Peering into the Milky Way by FAST: II. Ionized gas in the inner Galactic disk revealed by the piggyback line observations of the FAST GPPS survey

As one of the major components of the interstellar medium, the ionized gas in our Milky Way, especially the low-density diffuse component, has not been extensively observed in the radio band. The Galactic Plane Pulsar Snapshot (GPPS) survey covers the sky area within the Galactic latitude of ±10° around the Galactic plane visible by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), and the spectral line data are simultaneously recorded during the pulsar survey observations. With an integration time of 5 min for each beam, the GPPS survey project provides the most sensitive piggyback spectra for tens of radio recombination lines (RRLs) in the band of 1000–1500 MHz for H nα , He nα , C nα , as well as H nβ and H nγ . We processed the spectral data of RRLs, and obtained a sensitive averaged H nα RRL map of a sky area of 88 square degrees in the inner Galaxy of 33° ≼ l ≼ 55° and ∣ b ∣ ≼ 2°0. The final spectral data of the H nα RRLs have a spatial resolution of ∼3′, a spectral resolution of 2.2 km s −1 , and a typical spectral RMS noise of 0.25 mJy beam −1 or 6.3 mK in main-beam brightness temperature. The FAST GPPS H nα RRL observations are sensitive down to an emission measure of about 200 cm −6 pc if a 3 σ detection limit is required. The new H nα RRL map shows complex structural features dominated by a number of H ii regions and large extended diffuse ionized gas regions. We detect about 94% of the known H ii regions and confirm 43 WISE H ii regions in the observed sky area. Several large H ii regions or star-forming complexes in the distant outer Galaxy are resolved in the map of H nα RRLs. Extended RRL features of the diffuse ionized gas are detected. In addition, the GPPS piggyback spectral-line data also provide sensitive detection for other kinds of RRLs, such as the He nα , C nα , H nβ , and H nγ RRLs. The RRL data products of the GPPS survey will be published and updated at http://zmtt.bao.ac.cn/MilkyWayFAST/ .