The detection of a massive chain of dark HI clouds in the GAMA G23 Field

Abstract

We report on the detection of a large, extended H i cloud complex in the GAMA G23 field, located at a redshift of z ∼ 0.03, observed as part of the MeerHOGS campaign (a pilot survey to explore the mosaicing capabilities of MeerKAT). The cloud complex, with a total mass of 10 10.0 M , lies in proximity to a large galaxy group with M dyn ∼ 10 13.5 M . We identify seven H i peak concentrations, interconnected as a tenuous 'chain' structure, extending ∼400 kpc from east-to-west, with the largest (central) concentration containing 10 9.7 M in H i gas distributed across 50 kpc. The main source is not detected in ultra-violet, optical or infrared imaging. The implied gas mass-to-light (M HI /L r ) is extreme (>1000) even in comparison to other 'dark clouds'. The complex has very little kinematic structure (110 km s −1 ), making it difficult to identify cloud rotation. Assuming pressure support, the total mass of the central concentration is > 10 10.2 M , while a lower limit to the dynamical mass in the case of full rotational support is 10 10.4 M . If the central concentration is a stable structure, it has to contain some amount of unseen matter, but potentially less than is observed for a typical galaxy. It is, however, not clear whether the structure has any gravitationally stable concentrations. We report a faint UV-optical-infrared source in proximity to one of the smaller concentrations in the gas complex, leading to a possible stellar association. The system nature and origins is enigmatic, potentially being the result of an interaction with or within the galaxy group it appears to be associated with.