Figure: Velocity-integrated intensity maps of the Hnα RRLs recorded in a piggy-back mode in the FAST GPPS survey. The observation beam has a size of about 3 arcmin. The velocity range for the integration is from -40 km/s to 120 km/s.
Ionized gas is one of the major components of the Galactic interstellar medium (ISM). It is widely distributed in the Milky Way, from the Galactic center to the distant far outer Galaxy, making up about 20% of the total gas mass of the Milky Way. Observations of the Galactic ionized gas are crucial for understanding many astrophysical questions, such as the star formation and HII regions, the kinematics of ionized ISM, the electron density distribution and the spiral structure of the Galaxy, and the gaseous metallicity and recycling of materials in the Galactic ISM. However, in comparison to many surveys for atomic gas, molecular gas, and interstellar dust, previous surveys for ionized gas have to be improved in aspects of the observation sensitivity, spatial resolution, spectral resolution, and sky coverage. Especially the diffuse ionized gas in the Milky Way, it is the last major component of the Galaxy to be studied with sufficient sensitivity and resolution. Radio recombination lines (RRLs) are one of the best tracers of the diffuse ionized gas since they can probe the entire disk.
During observations of the Galactic Plane Pulsar Snapshot (GPPS) survey, a set of independent spectrum digital backends are connected to the 19-beam L-band receivers, and the four polarization products of spectral line data of 1024k channels covering the frequency band of 1000 - 1500 MHz are simultaneously recorded. The huge collecting area of FAST and the 5-minute long integration time for each beam make the GPPS piggyback spectral data the most sensitive resources for the detections of RRLs of hydrogen (H), helium (He) and Carbon (C) at L band.
With the data processing method developed by the survey team, we have processed the Hnα RRLs, and obtained a sensitive averaged Hnα RRL map for an area of 88 square degrees in the inner Galaxy (see Figure). The data of the Hnα RRLs has a spatial resolution of about 3 arcmin, a spectral resolution of 2.2 km/s, and a typical spectral rms noise of 0.25 mJy/beam. The GPPS Hnα RRL observations are sensitive down to an emission measure of about 200 cm^{-6} pc if a 3σ detection limit is required. The Hnα RRL map shows complex structural features dominated by a number of HII regions and large diffuse ionized gas regions.
We detect about 312 RRL components for 302 known HII regions (see data of Table 3 in paper, and also READme), i.e. 94% of the known HII regions, and confirm 43 HII region candidates in the observed sky area (see Table 4 in the paper ). Several large HII regions or star forming complexes in the distant outer Galaxy are resolved in the Hnα RRL map. Extended RRL features of the diffuse ionized gas are detected. In addition, the GPPS piggy-back line data also have the ability to provide sensitive detection for other kinds of RRLs, such as the Cnα, Henα, Hnβ and Hnγ RRLs, which will be given by the survey team in the near future. The GPPS survey piggyback spectral line data will promote our understanding of the characteristics of Galactic ionized gas.
The FAST GPPS survey piggyback RRL data products:
(v1.0 in a fits file) is released here.
Since data in some observations have much more RFIs, we will be continuously improving the data processing for the piggyback spectra of the FAST GPPS survey, e.g. on baseline fitting, RFIs cleaning etc, and will update the data product and release new version of the fits file.
You are welcome to use this most updated version of fits file.
If you find any problem or have any comments and suggestions, please contact us.