DECam MAGIC巡天：揭示Crater II矮星系的潮汐尾
The DECam MAGIC Survey: Uncovering the Tidal Tails of the Crater II Dwarf Galaxy
AI总结：
利用CaHK窄带测光数据，发现Crater II矮星系至少延伸7度的潮汐尾，并首次探测到其中心金属丰度梯度，表明该星系已损失超过25%的初始恒星质量。
摘要：
Crater II (CraII) 是一个大而低密度的矮球状星系，其不寻常的观测特性难以在冷暗物质模拟中重现。持续的潮汐瓦解可能有助于解释这些差异，最近发现的潮汐尾便是证据。本文展示了来自暗能量相机的Ca II H和K线金属丰度敏感窄带测光数据，覆盖了CraII中心及已识别潮汐尾的128平方度天区，作为Mapping the Ancient Galaxy in CaHK (MAGIC)巡天的一部分。我们的联合测光金属丰度、颜色-星等、自行和视差选择识别出162个CraII候选体。其中，37个候选体位于至少从CraII中心延伸7度（约95 kpc）的潮汐尾中，表明它已损失了至少25%的初始恒星质量。我们通过专用控制场确认了低污染率，并强调了CaHK数据能够揭示极低表面亮度的恒星特征，暗至约36 mag arcsec$^{-2}$。我们还首次在星系中心探测到金属丰度梯度（$-0.34\pm0.17~{\rm dex}~{\rm deg}^{-1}$），并推断出流宽度约为$w\sim 0.8^\circ$，比CraII的半光半径大约50%。在距CraII中心最远的指向中探测到候选体，表明潮汐尾延伸超出我们的覆盖范围。我们将CraII流与具有“核”和“尖点”暗物质晕前身的$N$体模型进行比较，确定CraII的密度分布仍然不明确，需要进一步建模。
英文摘要：
Crater II (CraII), a large and low-density dwarf spheroidal galaxy, has unusual observed properties that are difficult to reproduce in cold dark matter simulations. Ongoing tidal disruption may help explain the discrepancies, as evidenced by the recent discovery of tidal tails. Here we present metallicity-sensitive narrowband photometry of the Ca II H and K lines from the Dark Energy Camera, covering $128$ deg$^2$ across the center and identified tidal tails of CraII as part of the Mapping the Ancient Galaxy in CaHK (MAGIC) survey. Our combined photometric metallicity, color-magnitude, proper motion, and parallax selections identify 162 CraII candidates. Of these, 37 candidates are located in the tidal tails which extend at least $7^\circ$ ($\sim 95$ kpc) from the center of CraII, suggesting it has lost $\gtrsim 25$% of its initial stellar mass. We confirm low contamination rates with dedicated control fields and highlight the extremely low surface brightness stellar features that can be uncovered with CaHK data, as faint as $\sim 36$ mag arcsec$^{-2}$. We also make the first detection of a metallicity gradient ($-0.34\pm0.17~{\rm dex}~{\rm deg}^{-1}$) in the center of the galaxy and infer a stream width of $w\sim 0.8^\circ$, roughly 50% larger than the CraII half-light radius. The detection of candidates in the most distant CraII pointings from its center implies that the tidal tails extend beyond our footprint. We compare the CraII stream to $N$-body models with "cored" and "cuspy" dark matter halo progenitors, determining that CraII's density profile is still ambiguous and warrants further modeling.
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