The Rapid buildup of massive early-type galaxies: Supersolar metallicity, high velocity dispersion, and young age for an early-type galaxy at z = 3.35

Abstract

How massive early-type galaxies (ETGs) assembled their mass, on which timescales the star formation quenched, and when their supersolar metallicity has been established are still open and debated issues. Thanks to very deep spectroscopic observations carried out at the Large Binocular Telescope, we simultaneously measured stellar age, metallicity, and velocity dispersion for C1-23152, an ETG at redshift z = 3.352, corresponding to an epoch when the universe was ∼1.8 Gyr old. The analysis of its spectrum shows that this galaxy, hosting an active galactic nucleus (AGN), formed and assembled ∼2 × 1011 M o˙, shaping its morphology within the ∼600 Myr preceding the observations, since z ∼ 4.6. The stellar population has a mean mass-weighted age of Myr, and it is formed between ∼600 and ∼150 Myr before the observed epoch, the latter being the time since quenching. Its high stellar velocity dispersion, σ e = 409 ± 60 km s-1, confirms the high mass (M dyn = 2.2 (±0.4) × 1011 M o˙) and the high mass density (= Σ1kpc = 3.2 (±0.7) × 1010 M o˙ kpc-2), suggesting a fast dissipative process at its origin. The analysis points toward a supersolar metallicity, [Z/H] = 0.25, in agreement with the above picture, suggesting a star formation efficiency much higher than the replenishment time. However, subsolar-metallicity values cannot be firmly ruled out by our analysis. Quenching must have been extremely efficient to reduce the star formation to SFR < 6.5 M o˙ yr-1 in less than 150 Myr. This could be explained by the presence of the AGN, even if a causal relation cannot be established from the data. C1-23152 has the same stellar and physical properties of the densest ETGs in the local universe of comparable mass, suggesting that they are C1-23152-like galaxies that evolved to z = 0 unperturbed.