AEGIS: A Multi-wavelength Study of Spitzer Power-law Galaxies
  This paper analyzes a sample of 489 Spitzer/IRAC sources in the Extended
Groth Strip whose spectral energy distributions fit a red power law from 3.6 to
8.0 \micron. The median for sources with known redshift is <z>=1.6. Though all
or nearly all of the sample are likely to be active galactic nuclei, only 33%
were detected in the EGS X-ray survey (AEGIS-X) using 200 ks Chandra
observations. The detected sources are X-ray luminous with L_X > 10^43 erg/s
and moderately to heavily obscured with N_H > 10^22 cm^-2. Stacking the
X-ray-undetected sample members yields a statistically significant X-ray
signal, suggesting that they are on average more distant or more obscured than
sources with X-ray detections. The ratio of X-ray to mid-infrared fluxes
suggests that a substantial fraction of the sources undetected in X-rays are
obscured at the Compton-thick level, in contrast to the X-ray-detected sources,
all of which appear to be Compton-thin. For the X-ray-detected power-law
sources with redshifts, an X-ray luminosity L_X ~ 10^44 erg/s marks a
transition between low-luminosity, blue sources dominated by the host galaxy to
high-luminosity, red power-law sources dominated by nuclear activity.
X-ray-to-optical ratios, infrared variability, and 24 micron properties of the
sample are consistent with the identification of infrared power-law sources as
active nuclei, but a rough estimate is that only 22% of AGNs are selected by
the power law criteria. Comparison of the power-law selection technique and
various IRAC color criteria for identifying AGNs confirms that high-redshift
samples selected via simple IRAC colors may be heavily contaminated by
starlight-dominated objects.
