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 similar to 1.4 and z similar to 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 similar to 1.4 [z similar to 2.1]. We also find a tentative log sigma(v) -[Fe/H] relation at z similar to 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 similar to 1.4 [z similar to 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 similar to 0.2 dex lower at z = 1.4-2.1. We find no evolution in [Mg/Fe] out to z similar to 1.4, though the z similar to 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 similar to 2.1 galaxies forming the bulk of their stars over 150 Myr at z(form) similar to 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 similar to 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 similar to 2.1 may require further mechanisms, such as major mergers and/or central star formation.