Variability and Multiwavelength Detected AGN in the GOODS Fields
  We identify 85 variable galaxies in the GOODS North and South fields using 5
epochs of HST ACS V-band (F606W) images spanning 6 months. The variables are
identified through significant flux changes in the galaxy's nucleus and
represent ~2% of the survey galaxies. With the aim of studying the active
galaxy population in the GOODS fields, we compare the variability-selected
sample with X-ray and mid-IR AGN candidates. Forty-nine percent of the
variables are associated with X-ray sources identified in the 2Ms Chandra
surveys. Twenty-four percent of X-ray sources likely to be AGN are optical
variables and this percentage increases with decreasing hardness ratio of the
X-ray emission. Stacking of the non-X-ray detected variables reveals marginally
significant soft X-ray emission. Forty-eight percent of mid-IR power-law
sources are optical variables, all but one of which are also X-ray detected.
Thus, about half of the optical variables are associated with either X-ray or
mid-IR power-law emission. The slope of the power-law fit through the Spitzer
IRAC bands indicates that two-thirds of the variables have BLAGN-like SEDs.
Among those galaxies spectroscopically identified as AGN, we observe
variability in 74% of broad-line AGNs and 15% of NLAGNs. The variables are
found in galaxies extending to z~3.6. We compare the variable galaxy colors and
magnitudes to the X-ray and mid-IR sample and find that the non-X-ray detected
variable hosts extend to bluer colors and fainter intrinsic magnitudes. The
variable AGN candidates have Eddington ratios similar to those of X-ray
selected AGN.
