Ages and Metallicities of Quiescent Galaxies at 1.0 < z < 1.8 Derived from Deep Hubble Space Telescope Grism Data
Abstract
The ages and metallicities of massive quiescent galaxies contain important formation and quenching history information. When observed close to their quenching time we can better constrain these properties. This requires observations in the near-IR, which are sensitive to the (rest-frame) optical age and metallicity spectroscopic features (where ground-based observations suffer from strong sky backgrounds). Here, 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 12--orbit depth coverage with the WFC3/G102 grism covering ~ 8,000 < λ < 11,500 Å, at a spectral resolution of R ~ 210 taken as part of the CANDELS Lyman-α Emission at Reionization (CLEAR) survey. At 1.0 < z < 1.8, the spectra cover important stellar population features in the rest-frame optical, such as Ca HK, the 4000 Å break, Balmer lines (Hδ, Hγ, Hβ), and Mgb absorption features. We forward model 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, age, star-formation history, redshift, and dust. Using an iterative stacking procedure we combine 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 tz~1.1 = 3.2 ± 0.7 Gyr, tz~1.2 = 2.2 ± 0.6 Gyr, tz~1.3= 3.1 ± 0.6 Gyr, and tz~1.6= 2.0 ± 0.6 Gyr, for galaxies at z~1.1, 1.2, 1.3, and 1.6. This implies that most of the massive quiescent galaxies at 1 < z < 1.8 had formed > 68% of their stellar mass by a redshift of z > 2, supported by the comoving number densities of massive galaxies. The posteriors give metallicities of Zz~1.1 = 1.16 ± 0.29 Z⊙, Zz~1.2 = 1.05 ± 0.34 Z⊙, Zz~1.3 = 1.00 ± 0.31 Z⊙, and Zz~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.
- Publication:
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American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23331006E