The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies since z ∼ 2

We present the elemental abundances and ages of 19 massive quiescent galaxies at z ∼ 1.4 and z ∼ 2.1 from the Keck Heavy Metal Survey. The ultradeep LRIS and MOSFIRE spectra were modeled using a full-spectrum stellar population fitting code with variable abundance patterns. The galaxies have iron abundances between [Fe/H] = ‑0.5 and ‑0.1 dex, with typical values of ‑0.2 [‑0.3] at z ∼ 1.4 [z ∼ 2.1]. We also find a tentative $\mathrm{log}{\sigma }_{v}$ –[Fe/H] relation at z ∼ 1.4. The magnesium-to-iron ratios span [Mg/Fe] = 0.1–0.6 dex, with typical values of 0.3 [0.5] dex at z ∼ 1.4 [z ∼ 2.1]. The ages imply formation redshifts of z _form = 2–8. Compared to quiescent galaxies at lower redshifts, we find that [Fe/H] was ∼0.2 dex lower at z = 1.4–2.1. We find no evolution in [Mg/Fe] out to z ∼ 1.4, though the z ∼ 2.1 galaxies are 0.2 dex enhanced compared to z = 0–0.7. A comparison of these results to a chemical evolution model indicates that galaxies at higher redshift form at progressively earlier epochs and over shorter star formation timescales, with the z ∼ 2.1 galaxies forming the bulk of their stars over 150 Myr at z _form ∼ 4. This evolution cannot be solely attributed to an increased number of quiescent galaxies at later times; several Heavy Metal galaxies have extreme chemical properties not found in massive galaxies at z ∼ 0.0–0.7. Thus, the chemical properties of individual galaxies must evolve over time. Minor mergers also cannot fully account for this evolution as they cannot increase [Fe/H], particularly in galaxy centers. Consequently, the buildup of massive quiescent galaxies since z ∼ 2.1 may require further mechanisms, such as major mergers and/or central star formation.