The Rapid Build-up of Massive Early-type Galaxies. Supersolar Metallicity, High Velocity Dispersion and Young Age for an ETG at z=3.35

Abstract

Thanks to very deep spectroscopic observations carried out at the Large Binocular Telescope, we measured simultaneously stellar age, metallicity and velocity dispersion for C1-23152, an ETG at redshift $z$=3.352, corresponding to an epoch when the Universe was $\sim$1.8 Gyr old. The analysis of its spectrum shows that this galaxy, hosting an AGN, formed and assembled $\sim$2$\times$10${}^{11}$ M${}_{\odot }$ shaping its morphology within the $\sim$600 Myr preceding the observations, since $z$$\sim$4.6. The stellar population has a mean mass-weighted age 400${}_{-70}^{+30}$ Myr and it is formed between $\sim$600 Myr and $\sim$150 Myr before the observed epoch, this latter being the time since quenching. Its high stellar velocity dispersion, $\ddot{I}{ƒ}_e$=409$\pm$60 km s${}^{-1}$, confirms the high mass (M${}_{dyn}$=$2.2(\pm 0.4)$$\times$10${}^{11}$ M${}_{\odot }$) and the high mass density ($\hat{I}{\pounds }_e^{M^{\ast }}$=$\hat{I}{\pounds }_{1kpc}=3.2(\pm 0.7)\times 10^{10}$ M${}_{\odot }$ kpc${}^{-2}$), suggesting a fast dissipative process at its origin. The analysis points toward a supersolar metallicity, [Z/H]=0.25${}_{-0.10}^{+0.006}$, in agreement with the above picture, suggesting a star formation efficiency much higher than the replenishment time. However, sub-solar metallicity values cannot be firmly ruled out by our analysis. Quenching must have been extremely efficient to reduce the star formation to SFR$<$6.5 M${}_{\odot }$ 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 which evolved to $z=0$ unperturbed.

20 pages, 12 figures. Accepted for publication in ApJ (revised to match the ApJ version)