The FAST Hundred-deg² HI Deep (HD²) Survey

Deep HI Survey of 100 Square Degrees

Survey Overview

The FAST-HD² Survey (Hundred-deg² HI Deep Survey) is a medium-area deep neutral hydrogen (HI) survey carried out with the Five-hundred-meter Aperture Spherical Telescope (FAST). The full survey is designed to map a contiguous ~100 deg² region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes per beam and a uniform detection sensitivity of 0.28 mJy beam⁻¹ at 4.8 km s⁻¹ resolution.

HD2 Logo

Survey Highlights

  • Full survey area: ~100 deg² (pilot: 10 deg²)
  • Full survey sensitivity: 0.28 mJy beam⁻¹ at 4.8 km s⁻¹
  • Redshift coverage: z < 0.4
  • Beam size: ~2.9 arcmin at 1.42 GHz (19-beam receiver)
  • Minimum frequency resolution: 7.6 kHz (1.6 km s⁻¹ at z = 0)
  • Expected HI detection number density: more than 30 per deg²

Key Science Goals

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HI Mass Function

Extend the faint end of the HI mass function. FAST's superior sensitivity enables detection of low-mass, gas-rich dwarf galaxies that elude shallower surveys.

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Large-Scale Structure

Map the spatial distribution and clustering of HI galaxies across ~100 deg² within the DESI DR1 footprint. The survey volume balances cosmic variance and statistical power, providing a robust sample for studying HI in the cosmic web and Bootes Void.

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DESI Synergy & Scaling Relations

Combine HI detections and spectral stacking with DESI DR1 redshifts to measure HI gas fractions across stellar mass, color, SFR, and environment.

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Rare & Exotic Populations

Uncover rare systems invisible to shallow surveys: ultra-diffuse galaxies (UDGs), low surface brightness galaxies, HI-rich early-type galaxies, and dark gas clouds (RELHICs — Reionization-Limited HI Clouds) with no optical counterpart.

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Cosmic HI Density & Intensity Mapping

Constrain ΩHI out to z ~ 0.3 via HI spectral stacking and intensity mapping. FAST's depth enables detection of the average HI signal from faint galaxy populations across cosmic time.

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HI–Halo Mass Relation

Stack HI spectra of galaxy groups from the DESI Legacy Survey group catalog to measure the HI–halo mass relation and compare with simulations. This links neutral gas reservoirs to dark matter halos across a wide range of environments.

Quick Links

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Science Goals

Detailed description of the scientific objectives and expected outcomes.
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Observations

Observation strategy, setup, and current progress.
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Publications

Papers and publications resulting from the HD2 survey.
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Data Products

Download catalogs, spectra, and other data products.

Contact

For questions about the HD2 survey, please contact the survey PI or visit our people page.