Ionized gas clouds near the Sagittarius Arm tangent

Li-Gang Hou1, Jian Dong2 , Xu-Yang Gao1 and Jin-Lin Han1   2017-05-29 00:20:12

1 National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China;

2 Shanghai Astronomical Observatory, Chinese Academy of Sciences, Shanghai 200030, China

Received 2016 July 26; accepted 2017 February 6

Abstract Radio recombination lines (RRLs) are the best tracers of ionized gas. Simultaneous observations of multi-transitions of RRLs can significantly improve survey sensitivity. We conducted pilot RRL observations near the Sagittarius Arm tangent by using the 65-m Shanghai Tian Ma Radio Telescope (TMRT) equipped with broadband feeds and a digital backend. Six hydrogen RRLs (H96α−H101α) at C band (6289 MHz−7319 MHz) were observed simultaneously toward a sky area of 2◦ × 1.2◦ by using on-the-fly mapping mode. These transitions were then stacked together for detection of ionized gas. Star forming complexes G48.6+0.1 and G49.5−0.3 were detected in the integrated intensity map. We found agreements between our measured centroid velocities and previous results for the 21 known HII regions in the mapped area. For more than 80 cataloged HII region candidates without previous RRL measurements, we obtained new RRL spectra at 30 targeted positions. In addition, we detected 25 new discrete RRL sources with spectral S/N > 5σ, and they were not listed in the catalogs of previously known HII regions. The distances for 44 out of these 55 new RRL sources were estimated.

  Key words: Galaxy: disk—radio lines: ISM—ISM: HII regions

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