2025ApJ...993L..55A

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 ($\sim 1^{\prime} \rm{}\,\rm{}\,$radius, or ∼1.4 kpc), dynamically cold (W₅₀ = 12 km s^–1), nonrotating, and fairly massive, with an H I mass of ∼10⁶M_⊙. 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 10^3.5M_⊙. A more robust color-magnitude diagram-based analysis conservatively rules out a 10⁴M_⊙ 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.