The hyper-stable disc of UGC 8839

ABSTRACT The low surface brightness spiral UGC 8839 is nearly devoid of star formation aside from a large $\rm H\,{\small II}$ region complex located in the extreme outer disc. In order to understand the origin and nature of this complex, we compare new $\rm H\,\alpha$ 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 $\rm H\,{\small II}$ 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 $\rm H\,{\small II}$ regions in UGC 8839 shows a preference for larger galactocentric radii when compared to $\rm H\,{\small II}$ regions in the other galaxies in our sample. UGC 8839 dramatically highlights the relationship between the baryonic/dark matter ratio and disc 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 disc star formation that we see in the extended disc of UGC 8839.