科研之友
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摘要
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 Hnα, Henα, Cnα, as well as Hnβ and Hnγ. We processed the spectral data of RRLs, and obtained a sensitive averaged Hnα 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 Hnα RRLs have a spatial resolution of ～3′, a spectral resolution of2.2 km s-1, and a typical spectral RMS noise of 0.25 m Jy beam-1or 6.3 m K in main-beam brightness temperature. The FAST GPPS Hnα RRL observations are sensitive down to an emission measure of about 200 cm-6pc if a 3σ detection limit is required.The new Hnα RRL map shows complex structural features dominated by a number of Hii regions and large extended diffuse ionized gas regions. We detect about 94% of the known Hii regions and confirm 43 WISE Hii regions in the observed sky area.Several large Hii regions or star-forming complexes in the distant outer Galaxy are resolved in the map of Hnα 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 Henα, Cnα, Hnβ, and Hnγ RRLs. The RRL data products of the GPPS survey will be published and updated at http://zmtt.bao.ac.cn/Milky Way FAST/.
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