The Heavy Metal Survey: The Evolution of Stellar Metallicities, Abundance Ratios, and Ages of Massive Quiescent Galaxies since z ∼ 2
Aliza G. Beverage, Mariska Kriek, Katherine A. Suess, Charlie Conroy, Sedona H. Price, Guillermo Barro, Rachel Bezanson, Marijn Franx, Brian Lorenz, Yilun Ma, Lamiya Mowla, Imad Pasha, Pieter van Dokkum, Daniel R. Weisz
Abstract
Abstract 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 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:msub> <mml:mrow> <mml:mi>σ</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> </mml:msub> </mml:math> –[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.