# A Chandra and ALMA Study of X-ray-irradiated Gas in the Central ~100 pc of the Circinus Galaxy

## TL;DR

This study combines Chandra and ALMA observations to analyze the impact of X-ray irradiation from the AGN in the Circinus galaxy on surrounding dense molecular gas, revealing potential gas dissociation and star formation suppression.

## Contribution

It provides the first high-resolution spatial comparison of X-ray-irradiated gas and molecular gas in the central 100 pc of a galaxy with a Compton-thick AGN, highlighting X-ray effects on molecular gas.

## Key findings

- Molecular gas is faint where iron emission is bright, indicating possible dissociation.
- X-ray irradiation may suppress star formation near the AGN.
- Gas densities are consistent with X-ray dissociation models.

## Abstract

We report a study of X-ray-irradiated gas in the central ~100 pc of the Circinus galaxy, hosting a Compton-thick active galactic nucleus (AGN), at 10-pc resolution using Chandra and ALMA. Based on ~200 ksec Chandra/ACIS-S data, we created an image of the Fe Kalpha line at 6.4 keV, tracing X-ray-irradiated dense gas. The ALMA data in Bands 6 (~270 GHz) and 7 (~350 GHz) cover five molecular lines: CO(J=3--2), HCN(J=3--2), HCN(J=4--3), HCO^+(J=3--2), and HCO^+(J=4--3). The detailed spatial distribution of dense molecular gas was revealed, and compared to the iron line image. The molecular gas emission appeared faint in regions with bright iron emission. Motivated by this, we quantitatively discuss the possibility that the molecular gas is efficiently dissociated by AGN X-ray irradiation (i.e., creating an X-ray-dominated region). Based on a non-local thermodynamic equilibrium model, we constrained the molecular gas densities and found that they are as low as interpreted by X-ray dissociation. Furthermore, judging from inactive star formation (SF) reported in the literature, we suggest that the X-ray emission has potential to suppress SF, particularly in the proximity of the AGN.