MIGHTEE-HI: Discovery of an HI-rich galaxy group at z = 0.044 with
  MeerKAT
  We present the serendipitous discovery of a galaxy group in the XMM-LSS field
with MIGHTEE Early Science observations. Twenty galaxies are detected in HI in
this $z\sim0.044$ group, with a $3\sigma$ column density sensitivity of
$N_\mathrm{HI} = 1.6\times10^{20}\,\mathrm{cm}^{-2}$. This group has not been
previously identified, despite residing in a well-studied extragalactic legacy
field. We present spatially-resolved HI total intensity and velocity maps for
each of the objects, which reveal environmental influence through disturbed
morphologies. The group has a dynamical mass of
$\log_{10}(M_\mathrm{dyn}/\mathrm{M}_\odot) = 12.32$, and is unusually
gas-rich, with an HI-to-stellar mass ratio of
$\log_{10}(f_\mathrm{HI}^\mathrm{*}) = -0.2$, which is 0.7 dex greater than
expected. The group's high HI content, spatial, velocity, and identified galaxy
type distributions strongly suggest that it is in the early stages of its
assembly. The discovery of this galaxy group is an example of the importance of
mapping spatially-resolved HI in a wide range of environments, including galaxy
groups. This scientific goal has been dramatically enhanced by the high
sensitivity, large field-of-view, and wide instantaneous bandwidth of the
MeerKAT telescope.
