# HI absorption in nearby compact radio galaxies

## TL;DR

This study investigates neutral hydrogen absorption in 66 nearby compact radio galaxies, revealing diverse gas kinematics and linking HI features to galaxy type and activity, thus enhancing understanding of AGN-gas interactions.

## Contribution

It provides new HI absorption data for low-luminosity, nearby compact radio galaxies, expanding previous surveys and linking HI features to galaxy properties and X-ray absorption.

## Key findings

- 71% of detections show disturbed gas kinematics
- Narrow, deep HI features are linked to late-type, high-excitation galaxies
- Absorbed X-ray sources may indicate higher HI content in hosts

## Abstract

HI absorption studies yield information on both AGN feeding and feedback processes. This AGN activity interacts with the neutral gas in compact radio sources, which are believed to represent the young or recently re-triggered AGN population. We present the results of a survey for HI absorption in a sample of 66 compact radio sources at 0.040 < z < 0.096 with the Australia Telescope Compact Array. In total, we obtained seven detections, five of which are new, with a large range of peak optical depths (3% to 87%). Of the detections, 71% exhibit asymmetric, broad ($\Delta{v}_{\mathrm{FWHM}}$ > 100 km s$^{-1}$) features, indicative of disturbed gas kinematics. Such broad, shallow and offset features are also found within low-excitation radio galaxies which is attributed to disturbed circumnuclear gas, consistent with early-type galaxies typically devoid of a gas-rich disk. Comparing mid-infrared colours of our galaxies with HI detections indicates that narrow and deep absorption features are preferentially found in late-type and high-excitation radio galaxies in our sample. These features are attributed to gas in galactic disks. By combining XMM-Newton archival data with 21-cm data, we find support that absorbed X-ray sources may be good tracers of HI content within the host galaxy. This sample extends previous HI surveys in compact radio galaxies to lower radio luminosities and provides a basis for future work exploring the higher redshift universe.