标题： 猎户座II：再电离时期的化石，带有中心偏移的星团 显示英文标题

标题： Eridanus II: A Fossil from Reionization with an Off-Center Star Cluster

摘要： 我们展示了对超暗弱矮星系Eridanus II（Eri II）的深度哈勃空间望远镜（HST）测光数据。Eri II的绝对视星等为M_V = -7.1，位于距离339 kpc处，刚好在银河系的洛希半径之外。我们确定了Eri II的恒星形成历史，并测量了该星系及其星团的结构。我们发现，一个由两次恒星形成爆发组成的恒星形成历史，受到该星系光谱金属丰度分布的限制，准确描述了Eri II的恒星种群。最佳拟合模型表明，在早期时间恒星形成迅速终止，超过80%的恒星质量在z~6之前就已经存在。一小部分恒星可能年轻至8 Gyr，但这一种群并不具有统计显著性；蒙特卡罗模拟仅在15%的情况下恢复出小于9 Gyr的成分，其中它们平均占种群的7 +/- 4%。这些结果与再电离导致恒星形成停止的理论预期一致。HST的深度和角分辨率使我们能够证明，Eri II的星团相对于星系中心的投影距离为23 +/- 3 pc。这种偏移可能表明Eri II中存在一个小的（~50-75 pc）暗物质核心。此外，我们证明该星团具有高的椭率0.31 +0.05/-0.06，并且其取向与Eri II的取向在3 +/- 6度内一致，这可能是由于潮汐作用所致。该星团的恒星种群与Eri II本身的种群无法区分。 显示英文摘要

摘要： We present deep Hubble Space Telescope (HST) photometry of the ultra-faint dwarf galaxy Eridanus II (Eri II). Eri II, which has an absolute magnitude of M_V = -7.1, is located at a distance of 339 kpc, just beyond the virial radius of the Milky Way. We determine the star formation history of Eri II and measure the structure of the galaxy and its star cluster. We find that a star formation history consisting of two bursts, constrained to match the spectroscopic metallicity distribution of the galaxy, accurately describes the Eri II stellar population. The best-fit model implies a rapid truncation of star formation at early times, with >80% of the stellar mass in place before z~6. A small fraction of the stars could be as young as 8 Gyr, but this population is not statistically significant; Monte Carlo simulations recover a component younger than 9 Gyr only 15% of the time, where they represent an average of 7 +/- 4% of the population. These results are consistent with theoretical expectations for quenching by reionization. The HST depth and angular resolution enable us to show that Eri II's cluster is offset from the center of the galaxy by a projected distance of 23 +/- 3 pc. This offset could be an indication of a small (~50-75 pc) dark matter core in Eri II. Moreover, we demonstrate that the cluster has a high ellipticity of 0.31 +0.05/-0.06 and is aligned with the orientation of Eri II within 3 +/- 6 degrees, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.