The First RELHIC? Cloud-9 is a Starless Gas Cloud

Five-hundred-meter Aperture Spherical Telescope observations have recently identified a compact H I cloud (hereafter Cloud-9) in the vicinity of the spiral galaxy M94. This identification has been confirmed independently by Very Large Array and Green Bank Telescope observations. Cloud-9 has the same recession velocity as M94, and is therefore at a similar distance (∼4.4 Mpc). It is compact ( ∼1' radius, or ∼1.4 kpc), dynamically cold (W50 = 12 km s−1), nonrotating, and fairly massive, with an H I mass of ∼106 M⊙. Here we present deep Hubble Space Telescope/Advanced Camera for Surveys imaging designed to search for a luminous stellar counterpart. We visually rule out the presence of any dwarf galaxy with stellar mass exceeding 103.5 M⊙. A more robust color─magnitude diagram-based analysis conservatively rules out a 104 M⊙ stellar counterpart with 99.5−8.2+0.5 % confidence. The nondetection of a luminous component reinforces the interpretation that this system is a reionization-limited H I cloud (RELHIC); i.e., a starless dark matter halo filled with hydrostatic gas in thermal equilibrium with the cosmic ultraviolet background. Our results make Cloud-9 the leading RELHIC candidate of any known compact H I cloud. This provides strong support for a cornerstone prediction of the Lambda cold dark matter model, namely the existence of gas-filled starless dark matter halos on subgalactic mass scales, and constrains the present-day threshold halo mass for galaxy formation.