Examining the Nature of the Starless Dark Matter Halo Candidate Cloud-9 with Very Large Array Observations

Abstract Observations with the Five-hundred-meter Aperture Spherical Telescope recently detected H i 21 cm emission near M94, revealing an intriguing object, Cloud-9, without an optical counterpart. Subsequent analysis suggests that Cloud-9 is consistent with a gas-rich ( M H I ≈ 10 6 M ⊙ ), starless, dark matter (DM) halo of mass M 200 ≈ 5 × 10 9 M ⊙ . Using the Karl G. Jansky Very Large Array in D-array configuration, we present interferometric observations of Cloud-9, revealing it as a dynamically cold ( W 50 ≈ 12 km s −1 ), nonrotating, and spatially asymmetric system, exhibiting gas compression on one side and a tail-like structure toward the other—features likely originating from ram pressure. Our observations suggest Cloud-9 is consistent with a starless ΛCDM DM halo if the gas is largely isothermal. If interpreted as a faint dwarf, Cloud-9 is similar to Leo T, a nearby gas-rich galaxy that would fall below current optical detection limits at Cloud-9's distance ( d ≈ 5 Mpc). Further observations with the Hubble Space Telescope reaching magnitudes m g ≈ 30 would help identify such a galaxy or dramatically lower the current limits on its stellar mass ( M gal ≲ 10 5 M ⊙ ). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.