer to detect X-ray-quiet AGN are likely to find
more such obscured sources.
