CLEAR. I. Ages and Metallicities of Quiescent Galaxies at 1.0 < z < 1.8 Derived from Deep Hubble Space Telescope Grism Data

We use deep Hubble Space Telescope spectroscopy to constrain the metallicities and (light-weighted) ages of massive ({log}{M}_* /{M}_⊙ ≳ 10) galaxies selected to have quiescent stellar populations at 1.0 < z < 1.8. The data include twelve-orbit depth coverage with the WFC3/G102 grism covering ∼8000 < λ < 11,500 Å at a spectral resolution of R ∼ 210 taken as part of the CANDELS Lyα Emission at Reionization (CLEAR) survey. At 1.0 < z < 1.8, the spectra cover important stellar population features in the rest-frame optical. We simulate a suite of stellar population models at the grism resolution, fit these to the data for each galaxy, and derive posterior likelihood distributions for metallicity and age. We stack the posteriors for subgroups of galaxies in different redshift ranges that include different combinations of stellar absorption features. Our results give light-weighted ages of t _z∼1.1 = 3.2 ± 0.7 Gyr, t _z∼1.2 = 2.2 ± 0.6 Gyr, t _z∼1.3 = 3.1 ± 0.6 Gyr, and t _z∼1.6 = 2.0 ± 0.6 Gyr for galaxies at z ∼ 1.1, 1.2, 1.3, and 1.6, respectively. This implies that most of the massive quiescent galaxies at 1 < z < 1.8 had formed >68% of their stellar mass by z > 2. The posteriors give metallicities of Z _z∼1.1 = 1.16 ± 0.29 Z _⊙, Z _z∼1.2 = 1.05 ± 0.34 Z _⊙, Z _z∼1.3 = 1.00 ± 0.31 Z _⊙, and Z _z∼1.6 = 0.95 ± 0.39 Z _⊙. This is evidence that massive galaxies had enriched rapidly to approximately solar metallicities as early as z ∼ 3.