A pilot study of Galactic radio recombination lines using FAST: Identification of diffuse ionized gas clumps and off-arm star-forming regions

¹ National Astronomical Observatories, Chinese Academy of Sciences, 20A Datun Road, Chaoyang District, Beijing 100101, PR China
e-mail: bliu@nao.cas.cn
² Department of Astronomy and Institute of Interdisciplinary Studies, Hunan Normal University, Changsha, Hunan 410081, PR China
³ Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, PR China
e-mail: junzhiwang@gxu.edu.cn
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai, 200030, PR China
e-mail: zb@shao.ac.cn
⁵ Max Planck Institute for Astronomy, Kõnigstuhl 17, 69117 Heidelberg, Germany
⁶ Purple Mountain Observatory, Chinese Academy of Sciences, 8 Yuanhua Road, Nanjing, 210034, PR China

Received: 17 January 2024
Accepted: 11 July 2024

Abstract

Observing low-frequency decimeter hydrogen radio recombination lines (RRLs) with large single-dish telescopes, such as the Five-hundred-meter Aperture Spherical radio Telescope (FAST) in the L band, is a unique method for probing massive star formation on scales of hundreds of parsecs. This approach is particularly effective for detecting relatively weak and extended emissions from low-density gas ionized by massive stars. Deep, unbiased decimeter or centimeter RRL surveys with large single-dish telescopes can significantly enhance our understanding of the diffuse ionized gas along the Galactic plane. This, in turn, will improve our knowledge of the life cycle of matter in the interstellar medium and the dynamics of the Galaxy. In this context, we present a pilot project for such a blind L-band RRL survey targeting the Galactic plane and conducted using FAST. The results include the detection of RRL clumps and the identification of an off-arm active massive star-forming region near the Sagittarius-Carina arm. The ongoing and upcoming massive star formation in this region may be associated with the kink in the Sagittarius-Carina arm near 23° azimuth.