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Unlike spiral galaxies such as the Milky Way, the majority of the stars inmassive elliptical galaxies were formed in a short period early in the historyof the Universe. The duration of this formation period can be measured usingthe ratio of magnesium to iron abundance ([Mg/Fe]), which reflects the relativeenrichment 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-timemergers. Therefore, to directly constrain the initial star-formation period, wemust study galaxies at earlier epochs. The most distant galaxy for which[Mg/Fe] had previously been measured is at z~1.4, with[Mg/Fe]=0.45(+0.05,-0.19). A slightly earlier epoch (z~1.6) was probed bystacking the spectra of 24 massive quiescent galaxies, yielding an average[Mg/Fe] of 0.31+/-0.12. However, the relatively low S/N of the data and the useof index analysis techniques for both studies resulted in measurement errorsthat are too large to allow us to form strong conclusions. Deeper spectra ateven earlier epochs in combination with analysis techniques based on fullspectral fitting are required to precisely measure the abundance patternshortly after the major star-forming phase (z>2). Here we report a measurementof [Mg/Fe] for a massive quiescent galaxy at z=2.1. With [Mg/Fe]=0.59+/-0.11,this galaxy is the most Mg-enhanced massive galaxy found so far, having twicethe Mg enhancement of similar-mass galaxies today. The abundance pattern of thegalaxy is consistent with enrichment exclusively by core-collapse supernovaeand with a star-formation timescale of 0.1-0.5 Gyr - characteristics that aresimilar to population II stars in the Milky Way. With an average past SFR of600-3000 Msol/yr, this galaxy was among the most vigorous star-forming galaxiesin the Universe.

A massive, quiescent, population II galaxy at a redshift of 2.1