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26th of January 2016
HI emission and absorption in nearby, gas-rich galaxies
by Sarah Reeves (University of Sydney)
Reeves et al. (2016) present the detection of a new intervening HI absorption-line in the galaxy NGC 5156. The detection was made as part of a survey for intervening absorption in 16 nearby galaxies with the Australia Telescope Compact Array (ATCA); see Reeves et al. (2015). The absorption-line is deep and narrow (7 km/s) and arises at a distance of 19 kpc from the centre of the galaxy. High resolution 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). Given the narrowness of the absorption-line, it is most likely that the absorption is occurring against a single component. Future observations using VLBI spectroscopy would allow this to be confirmed, and help to constrain the size of the absorbing cloud. The results of the survey also provide important information in the preparation for the ASKAP-FLASH survey - for example the effect of background source structure on absorption-line detection rate.

Figure caption: Top: ATCA HI absorption-line spectrum with the original detection on the left and a follow-up observation with high spectral resolution on the right. Bottom: schematic of the two different scenarios which may be giving rise to the detected absorption-line: the absorption may be occurring against both components (left), or against just one of the two components (right).

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