2023ApJ...956....1B

Observations with the Five-hundred-meter Aperture Spherical Telescope have revealed the presence of a marginally resolved source of 21 cm emission from a location $\sim 50^{\prime} $ from the M94 galaxy, without a stellar counterpart down to the surface brightness limit of the DESI Imaging Legacy Survey (∼29.15 mag arcsec^–2 in the g band). The system (hereafter Cloud-9) has round column density isocontours and a line width consistent with thermal broadening from gas at T ∼ 2 × 10⁴ K. These properties are unlike those of previously detected dark H I clouds and similar to the expected properties of REionization-Limited-H I Clouds (RELHICs), namely, starless dark matter (DM) halos filled with gas in hydrostatic equilibrium and in thermal equilibrium with the cosmic ultraviolet background. At the distance of M94, d ∼ 4.7 Mpc, we find that Cloud-9 is consistent with being a RELHIC inhabiting a Navarro-Frenk-White (NFW) DM halo of mass M₂₀₀ ∼ 5 × 10⁹M_⊙ and concentration c_NFW ∼ 13. Although the agreement between the model and observations is good, Cloud-9 appears to be slightly, but systematically, more extended than expected for ΛCDM RELHICs. This may imply either that Cloud-9 is much closer than implied by its recessional velocity, v_CL9 ∼ 300 km s^–1, or that its halo density profile is flatter than NFW, with a DM mass deficit greater than a factor of 10 at radii r ≲ 1 kpc. Further observations may aid in constraining these scenarios better and help elucidate whether Cloud-9 is the first ever observed RELHIC, a cornerstone prediction of the ΛCDM model on the smallest scales.