Constraining Metallicity and Age for Massive Quiescent Galaxies in a Redshift Range of 1

Using HST/WFC3 grism spectroscopy from the CANDELS Lyman-alpha Emission at Reionization (CLEAR) survey, we constrain the metallicities and ages of massive quiescent galaxies, at z ~ 1.5. CLEAR provides deep spectroscopy (12 HST orbits per pointing) with the WFC3/G102 grism over the wavelength range ~ 7,500 < λ < 12,000 Å, at a spectral resolution of R ~ 200, within the GOODS-N and GOODS-S Deep regions of CANDELS. These data cover important age and metallicity sensitive spectral features for galaxies at 1 < z < 2, including the redshifted Ca HK lines, 4000 Å break, Balmer-series lines, and Hg+G features. We stack the G102 spectra of a stellar-mass limited sample of 34 quiescent galaxies, with log(M_*/M_⊙) > 10 and 1 < z < 2, and fit the spectra using two sets of stellar population synthesis models, BC03 (Bruzual & Charlot 2003) and FSPS (Flexible Stellar Population Synthesis, Conroy & Gunn 2010). From these fits, we construct probability distribution functions of age and metallicity for these galaxies, separated into two mass bins, 10 < log(M_*/M_⊙) < 10.9 and log(M_*/M_⊙) > 10.9. The model fits favor higher metallicity for the more massive quiescent galaxies, with Z/Z_⊙ ~ 1, with some systematics possibly leading from differences in the stellar population models. Therefore, there is no evidence for significant evolution in metallicity for the most massive quiescent galaxies since z ~ 1.5. The model fits to the lower mass quiescent galaxies favor lower metallicites, Z/Z_⊙ ~ 0.4, with an offset of ~ 0.3 dex from the present-day relation (e.g., Galazzi et al. 2005). For quiescent galaxies in this mass range, 10.0 < log(M_*/M_⊙) < 10.9, this requires evolution in metallicity, either as a result of continued chemical enrichment of current galaxies, or the formation of additional quiescent galaxies (presumably quenching of star-forming galaxies at z > 1), or a combination of the two.