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

TL;DR

This study investigates how hard X-ray irradiation from active galactic nuclei influences the interstellar medium, revealing potential negative feedback effects that alter molecular gas properties and possibly suppress star formation.

Contribution

It provides the first spatially resolved analysis linking X-ray irradiation to changes in molecular gas and suggests a negative feedback mechanism affecting star formation in AGN host galaxies.

Findings

Fe-Kα emission indicates X-ray-irradiated gas in external regions.

Higher X-ray luminosity correlates with decreased dense gas fraction.

X-ray irradiation may lead to gas evaporation, impacting star formation.

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.