Infrared and X-Ray Evidence of an AGN in the NGC 3256 Southern Nucleus
  We investigate signs of Active Galactic Nucleus (AGN) in the luminous
infrared galaxy NGC 3256 at both infrared and X-ray wavelengths. NGC 3256 has
double, the Northern and Southern, nuclei (hereafter, N and S nuclei,
respectively). We show that the Spitzer IRAC colors extracted at the S nucleus
are AGN-like, and the Spitzer IRS spectrum is bluer at <6um than at the N
nucleus. We built for the S nucleus an AGN-starburst composite model with a
heavily absorbed AGN to successfully reproduce not only the IRAC and IRS
specrophotometries at ~3arcsec but also the very deep silicate 9.7um absorption
observed at 0.36" scale by Diaz-Santos et al. We found a 2.2um compact source
at the S nucleus in a HST NICMOS image and identified its unresolved core (at
0.26" resolution) with the compact core in previous mid-infrared observations
at comparable resolution. The flux of the 2.2umm core is consistent with our
AGN spectral energy distribution model. We also analyzed a deeper than ever
Chandra X-ray spectrum of the unresolved (at 0.5" resolution) source at the S
nucleus. We found that a dual-component power-law model (for primary and
scattered ones) fits an apparently very hard spectrum with a moderately large
absorption on the primary component. Together with a limit on equivalent width
of a fluorescent Fe-K emission line at 6.4 keV, the X-ray spectrum is
consistent with a typical Compton-thin Seyfert 2. We therefore suggest that the
S nucleus hosts a heavily absorbed low-luminosity AGN.
