FASHI

The FAST All-Sky H I Survey

read as /fɑːʃ/ or “法师”

Overview

FAST Telescope

The FAST All-Sky H I Survey (FASHI) is a pioneering neutral hydrogen (H I) survey conducted using the Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world's most sensitive single-dish radio telescope. FASHI represents the first all-sky H I survey undertaken with FAST, leveraging the telescope's unprecedented sensitivity and wide field of view to conduct a comprehensive census of neutral atomic hydrogen in the local Universe.

Neutral hydrogen is the fundamental building block of galaxies, serving as the primary reservoir of fuel for star formation. Mapping the distribution and content of H I across the sky is essential for understanding the formation and evolution of galaxies, the growth of large-scale structure, and the cycling of baryonic matter in the cosmos. FASHI is designed to detect H I emission from galaxies out to redshifts z < 0.09, probing the local Universe with a sensitivity and source density that far surpasses previous surveys.

The survey is designed to map the entire sky accessible to FAST, covering declinations from −14° to +66° (~22,000 deg²) in the frequency range 1.0–1.5 GHz. As of the second data release (DR2), FASHI has mapped approximately 19,500 deg² of the sky, detecting over 156,000 extragalactic H I sources — making it the largest and most sensitive H I survey ever conducted.

Survey Strategy and Observations

FASHI adopts a flexible drift-scan observing strategy, optimized for FAST's schedule-filler observing mode. Observations are carried out at fixed declinations, enabling an efficient blind search for H I emission while maximizing the utilization of available telescope time. This strategy naturally leads to non-uniform sky coverage, with some regions observed multiple times and thus reaching greater sensitivity.

The survey utilizes FAST's 19-beam receiver array, with a half-power beamwidth of 2.9 arcmin at 1.4 GHz and pointing accuracy better than 15 arcsec. The spectral backend provides 500 MHz bandwidth with 64k channels, yielding a native resolution of 7.63 kHz (~1.6 km s−1 at 1.4 GHz). Data are recorded at 1-second integrations. Intensity calibration employs periodic noise-diode injections, with a beam-dependent gain of 13–17 K Jy−1 at 1400 MHz.

Data Releases

FASHI has released two major data releases, each representing significant milestones in the survey's progress and scientific impact.

FASHI DR1 (2024): The First Release of Catalog

Abstract: The FAST All Sky H I survey (FASHI) was designed to cover the entire sky observable by the Five-hundred-meter Aperture Spherical radio Telescope (FAST), spanning approximately 22,000 square degrees of declination between −14° and +66°, and in the frequency range of 1050–1450 MHz, with the expectation of eventually detecting more than 100,000 H I sources. Between August 2020 and June 2023, FASHI covered more than 7,600 square degrees, approximately 35% of the total sky observable by FAST. It has a median detection sensitivity of around 0.76 mJy beam−1 and a spectral line velocity resolution of ∼6.4 km s−1 at a frequency of ∼1.4 GHz. As of now, a total of 41,741 extragalactic H I sources have been detected in the frequency range 1305.5–1419.5 MHz, corresponding to a redshift limit of z ≲ 0.09. By cross-matching FASHI sources with the Siena Galaxy Atlas (SGA) and the Sloan Digital Sky Survey (SDSS) catalogs, we found that 16,972 (40.7%) sources have spectroscopic redshifts and 10,975 (26.3%) sources have only photometric redshifts. Most of the remaining 13,794 (33.0%) H I sources are located in the direction of the Galactic plane, making their optical counterparts difficult to identify due to high extinction or high contamination of Galactic stellar sources. Based on current survey results, the FASHI survey is an unprecedented blind extragalactic H I survey. It has higher spectral and spatial resolution and broader coverage than the Arecibo Legacy Fast ALFA Survey (ALFALFA). When completed, FASHI will provide the largest extragalactic H I catalog and an objective view of H I content and large-scale structure in the local universe.
FASHI DR1 sky coverage

FASHI DR2 (2026): The FASHI Catalog and the H I Mass Function

Abstract: The FAST All Sky H I survey (FASHI) conducted with the Five-hundred-meter Aperture Spherical radio Telescope (FAST) has mapped ∼19 500 deg2 of the sky north of DEC = −14°, detecting 156 411 extragalactic H I sources at z < 0.09 with a median sensitivity of 0.57 mJy beam−1 at a velocity resolution of 6.4 km s−1. The survey achieves unprecedented depth and area coverage, significantly improving upon previous single-dish surveys. Through a detailed completeness analysis that accounts for the survey's non-uniform sensitivity and line-width dependence, we construct a robust H I mass function (HIMF) using a completeness‑corrected sample of over 109 000 sources. The HIMF is robustly constrained down to MHI ∼ 106.2 M⊙. When systematic uncertainties are included, the HIMF is well described by a single-Schechter function with a characteristic mass log(Ms/h70−2 M⊙) = 9.89 ± 0.02, low-mass end slope α = −1.31 ± 0.02 and amplitude φs = (6.38 ± 0.49) × 10−3 h703 Mpc−3 dex−1. The derived cosmic H I density is ΩHI = (4.71 ± 0.03stat ± 0.40sys) × 10−4 h70−1. FASHI provides the most extensive and sensitive H I catalog to date, establishing an important benchmark for studies of gas accretion, galaxy evolution, and large-scale structure in the local universe.
FASHI DR2 sky coverage

Accessing FASHI Data

In addition to the catalog listed in the DR2 Technical Details table, integrated one-dimensional H I line spectra for all cataloged sources have been extracted from the reduced three-dimensional FITS data cubes. The full FASHI H I source catalog will be made publicly available upon publication of this paper.

The reduced one-dimensional spectral cubelets associated with individual sources are available upon request from the corresponding authors. Access to the reduced three-dimensional data cubes is currently restricted to internal collaboration use; however, a limited subset of these data may be provided upon reasonable request for specific scientific purposes.

All raw observational data from the FASHI project will be released to the public following a twelve-month proprietary period, in accordance with the FAST data release policy. The twelve-month proprietary period applies to each individual scan observation, rather than to the completion of the full survey.

FAST Data Policy Summary:

  • Proprietary Period: 12 months from the date of data collection for PI-time projects and priority/key science projects.
  • Data Access During Proprietary Period: External researchers may apply to the project PI or the FAST Scientific Committee for access to data during the proprietary period for collaborative or independent research.
  • Data Release After Proprietary Period: Data are publicly released by the FAST Operation and Development Center after the proprietary period and can be used without restriction.
  • Data Transfer: Due to bandwidth limitations, it is encouraged to make direct copies of released FAST data using hard disks at the FAST Data Center.
  • Acknowledgment: Publications using FAST data must include the proper acknowledgment: "This work has used the data from the Five-hundred-meter Aperture Spherical radio Telescope (FAST). FAST is a Chinese national mega-science facility, operated by the National Astronomical Observatories of Chinese Academy of Sciences (NAOC)."

Further details regarding FAST data access and release policies are available at FAST Data Policy.

FASHI DR2 Technical Details

Parameters Values
RA range0h ≤ RA ≤ 24h
DEC range−14° ≲ DEC ≲ +66°
Receiver19-beam array
Polarizations per beam2 linear (XX, YY)
Beam size (FWHM)2.9 arcmin at 1420 MHz
Gain13–17 K Jy−1
Tsys16–19 K
Total frequency coverage1050–1450 MHz
Released frequency range1305.7–1422.8 MHz
cz⊙ range−510.0 to 26320.6 km s−1
Redshift z rangez < 0.0878
Spectral channels65,536 (before smoothing)
Channel spacing7.629 kHz or 1.6 km s−1 at 1420 MHz
Spectral resolution6.4 km s−1 (after smoothing)
Median rms σrms~1.04 mJy at 6.4 km s−1 resolution
Median sensitivity fσ~0.57 mJy beam−1 at 6.4 km s−1 resolution
Coverage area19,500 deg2
Source number156,411

Notes: The definitions of the spectral rms σrms and the per-source detection sensitivity fσ are provided in Column 9 of Section 2.4.

Source Extraction

Source extraction for FASHI employs a two-step approach combining automated detection with visual validation:

  1. Blind detection: Initial candidates are identified using SoFiA (Serra et al. 2015; Westmeier et al. 2021) with a 4.5σ detection threshold, which was empirically chosen to provide the optimal trade-off between completeness and reliability.
  2. Optically guided extraction: Given the substantial sky overlap (>12,000 deg²) between FASHI and DESI/SDSS spectroscopic surveys, optical data are leveraged to complement source extraction and improve sample completeness. For each optical counterpart, an emission signal exceeding 3σ is required for detection.
  3. Visual validation: All candidates are inspected interactively using moment maps and integrated profiles to reject false detections and refine source boundaries.

FASHI DR1 Supplementary Materials

  1. The full paper can be downloaded in The FAST All Sky HI Survey (FASHI): The First Release of Catalog
  2. The complete Table 2 containing 41,741 sources can be accessed in FASHI Extragalactic H I Source Catalog
  3. The complete Table 3 containing 3,620 sources can be accessed in FASHI and ALFALFA Cross-Matching Catalog
  4. The complete Table 4 containing 14,072 sources can be accessed in Identified Optical Counterparts from the SGA Catalog
  5. The complete Table 5 containing 2,900 sources can be accessed in Identified Optical Counterparts from the SDSS Spectroscopic Catalog
  6. The complete Table 6 containing 10,975 sources can be accessed in Identified Optical Counterparts from the SDSS Photometric Catalog

Important Publications Related to FASHI

FASHI: An Untargeted Survey of the 21 cm H I Absorption Galaxies with FAST
Zhang, Chuan-Peng; Zhu, Ming; Jiang, Peng; Cheng, Cheng; Xu, Jin-Long; Yu, Nai-Ping; Liu, Xiao-Lan; Zhang, Bo
2025ApJS..276....6Z
FASHI: A Search for Extragalactic OH Megamasers with FAST
Zhang, Chuan-Peng; Cheng, Cheng; Zhu, Ming; Jiang, Peng
2024ApJ...971..131Z
The FAST All Sky H I Survey (FASHI): The First Release of Catalog
Zhang, Chuan-Peng; Zhu, M.; Jiang, P.; Cheng, C.; Wang, J.; Wang, J.; Xu, J.-L.; Liu, X.-L.; Yu, N.-P.; Qian, L.; Yu, H.; Ai, M.; Jing, Y.; Xu, C.; Liu, Z.; Guan, X.; Sun, C.; Yang, Q.; Huang, M.; Hao, Q.; FAST Collaboration
2024SCPMA..6719511Z

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© FASHI Collaboration · FAST Operation and Development Center, NAOC