The FAST-M31 HI Deep Survey

HI Mapping to the halo region of the Andromeda Galaxy

Survey Overview

The FAST-M31 Survey is a deep neutral hydrogen (HI) mapping project targeting the Andromeda Galaxy (M31) and its surrounding ~700 deg² halo region using the FAST telescope. The survey probes diffuse HI emission out to a projected radius of 160 kpc from M31's center, covering its satellite system, circumgalactic medium, and background extragalactic sources — making it the most sensitive wide-field HI survey of the Local Group to date.

M31 HI Halo Map

FAST HI moment 0 map of the M31 halo region integrated over the velocity range of -630 to -50 km s⁻¹. (Credit: NAOC-CCG)

Survey Highlights

  • Target: M31 halo, satellites, HVCs, and background HI galaxies
  • Coverage: ~700 deg² (radius ~160 kpc from M31 center)
  • Radial velocity range: -1000 – 24,000 km s⁻¹ (z < 0.08)
  • Map rms sensitivity: 0.8 mJy beam⁻¹ at 4.8 km s⁻¹
  • Beam size: ~2.9 arcmin at 1.42 GHz (~660 pc at M31)
  • Velocity resolution: 1.6 km s⁻¹ (W band), 4.8 km s⁻¹ (rebinned), 10 km s⁻¹ (Hann-smoothed)
  • Background HI detections: ~10,000 sources at z < 0.08

Why M31?

The Andromeda Galaxy offers unique advantages as a laboratory for resolved HI science in the Local Group:

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Proximity

At ~780 kpc, M31 is close enough to resolve individual HI features at ~660 pc per FAST beam — bridging the gap between resolved Local Group studies and distant galaxy surveys.

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Extended Halo

FAST's large beam and single-dish sensitivity are ideal for detecting diffuse, extended HI emission in M31's halo and CGM — structures resolved out by interferometers like VLA and WSRT.

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Rich Interaction History

M31's satellite system — including M33, NGC 205, Andromeda II, and dozens of dwarf galaxies — shows clear signs of tidal stripping and past interactions, traceable in HI at FAST's sensitivity.

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Background Survey Bonus

The deep integration over 700 deg² yields ~10,000 background HI galaxy detections — far exceeding ALFALFA and FASHI coverage in the same field — enabling independent cosmological studies.

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 M31 survey.
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Data Products

Download cubes, maps, and other data products.

Contact

For questions about the M31 survey, please contact the survey PI or visit our people page.