  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$$\sim$4.6. The stellar population has
a mean mass-weighted age 400$^{+30}_{-70}$ Myr and it is formed between
$\sim$600 Myr and $\sim$150 Myr before the observed epoch, this latter being
the time since quenching. Its high stellar velocity dispersion,
$\sigma_e$=409$\pm$60 km s$^{-1}$, confirms the high mass
(M$_{dyn}$=$2.2(\pm0.4)$$\times$10$^{11}$ M$_\odot$) and the high mass density
($\Sigma_e^{M^*}$=$\Sigma_{1kpc}=3.2(\pm0.7)\times10^{10}$ M$_\odot$
kpc$^{-2}$), suggesting a fast dissipative process at its origin. The analysis
points toward a supersolar metallicity, [Z/H]=0.25$^{+0.006}_{-0.10}$, 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.

Astrophysics of Galaxies