La Silla-QUEST 天琴座RR型变星巡天 II：Crater II 潮汐流
La Silla-QUEST RR Lyrae Star Survey II: The Crater II Tidal Streams
AI总结：
本研究利用改进的La Silla-QUEST巡天数据，在Crater II矮星系周围发现延伸的潮汐流，结合Gaia数据识别RGB候选星，估计该星系已损失约30%以上恒星质量。
摘要：
我们描述了La Silla-QUEST天琴座RR型变星（RRLS）巡天中的测光改进，这些改进使其能够达到距太阳（$d_{\odot}$）约140 kpc的距离。我们报告了对大型、低表面亮度矮球状星系Crater II周围约300平方度天区的巡天结果。在$d_{\odot}$ > 80 kpc处，我们发现了一个大的RRLS过密度区域，其延伸超出了传统上用于Crater II的等照度轮廓。这些RRLS中的大多数（34个）在天空上呈线性分布，延伸超过15度，穿过Crater II并沿其自行方向定向。我们假设这种不太可能出现的分布追踪了与Crater II相关的延伸潮汐流。为了验证这一点，我们搜索了应该存在于这些流中的其他Crater II恒星族群。利用Gaia自行数据，我们在Crater II之外分离出约17颗候选恒星，它们与来自Crater II红巨星分支（RGB）的明亮恒星一致。它们的空间分布与RRLS的分布一致。推断出的流很长，距离范围从银河系中心约80至135 kpc。它们相对于我们的视线方向以较小的角度（约25度）定向，这意味着一些流恒星可能投影到星系主体上。通过比较流中发现的RRLS和RGB候选恒星数量与星系主体中的数量，我们估计Crater II已经损失了其恒星质量的约30%以上。
英文摘要：
We describe photometry improvements in the La Silla--Quest RR Lyrae star (RRLS) survey that enable it to reach distances from the Sun ($d_{\odot}$) $\sim 140$ kpc. We report the results of surveying $\sim 300~ {\rm deg}^2$ of sky around the large, low-surface-brightness Crater II dwarf spheroidal galaxy. At $d_{\odot}$ $> 80$ kpc, we find a large overdensity of RRLS that extends beyond the traditional isophotal contours used for Crater II. The majority of these RRLS (34) have a linear distribution on the sky, extending over $15^{\circ}$, that runs through Crater II and is oriented along Crater II's proper motion vector. We hypothesize that this unlikely distribution traces extended tidal streams associated with Crater II. To test this, we search for other Crater II stellar populations that should be in the streams. Using Gaia proper motion data, we isolate $\approx$ 17 candidate stars outside of Crater II that are consistent with being luminous stars from the Crater II Red Giant Branch (RGB). Their spatial distribution is consistent with the RRLS one. The inferred streams are long, spanning a distance range $\sim 80 - 135$ kpc from the Galactic Centre. They are oriented at a relatively small angle relative to our line-of-sight ($\sim 25^{\circ}$), which means some stream stars are likely projected onto the main body of the galaxy. Comparing the numbers of RRLS and RGB candidate stars found in the streams to those in the main galaxy, we estimate Crater II has lost $\gtrsim 30\%$ of its stellar mass.
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