A massive, quiescent, population II galaxy at a redshift of 2.1
  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]), 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 z~1.4, with
[Mg/Fe]=0.45(+0.05,-0.19). A slightly earlier epoch (z~1.6) was probed by
stacking 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 use
of index analysis techniques for both 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 z=2.1. 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 0.1-0.5 Gyr - characteristics that are
similar to population II stars in the Milky Way. With an average past SFR of
600-3000 Msol/yr, this galaxy was among the most vigorous star-forming galaxies
in the Universe.
