Peering into the Milky Way by FAST: I. Exquisite Hi structures in the inner Galactic disk from the piggyback line observations of the FAST GPPS survey

Abstract

Neutral hydrogen (Hi) is the fundamental component of the interstellar medium. The Galactic Plane Pulsar Snapshot (GPPS) survey is designed for hunting pulsars by using the Five-hundred-meter Aperture Spherical radio Telescope (FAST) from the visible Galactic plane within |b| ≤ 10 • . The survey observations are conducted with the L-band 19-beam receiver in the frequency range of 1.0 − 1.5 GHz, and each pointing has an integration time of 5 minutes. The piggyback spectral data simultaneously recorded during the FAST GPPS survey are great resources for studies on the Galactic Hi distribution and ionized gas. We process the piggyback Hi data of the FAST GPPS survey in the region of 33 • ≤ l ≤ 55 • and |b| ≤ 2 • . The rms of the data cube is found to be approximately 40 mK at a velocity resolution of 0.1 km s −1 , placing it the most sensitive observations of the Galactic Hi by far. The high velocity resolution and high sensitivity of the FAST GPPS Hi data enable us to detect weak exquisite Hi structures in the interstellar medium. Hi absorption line with great details can be obtained against bright continuum sources. The FAST GPPS survey piggyback Hi data cube will be released and updated on the web: http://zmtt.bao.ac.cn/MilkyWayFAST/.

The GPPS HI survey—A deeper view of the Milky Way galaxy

“…The "exquisite HI structures" of the paper title can be seen in Figure 10 of Hong et al [1]. This is where the advantage of better resolution and sensitivity of the FAST tele-129706-2 scope is indispensable to the discovery of small and faint features in the interstellar gas.…”

“…The colourful images of the sky shown in Figures 5-7 of Hong et al [1] have wonderful detail and high dynamic range, i.e., contrast. Looking at the bottom panels of Figure 6 (120 km/s > V > 100 km/s at the bottom, 100 km/s > V > 80 km/s just above, and 80 km/s > V > 60 km/s above that) shows how the rotation of the Galaxy gradually shifts the line emission to low longitudes (right hand side of the figures).…”

“…The paper in this volume by Hong et al [1] describing a new survey of the radio frequency spectral line of hydrogen, that astronomers call the HI line, shows some dramatic images of the Milky Way galaxy. The Five-hundred-meter Aperture Spherical radio Telescope, FAST, has done this survey as part of the Galactic Plane Pulsar Snapshot (GPPS) Survey [2].…”

“…The Five-hundred-meter Aperture Spherical radio Telescope, FAST, has done this survey as part of the Galactic Plane Pulsar Snapshot (GPPS) Survey [2]. The primary goal is to find pulsars in the Galactic plane, 500 have been found already, but the survey has secondary applications to mapping the neutral atomic interstellar medium with the HI line [1], the ionized medium using recombination lines [3], and the magnetic field through measurement of Faraday rotation of pulsars [4]. This special edition includes papers describing results of all these projects; the background and significance of the HI data are considered in this News & Views piece.…”

“…Matching absorption and emission spectra, described briefly in sect. 4.4 of Hong et al [1], will allow the temperature of the interstellar hydrogen to be measured. These subjects will surely be topics for future dissertations for Ph.D. theses, and there will be scope for creative, completely original research with the survey results.…”