暗弱M31卫星星系仙女座XVI与仙女座XXVIII的恒星金属丰度及梯度
Stellar Metallicities and Gradients in the Faint M31 Satellites Andromeda XVI and Andromeda XXVIII
AI总结：
本文利用哈勃空间望远镜窄带成像首次测量了仙女座XVI和仙女座XXVIII中约300颗恒星的金属丰度及径向梯度，发现两者均呈负梯度且无富金属端截断，结果与FIRE模拟中恒星反馈产生暗物质核的预测一致。
摘要：
我们利用对金属丰度敏感的钙H&K窄带哈勃空间望远镜成像，在两个暗弱的M31矮星系——仙女座XVI（$M_V = -7.5$）和仙女座XXVIII（$M_V = -8.8$）中给出了约300颗恒星的金属丰度测量结果。这是仙女座XVI中首批单颗恒星金属丰度（95颗恒星）。我们的仙女座XXVIII样本（191颗恒星）比文献中的金属丰度数据增加了约15倍。对于仙女座XVI，我们测得$\langle \mbox{[Fe/H]}\rangle = -2.17^{+0.05}_{-0.05}$，$σ_{\mbox{[Fe/H]}}=0.33^{+0.07}_{-0.07}$，以及$\nabla_{\mbox{[Fe/H]}} = -0.23\pm0.15$ dex $R_e^{-1}$。我们发现仙女座XVI比光度相近的银河系超暗矮星系更富金属，这可能是其异常延长的恒星形成历史所致。对于仙女座XXVIII，我们测得$\langle \mbox{[Fe/H]}\rangle = -1.95^{+0.04}_{-0.04}$，$σ_{\mbox{[Fe/H]}}=0.34^{+0.07}_{-0.07}$，以及$\nabla_{\mbox{[Fe/H]}} = -0.46 \pm 0.10$ dex $R_e^{-1}$，使其落在矮星系质量-金属丰度关系上。两个星系均不具有金属丰度分布函数在富金属端突然截断的特征，表明恒星形成是逐渐衰减的。这些恒星金属丰度梯度测量是银河系晕外暗弱（$L \lesssim 10^6~L_{\odot}$）星系中最早的一批测量之一。两个星系的梯度均与FIRE模拟的预测一致，在该模拟中，年龄-梯度强度关系是恒星反馈产生暗物质核的观测结果。我们提供了一个用于后续光谱跟踪观测的星表，其中包括19颗极贫金属（$\mbox{[Fe/H]} < -3.0$）候选恒星，它们占仙女座XVI金属丰度分布函数的7%，占仙女座XXVIII的6%。
英文摘要：
We present $\sim300$ stellar metallicity measurements in two faint M31 dwarf galaxies, Andromeda XVI ($M_V = -7.5$) and Andromeda XXVIII ($M_V = -8.8$) derived using metallicity-sensitive Calcium H & K narrow-band Hubble Space Telescope imaging. These are the first individual stellar metallicities in And~XVI (95 stars). Our And~XXVIII sample (191 stars) is a factor of $\sim15$ increase over literature metallicities. For And~XVI, we measure $\langle \mbox{[Fe/H]}\rangle = -2.17^{+0.05}_{-0.05}$, $σ_{\mbox{[Fe/H]}}=0.33^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.23\pm0.15$ dex $R_e^{-1}$. We find that And XVI is more metal-rich than MW UFDs of similar luminosity, which may be a result of its unusually extended star formation history. For And XXVIII, we measure $\langle \mbox{[Fe/H]}\rangle = -1.95^{+0.04}_{-0.04}$, $σ_{\mbox{[Fe/H]}}=0.34^{+0.07}_{-0.07}$, and $\nabla_{\mbox{[Fe/H]}} = -0.46 \pm 0.10$~dex~$R_e^{-1}$, placing it on the dwarf galaxy mass-metallicity relation. Neither galaxy has a metallicity distribution function with an abrupt metal-rich truncation, suggesting that star formation fell off gradually. The stellar metallicity gradient measurements are among the first for faint ($L \lesssim 10^6~L_{\odot}$) galaxies outside the Milky Way halo. Both galaxies' gradients are consistent with predictions from the FIRE simulations, where an age-gradient strength relationship is the observational consequence of stellar feedback that produces dark matter cores. We include a catalog for community spectroscopic follow-up, including 19 extremely metal poor ($\mbox{[Fe/H]} < -3.0$) star candidates, which make up 7% of And~XVI's MDF and 6% of And~XXVIII's.
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