Not So Round: VLA Observations of the Starless Dark Matter Halo Candidate Cloud-9

Observations with FAST recently detected HI 21-cm emission near M94, revealing an intriguing object, Cloud-9, without an optical counterpart. Subsequent analysis suggests Cloud-9 is consistent with a gas-rich (MHI ≈ 10⁶ \ M_), starless dark matter (DM) halo of mass M₂₀₀ ≈ 5 × 10⁹ \ M_. Using VLA in D-array configuration, we present interferometric observations of Cloud-9 revealing it as a dynamically cold (W₅₀ ≈ 12 \ km \ s⁻¹), non-rotating, and spatially-asymmetric system, exhibiting gas compression on one side and a tail-like structure towards the other, features likely originating from ram pressure. Our observations suggest Cloud-9 is consistent with a starless ΛCDM dark matter 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 HST reaching magnitudes mg ≈ 30 would help identify such a galaxy or dramatically lower current limits to its stellar mass (Mgal 10⁵ \ M_). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.