FAST and Dark: A catalogue of Dark Galaxy Candidates within 50 Mpc

Abstract Using the first data release of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) All-Sky H i survey (FASHI), we compile a catalogue of 70 dark galaxy candidates (DGCs) within 50 Mpc. We select DGCs without an identified optical counterpart at a limiting g-band magnitude of ∼ 28 mag arcsec−2 in the DESI Legacy Survey, using both automatic cross-checking with optical catalogues and visual inspection of the colour images. After validating our DGCs, excluding potential spurious detections, issues in the registered position of the H i sources, and possible Radio Frequency Interferences (RFIs), we analyse their distribution over the surveyed sky, H i mass, linewidths, and inferred distance. They appear evenly distributed across the surveyed area, with no apparent bias to isolation. We did not find any DGC within the Local Volume (11 Mpc) in the sky surveyed by this first release of FASHI. We compare the observed properties of DGCs with those of galaxies with optical counterparts, finding that DGCs tend to have higher linewidths for a given H i mass. We discuss our DGCs in light of theoretical works, and compare them with other observational samples from previous H i surveys. This work presents a catalogue of dark galaxy candidates, which can serve as a basis for follow-up studies.