Hard X-ray Irradiation Potentially Drives Negative AGN Feedback by Altering Molecular Gas Properties

Abstract

To investigate the role of active galactic nucleus (AGN) X-ray irradiation on the interstellar medium (ISM), we systematically analyzed Chandra and ALMA CO($J$=2-1) data for 26 ultra-hard X-ray ($>$ 10 keV) selected AGNs at redshifts below 0.05. While Chandra unveils the distribution of X-ray-irradiated gas via Fe-K$\alpha$ emission, the CO($J$=2-1) observations reveal that of cold molecular gas. At high resolutions $\lesssim$ 1 arcsec, we derive Fe-K$\alpha$ and CO($J$=2-1) maps for the nuclear 2 arcsec region, and for the external annular region of 2 arcsec-4 arcsec, where 2 arcsec is $\sim$ 100-600 pc for most of our AGNs. First, focusing on the external regions, we find the Fe-K$\alpha$ emission for six AGNs above 2$\sigma$. Their large equivalent widths ($\gtrsim$ 1 keV) suggest a fluorescent process as their origin. Moreover, by comparing 6-7 keV/3-6 keV ratio, as a proxy of Fe-K$\alpha$, and CO($J$=2-1) images for three AGNs with the highest significant Fe-K$\alpha$ detections, we find a possible spatial separation. These suggest the presence of X-ray-irradiated ISM and the change in the ISM properties. Next, examining the nuclear regions, we find that (1) The 20-50 keV luminosity increases with the CO($J$=2-1) luminosity. (2) The ratio of CO($J$=2-1)-to-HCN($J$=1-0) luminosities increases with 20-50 keV luminosity, suggesting a decrease in the dense gas fraction with X-ray luminosity. (3) The Fe-K$\alpha$-to-X-ray continuum luminosity ratio decreases with the molecular gas mass. This may be explained by a negative AGN feedback scenario: the mass accretion rate increases with gas mass, and simultaneously, the AGN evaporates a portion of the gas, which possibly affects star formation.