Infrared and X-ray evidence of an AGN in the NGC 3256 southern nucleus

We investigate signs of an active galactic nucleus (AGN) in the luminous infrared (IR) galaxy NGC 3256 at both IR and X-ray wavelengths. NGC 3256 has double nuclei: the northern and southern (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 $<6$ μm 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 $\simeq 3$″, but also the very deep silicate 9.7 μm absorption observed at a 0.″ 36 scale by Díaz-Santos et al. We found a 2.2 μm compact source at the S nucleus in an 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 2.2 μm 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.