The Active Nucleus of IC4970: A Nearby Example of Merger-Induced Cold-Gas Accretion

TL;DR

This study reveals that the galaxy IC4970 hosts a highly obscured AGN fueled by cold-gas accretion, triggered by its interaction with the nearby galaxy NGC6872, demonstrated through X-ray and infrared observations.

Contribution

It provides the first detailed multi-wavelength evidence of a merger-induced cold-gas accretion fueling an obscured AGN in a nearby galaxy.

Findings

IC4970 hosts a variable, heavily obscured AGN.

The AGN's X-ray spectrum shows a strong Fe Kalpha line.

Cold-gas accretion likely powers the AGN, not hot interstellar gas.

Abstract

We present results from Chandra X-ray and Spitzer mid-infrared observations of the interacting galaxy pair NGC6872/IC4970 in the Pavo galaxy group and show that the smaller companion galaxy IC4970 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 1.7 x 10^{42} erg/s (bright state) on ~100 ks timescales. The X-ray spectrum of the is heavily absorbed (N_H = 3 x 10^{23} cm^{-2}) for power law models with Gamma = 1.5-2.0 and shows a clear 6.4 keV Fe Kalpha line with equivalent width of 144-195 eV. Limits on the diffuse emission in IC4970 from Chandra X-ray data suggest that the available power from Bondi accretion of hot interstellar gas may be an order of magnitude too small to power the AGN. Spitzer images show that 8 micron nonstellar emission is concentrated in the central 1 kpc of IC4970, 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 IC4970 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 IC4970 with the cold-gas rich spiral galaxy NGC6872.