用UNIONS发现遥远的Milky Way化石：NGC 5466及其恒星流
Uncovering fossils of the distant Milky Way with UNIONS: NGC 5466 and its stellar stream
摘要：
我们研究的空间集群的蓝色水平分支（BHB）恒星的$\textit{u}$ -带的加拿大-法国成像调查（CFIS，紫外近红外光学北方调查，或UNIONS）。所有主要的恒星群都与先前已知的卫星有关，其中包括NGC 5466，一个遥远的（16 kpc）球状星团。据报道，NGC 5466拥有一个很长的恒星流，尽管之前没有发现过该恒星流的单个成员。使用BHB和更多的红巨星分支星交叉匹配到$\textit{Gaia}$数据发布2，我们确定延长潮汐尾从NGC 5466的空间和运动学相干。有趣的是，我们发现这个流并不遵循与之前在离集群很远的地方检测到的路径相同的路径。我们跟踪流横跨31 $^{\circ}$的天空，并表明它表现出非常强的距离梯度范围从10 $<$ R $_{helio}$ $<$ 30千秒差距。我们比较我们的观察到简单的动力学模型的流，发现他们能够广泛地再现的整体路径和运动学。事实上，NGC 5466是如此遥远，追踪了广泛的银河系距离，有一个确定的祖先，似乎最近与银河系的磁盘有过相互作用，使其成为银河系动力学建模的独特测试案例。
英文摘要：
We examine the spatial clustering of blue horizontal branch (BHB) stars from the $\textit{u}$-band of the Canada-France Imaging Survey (CFIS, a component of the Ultraviolet Near-Infrared Optical Northern Survey, or UNIONS). All major groupings of stars are associated with previously known satellites, and among these is NGC 5466, a distant (16 kpc) globular cluster. NGC 5466 reportedly possesses a long stellar stream, although no individual members of the stream have previously been identified. Using both BHBs and more numerous red giant branch stars cross-matched to $\textit{Gaia}$ Data Release 2, we identify extended tidal tails from NGC 5466 that are both spatially and kinematically coherent. Interestingly, we find that this stream does not follow the same path as the previous detection at large distances from the cluster. We trace the stream across 31$^{\circ}$ of sky and show that it exhibits a very strong distance gradient ranging from 10 $<$ R$_{helio}$ $<$ 30 kpc. We compare our observations to simple dynamical models of the stream and find that they are able to broadly reproduce the overall path and kinematics. The fact that NGC 5466 is so distant, traces a wide range of Galactic distances, has an identified progenitor, and appears to have recently had an interaction with the Galaxy's disk, makes it a unique test-case for dynamical modelling of the Milky Way.
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