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.

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UGC 8839的超稳定光盘

除了较大的| $ \ rm H \，{\ small II} $ |外，低表面亮度螺旋UGC 8839几乎没有恒星形成。位于极外盘的区域复合体。为了理解该复合体的起源和性质，我们比较新的| $ \ rm H \，\ alpha $ |。以及UGC 8839的存档宽带图像与其他四个旋涡星系的相似数据。我们得出结论，UGC 8839中极轴外恒星的形成很可能是由于超稳定圆盘在所有半径上都由暗物质主导，而Toomre参数仅在极外圆盘中达到最小值。使用设计为对低表面亮度对象的陷阱和小数量统计数据特别不敏感的分析策略，我们确定当| $ \ rm H \，{\ small II} $时，UGC 8839中这种复合物的存在不是异常。| 区域光度函数由幂定律建模，表明它是本机结构而不是合并卫星。但是，我们发现| $ \ rm H \，{\ small II} $ |的全部人口。与| $ \ rm H \，{\ small II} $ |相比，UGC 8839中的区域显示出较大的偏心半径。我们样本中其他星系的区域。UGC 8839突显了重子/暗物质比率与圆盘稳定性之间的关系。三体相互作用类似于按比例缩小版本的怀疑是马林1号星的相互作用，这与我们在UGC 8839扩展盘中看到的极端外盘恒星形成是一致的。