2025A&A...701A..73S

Context. The origin of extragalactic, almost dark H I clouds with extreme gas-to-stellar mass ratios remains poorly understood.
Aims. We investigated the nature and fate of the almost dark cloud AGC 226178, projected within the Virgo cluster, which exhibits an H I-to-stellar mass ratio of approximately 1000.
Methods. We present deep single-dish H I mapping from the Five-hundred-meter Aperture Spherical Telescope (FAST), complemented by high-resolution interferometric data from the Very Large Array (VLA), as part of the Atomic gas in Virgo Interacting Dwarf galaxies (AVID) project. Together, these observations provide the highest-quality H I analysis to date of an almost dark cloud, in terms of the combination of spatial resolution and sensitivity.
Results. The FAST data reveal a short low-velocity tail extending towards the dwarf galaxy VCC 2034, which was previously proposed as a possible origin for AGC 226178. However, VCC 2034 itself exhibits a line-of-sight asymmetric H I feature and a cometary morphology that indicates a stripping event unrelated to AGC 226178. VLA observations reveal a velocity gradient across AGC 226178, along with a clumpy internal structure. The velocity dispersion within the cloud exceeds the thermal line width, indicating the presence of turbulence and/or unresolved random motions. The whole cloud cannot be gravitationally bound by the observed atomic gas alone. The resolved H I clumps follow the standard H I mass-star formation rate relation and a tight mass-size relation: those associated with star formation reach surface densities above the theoretical threshold for self-shielding.
Conclusions. We conclude that AGC 226178 is a free-floating H I cloud of unknown origin. The system appears to be in the process of disintegration. It is likely located well outside the Virgo cluster, as the preservation of its extended H I morphology within the cluster environment would otherwise require a substantial reservoir of unseen molecular gas with a mass exceeding that of the observed H I content. While confinement pressure from the hot intracluster medium may contribute to its stability, it is unlikely to be the dominant factor preventing its disruption.