2024/06/26 by Benítez-Llambay, Alejandro, Dutta, Rajeshwari, Fumagalli, Michele +1
#Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences
paper · doi:10.48550/arxiv.2406.18643
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 ≈ 106 M\odot), starless dark matter (DM) halo of mass M200 ≈ 5 × 109 M\odot. Using VLA in D-array configuration, we present interferometric observations of Cloud-9 revealing it as a dynamically cold (W50 ≈ 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 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 \rm Mpc). Further observations with HST reaching magnitudes mg ≈ 30 would help identify such a galaxy or dramatically lower current limits to its stellar mass (M\rm gal \lesssim 105 M\odot). Cloud-9 thus stands as the firmest starless DM halo candidate to date or the faintest galaxy known at its distance.