Discovery of a bright-dark binary galaxy in a nearby galaxy group

Abstract One of the puzzles in extragalactic astronomy is the number disparity between the standard cold dark matter (CDM) predictions of low-mass dwarf galaxy and observations. It is often referred to as the missing satellite problem. With a large amount of deep optical surveys, the disparity has considerably decreased, but such problem remains open. The existence of ``dark galaxies", being gas-rich but nearly starless, can offers a standard way to explain the missing satellite problem. To date, many objects are considered as candidates of the dark galaxies, however, none of the candidates can be confirmed with solid evidence. Here we report the discovery of an optically bright-dark binary galaxy in the hub position of a filament in nearby galaxy group NGC 1023, based on HI observation using the Five-hundred-meter Aperture Spherical radio Telescope (FAST). The optically bright member of this binary is the known dwarf galaxy NGC 1023A, while the other member is a newly detected dark galaxy FAST 0240+3905. Besides, the binary galaxy shows a Keplerian rotation, implying that two members of the binary is merging. Our result suggests that even if dark galaxies exist, they can merge with other nearby galaxies in a short time scale. Being merged could provide another important clues to the missing satellite problem.