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The nature of Cloud-9: a compact core embedded in a diffuse envelope
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DOI：10.1093/mnras/stag1293.png)
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Abstract
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We present new H i observations of Cloud-9 with the Five-hundred-meter Aperture Spherical Telescope (FAST), measuring total flux of 0.35 ± 0.03 Jy km s−1. Combined with Very Large Array (VLA) data, the H i shows a two-component structure: a compact, kinematically quiescent core and an extended envelope. The outer component exhibits a coherent velocity gradient of ~25 km s−1 across ~7 kpc. The position–velocity structure does not support rotation; instead, the gradient is consistent with ordered large-scale motion, induced by environmental interaction with M94. The coexistence of a compact, kinematically quiescent core and an extended structured envelope is difficult to reconcile with a unbound or purely gaseous system. While the extended component is more naturally interpreted as environmentally shaped gas, the compact core suggests gravitational confinement beyond that provided by the observed baryonic content, without requiring strict long-term dynamical equilibrium. Deep optical limits further rule out a significant stellar component. Taken together, our results indicate that Cloud-9 is most naturally interpreted as a dark matter-dominated system undergoing environmental interaction. The observed morphology and kinematics are consistent with a stripped RELHIC scenario, in which a compact gas-rich core is embedded within an extended envelope shaped by ram-pressure interaction with the circumgalactic medium of M94.
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