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

Not So Round: VLA Observations of the Starless Dark Matter Halo Candidate Cloud-9
Alejandro Ben’itez-Llambay, Rajeshwari Dutta, Michele Fumagalli, Julio F. Navarro
arXiv:2406.18643v1 Announce Type: new
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 ($dapprox 5 rm Mpc$). 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.arXiv:2406.18643v1 Announce Type: new
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 ($dapprox 5 rm Mpc$). 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.