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

Abstract

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 ($M_{\rm HI} \approx 10^{6} \ M_{\odot}$), starless dark matter (DM) halo of mass $M_{200} \approx 5 \times 10^{9} \ M_{\odot}$. Using VLA in D-array configuration, we present interferometric observations of Cloud-9 revealing it as a dynamically cold ($W_{50} \approx 12 \rm \ km \ s^{-1}$), 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 $\Lambda$CDM 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 ($d\approx 5 \rm \ Mpc$). Further observations with HST reaching magnitudes $m_{g} \approx 30$ would help identify such a galaxy or dramatically lower current limits to its stellar mass ($M_{\rm gal} \lesssim 10^{5} \ M_{\odot}$). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.