The FAST Hundred-deg 2 H I Deep (HD 2 ) Survey: Early Results from the Pilot Survey

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Abstract The Hundred-deg 2 H I Deep (HD 2 ) survey carried out with the Five-hundred-meter Aperture Spherical Telescope is planned to map a contiguous region within the DESI DR1 footprint, achieving an effective integration time of 20 minutes for each pointing and a uniform detection sensitivity of 0.28 mJy beam −1 at 4.8 km s −1 resolution. We present early results from the pilot HD 2 survey: a 10 deg 2 field overlapping with the Hyper Suprime-Cam Subaru Strategic Program and the DESI early data release SV3, observed with an integration time of 7.3 minutes beam −1 and an rms of 0.45 mJy beam −1 at 4.8 km s −1 resolution. We identify 339 H I sources at z < 0.09, corresponding to ∼34 detections per deg 2 , nearly 6 times higher than the detection rate of the wide-field surveys. Optical counterparts are primarily identified using DESI redshifts, yielding a matching rate and correctness exceeding 90% for galaxies with r < 19.5 mag, a substantial improvement over the Sloan Digital Sky Survey. Under the constraint of r < 17.8 mag and 0.01 < z < 0.05, nearly 50% of galaxies in the DESI Bright Galaxy Survey samples have H I detections in this pilot survey. The optical properties of these H I -detected galaxies span nearly the entire parameter range of the DESI sample. The gas fraction scaling relations versus stellar mass, stellar mass surface density, near-UV (NUV) − r , and specific star formation rate are consistent with previous surveys, e.g., ALFALFA, DINGO, and xGASS. These results justify the feasibility of the full HD 2 survey, which will build a high-completeness H I census over a contiguous area to probe the cold gas scaling relations of galaxies over different scales.

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