HI emission and absorption in nearby, gas-rich galaxies

TL;DR

This study searched for HI absorption in six nearby, gas-rich galaxies using ATCA, finding no detections possibly due to faint background sources or high spin temperature to covering factor ratios, informing future survey strategies.

Contribution

The paper provides new observational data on HI absorption in nearby galaxies and discusses the impact of background source structure on detection rates, guiding future survey designs.

Findings

No intervening HI absorption detected in the sample.

Faint background sources likely caused non-detections in three cases.

High $T_{S}/f$ ratio needed to explain non-detection in one galaxy.

Abstract

We present the results of a targeted search for intervening HI absorption in six nearby, gas-rich galaxies using the Australia Telescope Compact Array (ATCA). The sightlines searched have impact parameters of 10-20 kpc. By targeting nearby galaxies we are also able to map their HI emission, allowing us to directly relate the absorption-line detection rate to the extended HI distribution. The continuum sightlines intersect the HI disk in four of the six galaxies, but no intervening absorption was detected. Of these four galaxies, we find that three of the non-detections are the result of the background source being too faint. In the fourth case we find that the ratio of the spin temperature to the covering factor ($T_{\mathrm{S}}/f$) must be much higher than expected ($\gtrsim$5700 K) in order to explain the non-detection. We discuss how the structure of the background continuum sources may have affected the detection rate of HI absorption in our sample, and the possible implications for future surveys. Future work including an expanded sample, and VLBI observations, would allow us to better investigate the expected detection rate, and influence of background source structure, on the results of future surveys.