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

Neutral hydrogen (H i ) 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 receivers in the frequency range of 1.0–1.5 GHz, and each pointing has an integration time of 5 min. The piggyback spectral data simultaneously recorded during the FAST GPPS survey are great resources for studies on the Galactic H i distribution and ionized gas. We process the piggyback H i 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 H i by far. The high velocity resolution and high sensitivity of the FAST GPPS H i data enable us to detect weak exquisite H i structures in the interstellar medium. H i absorption line with great details can be obtained against bright continuum sources. The FAST GPPS survey piggyback H i data cube will be released and updated on the web: http://zmtt.bao.ac.cn/MilkyWayFAST/ .