THE ACTIVE NUCLEUS OF IC 4970: A NEARBY EXAMPLE OF MERGER-INDUCED COLD-GAS ACCRETION

摘要

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC 6872/IC 4970 in the Pavo galaxy group and show that the smaller companion galaxy IC 4970 hosts a highly obscured active galactic nucleus (AGN). The 0.5-10 keV X-ray luminosity of the nucleus is variable, increasing by a factor 2.9 to a luminosity of 1.7 × 10~(42) erg s~(-1) (bright state) on ～100 ks timescales. The X-ray spectrum of the bright state is heavily absorbed (N_H = 3 × 10~(23) cm~(-2) for power-law models with Γ = 1.5-2.0) and shows a clear 6.4 keV Fe Kα line with equivalent width of 144-195 eV. Limits on the diffuse emission in IC 4970 from Chandra X-ray data suggest that the available power from Bondi accretion of IC 4970's hot interstellar gas may be an order of magnitude too small to power the AGN. Spitzer images show that 8 μm nonstellar emission is concentrated in the central 1 kpc of IC 4970, consistent with high obscuration in this region. The mid-infrared colors of the nucleus are consistent with those expected for a highly obscured AGN. Taken together these data suggest that the nucleus of IC 4970 is a Seyfert 2, triggered and fueled by cold material supplied to the central supermassive black hole as a result of the off-axis collision of IC 4970 with the cold-gas-rich spiral galaxy NGC 6872.