HI emission and absorption in nearby, gas-rich galaxies II. -- sample completion and detection of intervening absorption in NGC 5156

TL;DR

This study surveys 10 nearby gas-rich galaxies for 21cm HI absorption, detecting one new intervening absorption in NGC 5156, and relates absorption detection to HI distribution and source structure.

Contribution

It completes a sample of HI absorption surveys in nearby galaxies and links absorption detection to HI distribution and background source structure.

Findings

Detected HI absorption in NGC 5156 at 19 kpc impact parameter.

Most non-detections are due to sightlines not intersecting dense HI regions.

Absorption likely occurs against a single component of the background source.

Abstract

We present the results of a survey for intervening 21cm HI absorption in a sample of 10 nearby, gas-rich galaxies selected from the HI Parkes All-Sky Survey (HIPASS). This follows the six HIPASS galaxies searched in previous work and completes our full sample. In this paper we searched for absorption along 17 sightlines with impact parameters between 6 and 46 kpc, making one new detection. We also obtained simultaneous HI emission-line data, allowing us to directly relate the absorption-line detection rate to the HI distribution. From this we find the majority of the non-detections in the current sample are because sightline does not intersect the HI disc of the galaxy at sufficiently high column density, but that source structure is also an important factor. The detected absorption-line arises in the galaxy NGC 5156 ($z = 0.01$) at an impact parameter of 19 kpc. The line is deep and narrow with an integrated optical depth of 0.82 km s$^{-1}$. High resolution Australia Telescope Compact Array (ATCA) images at 5 and 8 GHz reveal that the background source is resolved into two components with a separation of 2.6 arcsec (500 pc at the redshift of the galaxy), with the absorption likely occurring against a single component. We estimate that the ratio of the spin temperature and covering factor, $T_{\mathrm{S}}/f$, is approximately 950 K in the outer disc of NGC 5156, but further observations using VLBI would allow us to accurately measure the covering factor and spin temperature of the gas.