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23rd of June 2021
The Southern HII Region Discovery Survey
by Wenger et al.
HII ("H-two") regions are the zones of ionized gas surrounding young, massive stars. These nebulae are tracers of spiral arms and gas-phase metallicity structure in galaxies. Their physical properties (e.g., electron density, electron temperature, metallicity) inform our understanding of high-mass star formation, the interstellar medium (ISM), and Galactic chemical evolution. A complete census of Galactic H ii regions will place constraints on models of Milky Way formation and evolution, but a lack of sensitive surveys in the southern sky has left this census incomplete.

The Southern HI Region Discovery Survey (SHRDS) is a 900 hour ATCA 4--10 GHz radio continuum and radio recombination line survey of Galactic HII regions and infrared-identified HII region candidates in the southern sky. Radio recombination lines (RRL) are produced when atoms (most commonly, hydrogen) cascade into a series of successively lower ionisation states. Wenger et al. have recently published SHRDS data from a search for new HII regions between Galactic longitudes of 259 degrees and 346 degrees, within 4 degrees of the Galactic plane. They detect radio continuum emission toward 730 targets, including previously known nebulae and HII region candidates. By averaging <18 RRL transitions, they detect RRL emission toward 206 previously known HII regions and 436 HII region candidates. Including the northern sky surveys, over the last decade the HII Region Discovery Surveys have more than doubled the number of known Galactic HII regions. The census of HII regions in the WISE infra-red Catalog of Galactic HII Regions is now complete for nebulae with 9 GHz continuum flux densities greater than 250 mJy.

The image above shows the largest SHRDS radio continuum mosaic overlaid on 3.4 (blue), 12 (green), and 22 (red) micro-metre data from the WISE All-Sky Survey. Black regions in the image are saturated in at least one WISE band. The black contours are the mosaicked 4 GHz bandwidth radio continuum data at 5, 10, 20, 50, 100, and 200 times the image rms. The red contour represents the 10% primary beam response and the extent of the SHRDS data. Each white cross is the location of the brightest pixel associated with a WISE Catalog source and each white contour is the watershed region for that source. The filled white circle in the lower left is the smoothed resolution of the radio continuum data.

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