Abstract
We present the first spectroscopic confirmation of an ultra-massive galaxy at
redshift z>3 using data from Keck-NIRSPEC, VLT-Xshooter, and GTC-Osiris. We
detect strong OIII and Ly$\alpha$ emission, and weak OII, CIV, and HeII,
placing C1-23152 at a spectroscopic redshift of $z_spec$=3.351. The modeling
of the emission-line corrected spectral energy distribution results in a
best-fit stellar mass of $M_*=3.1^+0.6_-0.7\times10^11 M_ødot$, a
star-formation rate of <7 $M_ødot yr^-1$, and negligible dust extinction.
The stars appear to have formed in a short intense burst ~300-500 Myr prior to
the observation epoch, setting the formation redshift of this galaxy at z~4.1.
From the analysis of the line ratios and widths, and the observed flux at
24$\mu$m, we confirm the presence of a luminous hidden active galactic nucleus
(AGN), with bolometric luminosity of ~$10^46erg s^-1$. Potential
contamination to the observed SED from the AGN continuum is constrained,
placing a lower limit on the stellar mass of $2\times10^11 M_ødot$.
HST/WFC3 $H_160$ and ACS $I_814$ images are modeled, resulting in an
effective radius of $r_e$~1 kpc in the $H_160$ band and a Sersic index
n~4.4. This object may be a prototype of the progenitors of local most massive
elliptical galaxies in the first 2 Gyr of cosmic history, having formed most of
its stars at z>4 in a highly dissipative, intense, and short burst of star
formation. C1-23152 is completing its transition to a post-starburst phase
while hosting a powerful AGN, potentially responsible for the quenching of the
star formation activity.
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