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

FAST observations have recently identified a compact HI cloud (hereafter Cloud-9) in the vicinity of the spiral galaxy M94. This identification has been confirmed independently by VLA and GBT 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), non-rotating, and fairly massive, with an HI mass of ~ 10⁶ M_. Here we present deep HST/ACS imaging designed to search for a luminous stellar counterpart. We visually rule out the presence of any dwarf galaxy with stellar mass exceeding 103.5M_{}$. A more robust color-magnitude diagram-based analysis rules out a 10$⁴M_ stellar counterpart with 99.5$%$ confidence. The non-detection of a luminous component reinforces the interpretation that this system is a Reionization-Limited HI 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 HI cloud. This provides strong support for a cornerstone prediction of the $Λ$CDM model, namely the existence of gas-filled starless dark matter halos on sub-galactic mass scales, and constrains the present-day threshold halo mass for galaxy formation.