2016MNRAS.457.2613R

We present the results of a survey for intervening 21 cm H i absorption in a sample of 10 nearby, gas-rich galaxies selected from the H i 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 H i emission-line data, allowing us to directly relate the absorption-line detection rate to the H i distribution. From this, we find the majority of the non-detections in the current sample are because sightline does not intersect the H i 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. 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_S/f, is approximately 950 K in the outer disc of NGC 5156, but further observations using very long baseline interferometry would allow us to accurately measure the covering factor and spin temperature of the gas.