Unveiling the nature and fate of the almost-dark cloud AGC 226178 through H I mapping

¹ Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
² School of Astronomy and Space Science, University of Science and Technology of China, Hefei 230026, China
³ School of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁴ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
⁵ Departamento de Fisica, Universidad Tecnica Federico Santa Maria, Avenida España, 1680 Valparaíso, Chile
⁶ Millenium Nucleus for Galaxies (MINGAL), Valparaíso, Chile
⁷ Department of Astronomy and Space Science, Graduate School, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, Republic of Korea
⁸ Department of Physics and Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-gu, Seoul, Republic of Korea
⁹ Department of Physics, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong SAR, China
¹⁰ National Research Council of Canada, Herzberg Astronomy and Astrophysics Research Centre, Victoria, BC V9E 2E7, Canada
¹¹ Aix Marseille Univ., CNRS, CNES, LAM, Marseille, France
¹² Université de Strasbourg, CNRS, Observatoire Astronomique de Strasbourg, UMR 7550, Strasbourg, France
¹³ Department of Astronomy & Center for Galaxy Evolution Research, Yonsei University, Seoul 03722, Republic of Korea
¹⁴ University of Calgary, 2500 University Drive NW, Calgary, Alberta T2N 1N4, Canada
¹⁵ Department of Astronomy, Peking University, Beijing, China
¹⁶ Kavli Institute for Astronomy and Astrophysics, Peking University, Beijing, China
¹⁷ Yunnan Observatories, Chinese Academy of Sciences, Kunming 652016, PR China
¹⁸ State Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100101, China

^⋆ Corresponding author: hzhang18@ustc.edu.cn

Received: 14 April 2025
Accepted: 28 June 2025

Abstract

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.