FATHOMER Survey. III. Preliminary H <scp>I</scp> Galaxy Identification Results

Abstract We present the H i galaxy observation results of FATHOMER, a pilot drift scan survey by the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The survey comprises 28 hr of observations over seven nights in 2021, covering a 60 deg 2 sky area in the frequency range 1.05–1.45 GHz. The H i galaxies are identified using both a matched-filtering algorithm and the SoFiA source-finding pipeline, which yield consistent detections. We derive the velocity width ( W 50 ), flux density, and Hi mass for detected galaxies. A total of 702 galaxies are identified with H i mass above 10 6.2 M ⊙ , signal-to-noise ratio greater than 5, and redshift z &lt; 0.09. Among these, 331 are previously known from the ALFALFA survey. Of the newly detected sources, 9 have spectroscopic confirmation from the Sloan Digital Sky Survey (SDSS), 285 are matched to SDSS or DESI photometric data, and 77 lack optical counterparts–possible candidates for dark or faint galaxies. Comparison with ALFALFA shows that FAST enables the detection of galaxies at higher redshifts and with lower Hi fluxes, despite the radio frequency interference (RFI) and partial data masking. A preliminary Hi mass function analysis reveals a higher characteristic mass and steeper low-mass slope than ALFALFA, indicating FAST’s enhanced sensitivity to massive and distant H i systems. These results demonstrate FAST’s strong potential for future deep H i surveys and highlight the importance of improved RFI mitigation and completeness correction.