Eridanus II: A Fossil from Reionization with an Off-center Star Cluster
Eridanus II: A Fossil from Reionization with an Off-center Star Cluster
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DOI：
10.3847/1538-4357/abd31b
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发表时间：
2020-11
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影响因子：
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通讯作者：
J. Simon;T. Brown;A. Drlica-Wagner;Ting S. Li;R. Avila;K. Bechtol;G. Clementini;D. Crnojević
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文献类型：
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作者：
J. Simon;T. Brown;A. Drlica-Wagner;Ting S. Li;R. Avila;K. Bechtol;G. Clementini;D. Crnojević
介绍了超暗矮星Eridanus II(Eri II)的深哈勃太空望远镜(HST)测光技术。ERIII的绝对震级为MV=−7.1，距离为339Kpc，刚好超出银河系的维里半径。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，由两次爆发组成的恒星形成历史，约束为匹配星系的光谱金属丰度分布，准确地描述了Eri II恒星群。最佳拟合模型意味着恒星形成早期的快速截断，在z∼6之前，恒星质量的80%已经到位。一小部分恒星可能只有8 Gyr，但这个群体在统计学上并不显著；蒙特卡罗模拟只在15%的时间内恢复到9 Gyr以下的恒星成分，它们平均占总数的7±4%。这些结果与再电离猝灭的理论预期是一致的。HST深度和角分辨率使我们能够证明Eri II星系团偏离星系中心的投影距离为23±3pC。这个偏移量可能是Eri II中一个小的(∼50-75pC)暗物质核心的迹象。此外，我们还证明了该星系团具有很高的椭圆度，并且在3°±6°范围内与Eri II的方向一致，这可能是由于潮汐的原因。该星系团的恒星群与Eri II本身难以区分。
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 and is aligned with the orientation of Eri II within 3° ± 6°, likely due to tides. The stellar population of the cluster is indistinguishable from that of Eri II itself.