The Hyper-Stable Disc of UGC 8839
  The low surface brightness (LSB) spiral UGC 8839 is nearly devoid of star
formation aside from a large HII region complex located in the extreme outer
disc. In order to understand the origin and nature of this complex, we compare
new H{\alpha} and archival broad-band images of UGC 8839 to similar data for
four other spiral galaxies. We conclude that the extreme off-axis star
formation in UGC 8839 is likely due to a hyper-stable disc that is dark matter
dominated at all radii, with the Toomre parameter reaching a minimum only in
the extreme outer disc. Using analysis strategies designed to be particularly
insensitive to the pitfalls of low-surface brightness objects and small number
statistics, we determine that the presence of this complex in UGC 8839 is not
exceptional when the HII region luminosity function is modelled by a power law,
suggesting that it is a native structure and not a merging satellite. However,
we find that the entire population of HII regions in UGC 8839 shows a
preference for larger galactocentric radii when compared to HII regions in the
other galaxies in our sample. UGC 8839 dramatically highlights the relationship
between the baryonic/dark matter ratio and disk stability. A three-body
interaction, similar to a scaled-down version of the interaction suspected to
be responsible for Malin 1, is consistent with the extreme outer disk star
formation that we see in the extended disk of UGC 8839.
