Department of Astronomy, University of California, Berkeley, California 94720, USA.
Department of Astronomy, Harvard University, Cambridge, Massachusetts, USA.
Nature. 2016 Dec 7;540(7632):248-251. doi: 10.1038/nature20570.
Unlike spiral galaxies such as the Milky Way, the majority of the stars in massive elliptical galaxies were formed in a short period early in the history of the Universe. The duration of this formation period can be measured using the ratio of magnesium to iron abundance ([Mg/Fe]) in spectra, which reflects the relative enrichment by core-collapse and type Ia supernovae. For local galaxies, [Mg/Fe] probes the combined formation history of all stars currently in the galaxy, including younger and metal-poor stars that were added during late-time mergers. Therefore, to directly constrain the initial star-formation period, we must study galaxies at earlier epochs. The most distant galaxy for which [Mg/Fe] had previously been measured is at a redshift of z ≈ 1.4, with [Mg/Fe] = . A slightly earlier epoch (z ≈ 1.6) was probed by combining the spectra of 24 massive quiescent galaxies, yielding an average [Mg/Fe] = 0.31 ± 0.12 (ref. 7). However, the relatively low signal-to-noise ratio of the data and the use of index analysis techniques for both of these studies resulted in measurement errors that are too large to allow us to form strong conclusions. Deeper spectra at even earlier epochs in combination with analysis techniques based on full spectral fitting are required to precisely measure the abundance pattern shortly after the major star-forming phase (z > 2). Here we report a measurement of [Mg/Fe] for a massive quiescent galaxy at a redshift of z = 2.1, when the Universe was three billion years old. With [Mg/Fe] = 0.59 ± 0.11, this galaxy is the most Mg-enhanced massive galaxy found so far, having twice the Mg enhancement of similar-mass galaxies today. The abundance pattern of the galaxy is consistent with enrichment exclusively by core-collapse supernovae and with a star-formation timescale of 0.1 to 0.5 billion years-characteristics that are similar to population II stars in the Milky Way. With an average past star-formation rate of 600 to 3,000 solar masses per year, this galaxy was among the most vigorous star-forming galaxies in the Universe.
与银河系等螺旋星系不同，大多数大质量椭圆星系中的恒星是在宇宙早期的一个短时间内形成的。可以通过测量镁铁比（[Mg/Fe]）来测量这个形成期的持续时间，镁铁比反映了核心坍缩和 Ia 型超新星的相对富集。对于本地星系，[Mg/Fe] 探测到了星系中所有恒星的综合形成历史，包括在后期合并中添加的年轻和贫金属恒星。因此，要直接约束初始恒星形成期，我们必须研究早期的星系。之前已经测量过 [Mg/Fe] 的最遥远星系的红移约为 z ≈ 1.4，[Mg/Fe] = . 通过组合 24 个大质量静止星系的光谱探测到更早的时期（z ≈ 1.6），得到平均 [Mg/Fe] = 0.31 ± 0.12（参考文献 7）。然而，这些研究中数据的信噪比相对较低，并且对两种技术都使用了索引分析技术，这导致了太大的测量误差，以至于我们无法得出有力的结论。需要在更早的时期获得更深入的光谱，并结合基于全谱拟合的分析技术，以便在主要恒星形成阶段（z > 2）之后不久精确测量丰度模式。在这里，我们报告了在红移 z = 2.1 处的一个大质量静止星系的 [Mg/Fe] 测量结果，当时宇宙的年龄为 30 亿年。该星系的 [Mg/Fe] = 0.59 ± 0.11，是迄今为止发现的镁增强程度最高的大质量星系，其镁增强程度是当今类似质量星系的两倍。星系的丰度模式与仅由核心坍缩超新星引起的富集一致，并且恒星形成时间尺度为 0.1 到 0.5 亿年-这与银河系中的 II 型恒星相似。该星系的平均过去恒星形成率为每年 600 到 3000 个太阳质量，是宇宙中最活跃的恒星形成星系之一。
