7th Asia-Pacific Conference on Plasma Physics, 12-17 Nov, 2023 at Port Messe Nagoya

New details of interstellar medium revealed by
the FAST Galactic Plane Snapshot survey

J.L. Han 1,2 , J. Xu 1 ,Tao Hong 1 , L.G. Hou 1 , and X.Y. Gao 1,21National Astronomical Observatories, Chinese Academy of Sciences,2
School of Astronomy, University of Chinese Academy of Sciencese-mail (speaker): hjl@nao.cas.cn
Many aspects of the interstellar medium in our Milky
Way galaxy has not yet revealed clearly. Sensitive radioobservations can detect the neutron and ionizedhydrogen through spectral line observations, and probemagneto-ionized gas through the polarization
observations of polarized radio sources such as pulsars
and background radio sources. Radio images can also
show the shocked gas by supernova remnants. The
Five-hundred-meter Aperture Spherical Telescope (FAST)
built by China has a 300 m illumination aperture, and is
the most sensitive single-dish radio telescope in theworld because it is equipped with a highly sensitive
L-band 19-beam cryogenic receiver hat has a systemtemperature of 22 K. It is an excellent instrument for
hunting pulsars and exploring the Galactic interstellar
medium.
We started the FAST Galactic Plane Pulsar Snapshot(GPPS) survey[1] at the beginning of 2020, and plan tocover the Galactic plane in the Galactic latitude of ±10°in the visible sky by FAST (zenith angle less than 28.5°)
with a huge number of 3’ beams in the L-band, each has
an integration time of 5 minutes. Up to now, we has
successfully discovered more than 600 pulsars (seehttp://zmtt.bao.ac.cn/GPPS/). During the pulsar surveyobservations, the piggyback spectral line data in the Lband of 1000— 1500 MHz are recorded simultaneouslywith the digital spectroscopy backends connected to theL-band 19-beam receiver.
The Galactic magnetic fields which permeate theinterstellar medium of the whole galaxy are extremely
difficult to measure. Rested on the high sensitivity ofFAST, we measure polarization and Faraday rotation ofpreviously known pulsars and newly discovered pulsars,
we get magnetic field strength in the Galactic halo as
being about 2μG[2] . Newly determined Faraday rotation
measures of pulsars by FAST gives evidence for themagnetic field reversals along the spiral arms in farther
regions of the first quadrant in the Milky Way.
By processing the piggyback spectral line data, we
get the sky distribution of neutral hydrogen (HI) gas[3] in
88 square degrees between the Galactic longitude of
33°to 55°and within the Galactic latitude of ±2°. Though
the fine calibration is still under way, the results
available are already the most sensitive for detection ofHI gas clouds to date, showing unprecedented detailabout the distribution of HI gas.
The ionized gas of interstellar space is the last major
component of the Milky Way that remains unexplored indetail. We processed the hydrogen radio recombinationlines (RRL) in the GPPS spectral line data in the samesky area as the HI data[4] , revealing luminous regions

Figure: The magnetic field structure in a much large region
in the Galactic disk revealed by the new measurements(color symbols) of pulsar Faraday effect by the FAST.

ionized by bright stars and diffuse ionized gas (DIG) of
unknown origin. The data are indispensable for thestudies of the ecological cycle of gas and star formationin the Milky Way. The results will be introduced by
L.H. Hou in the next talk.
In addition, we tested the scanning observations by
FAST for radio continuum radiation of our Galaxy. The
results in a sky area of 5°×7° confirm that two large faintradio emission structures (G203.1+6.6 and G206.7+5.9)
as being nearby shell-type supernova remnants[5], one ofwhich is produced by supernova explosion very close tothe Sun at only about 1,400 light-years.
References[1] J. L. Han et al, Res. Astron. Astrophys. 21, 107(2021)[2] J. Xu et al, Sci. China-Phys. Mech. Astron. 65,129704 (2022)[3] T. Hong et al, Sci. China-Phys. Mech. Astron. 65,129702 (2022)[4] L.G. Hou et al, Sci. China-Phys. Mech. Astron. 65,129703 (2022)[5] X.Y. Gao et al, Sci. China-Phys. Mech. Astron. 65,129705 (2022)