AGC 322753: A dark galaxy candidate detected by ALFALFA and FASHI

ABSTRACT We report the possible detection of a dark galaxy candidate (AGC 322753), via the cross-matching of the Arecibo Legacy Fast ALFA (ALFALFA) and the FAST All Sky H i (FASHI) H i blind surveys. It is detected by both surveys at the same recessional velocity, with a comparable H i mass of $2 \times 10^{9} \ {\rm M}_\odot$, and with similar H i spectral features. The ALFALFA and FASHI spectra show a double-peaked profile, compatible with a disc-like ordered rotation, and an H i linewidth at 50 per cent of the peak (W$_{50}$) of $\sim 85$ km s$^{-1}$. The common intersection of the two beams allows the source position to be constrained with much greater precision than in either survey individually. Being detected by both radio telescopes, a radio frequency interference origin is excluded. No evidence of an optical counterpart was found down to $\sim$ 27.5 mag arcsec$^{-2}$ in the DESI Legacy Imaging Surveys, which leads to a stellar mass limit of $M_\bigstar \lesssim 5 \times 10^7 \ {\rm M}_\odot$. Given this stellar mass limit, AGC 322753 has $M_{\rm H\,{\small I}}/M_\bigstar \gtrsim 40$. The H i mass is higher than expected for totally starless haloes; this could imply that AGC 322753 is not completely ‘dark’ but could host a luminous counterpart below the current optical detection limit.