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

Abstract

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]) 1-4 , 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 5 . 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 6 is at a redshift of 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 7 . However, the relatively low signal-to-noise ratio 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 a redshift of z = 2.1, when the Universe was 3 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 starforming galaxies in the Universe.We observed the galaxy COSMOS-11494 with the near-infrared multi-object spectrograph MOSFIRE on the Keck I Telescope 8 . It was also observed by two other programmes 9, 10 , and so we incorporated these publicly available archival data. COSMOS-11494 was selected from the 3D-HST survey 11,12 . With a stellar mass M given by log10 M/M⊙ = 11.5 ± 0.1, COSMOS-11494 is among the most massive galaxies at its redshift, and it has a very low star-formation rate of less than 0.6M⊙/ yr (see Methods). Similarly to the typical massive, quiescent galaxy at this redshift, it is smaller than its local counterparts of the same mass, with an effective radius of 2.1 kpc 13 . The MOS-

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