Unveiling the nature and fate of the almost-dark cloud AGC 226178 through HI mapping
arXiv:2506.23232 · doi:10.1051/0004-6361/202555150
Abstract
The origin of extragalactic, almost dark HI clouds with extreme gas-to-stellar mass ratios remains poorly understood. We investigate the nature and fate of the "almost dark" cloud AGC 226178, projected within the Virgo cluster, with an HI-to-stellar mass ratio of ~1000. We present deep single-dish HI 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. These observations provide the highest-quality HI analysis to date of such a cloud, combining resolution and sensitivity. FAST data reveal a short, low-velocity tail toward the dwarf galaxy VCC 2034, previously proposed as a possible origin for AGC 226178. However, VCC 2034 shows a line-of-sight asymmetric HI feature and cometary morphology indicating a stripping event unrelated to AGC 226178. VLA data reveal a velocity gradient across AGC 226178 and a clumpy internal structure. The velocity dispersion exceeds the thermal linewidth, implying turbulence or unresolved motions. The cloud cannot be gravitationally bound by atomic gas alone. The resolved HI clumps follow standard HI mass-star formation rate and mass-size relations, with those forming stars reaching surface densities above the threshold for self-shielding. We conclude that AGC 226178 is a free-floating HI 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 HI morphology within the cluster environment would otherwise require a substantial reservoir of unseen molecular gas with a mass exceeding that of the observed HI content. While confinement pressure from the hot intracluster medium may aid its stability, it is unlikely to be the dominant factor preventing its disruption.
18 pages, 14 figures. Accepted for Publication in A&A
References in corpus (33)
- The Five-Hundred-Meter Aperture Spherical Radio Telescope (FAST) Project
- Ram Pressure Stripping in High-Density Environments
- WALLABY -- An SKA Pathfinder HI Survey
- Ram pressure stripping of disc galaxies orbiting in clusters. I. Mass and radius of the remaining gas disc
- The universal distribution of halo interlopers in projected phase space. Bias in galaxy cluster concentration and velocity anisotropy?
- NGC 4254: An Act of Harassment Uncovered by the Arecibo Legacy Fast ALFA Survey
- Tidal debris from high-velocity collisions as fake dark galaxies: A numerical model of VirgoHI21
- The ALFALFA "Almost Darks" Campaign: Pilot VLA HI Observations of Five High Mass-to-Light Ratio Systems
- FAST: Its Scientific Achievements and Prospects
- The FAST all sky HI survey (FASHI): The first release of catalog
- The properties of "dark" ΛCDM halos in the Local Group
- (Almost) Dark HI Sources in the ALFALFA Survey: The Intriguing Case of HI1232+20
- The Arecibo Legacy Fast ALFA Survey: VI. Second HI Source Catalog of the Virgo Cluster Region
- Systematically Measuring Ultra-Diffuse Galaxies (SMUDGes). V. The Complete SMUDGes Catalog and the Nature of Ultra-Diffuse Galaxies
- It's Cloud's Illusions I Recall: Mixing Drives the Acceleration of Clouds from Ram Pressure Stripped Galaxies
- On structure and kinematics of the Virgo cluster of galaxies
- 21-cm synthesis observations of VIRGOHI 21 - a possible dark galaxy in the Virgo Cluster
- MIGHTEE-HI: The baryonic Tully-Fisher relation over the last billion years
- Gas dynamics in dwarf galaxies as testbeds for dark matter and galaxy evolution
- Star Formation and Quenching of Central Galaxies from Stacked HI Measurements
- FEASTS: IGM cooling triggered by tidal interactions through the diffuse HI phase around NGC 4631
- An almost dark galaxy with the mass of the Small Magellanic Cloud
- HiFAST: an HI data calibration and imaging pipeline for FAST
- A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).XIII. The role of ram-pressure stripping in transforming the diffuse and ultra-diffuse galaxies in the Virgo cluster
- The existence and detection of optically dark galaxies by 21cm surveys
- A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).X. Formation of a red ultra-diffuse galaxy and an almost dark galaxy during a ram-pressure stripping event
- Discovery of an isolated dark dwarf galaxy in the nearby universe
- Hydrodynamic simulations of an isolated star-forming gas cloud in the Virgo cluster
- The ALFALFA Almost-Dark Galaxy AGC~229101: A Two Billion Solar Mass HI Cloud with a Very Low Surface Brightness Optical Counterpart
- The MeerKAT Fornax Survey. III. Ram-pressure stripping of the tidally interacting galaxy NGC 1427A in the Fornax cluster
- AGC 226178 and NGVS 3543: Two deceptive dwarfs towards Virgo
- A Virgo Environmental Survey Tracing Ionised Gas Emission (VESTIGE).XI. Two dimensional H kinematics of the edge-on ram pressure stripped galaxy NGC 4330
- Young, blue, and isolated stellar systems in the Virgo Cluster. I. 2-D Optical spectroscopy