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 HI cloud complex in the GAMA G23 field, located at a redshift of $z\sim 0.03$, 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 $10^{10.0}{M}_{\odot }$, lies in proximity to a large galaxy group with $M_{\mathrm{dyn}}\sim 10^{13.5}{M}_{\odot }$. We identify seven HI peak concentrations, interconnected as a tenuous 'chain' structure, extending $\sim 400\mathrm{kpc}$ from east-to-west, with the largest (central) concentration containing $10{}^{9.7}{M}_{\odot }$ in HI gas distributed across $50\mathrm{kpc}$. The main source is not detected in ultra-violet, optical or infrared imaging. The implied gas mass-to-light ($M_{\mathrm{HI}}$/$L_{\mathrm{r}}$) is extreme ($>$1000) even in comparison to other 'dark clouds'. The complex has very little kinematic structure ($110\mathrm{km}{\mathrm{s}}^{-1}$), making it difficult to identify cloud rotation. Assuming pressure support, the total mass of the central concentration is $>10^{10.2}{M}_{\odot }$, while a lower limit to the dynamical mass in the case of full rotational support is $10^{10.4}{M}_{\odot }$. 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.

25 pages, 10 figures, accepted for publication in ApJ, minor corrections

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