| Summary: | We present the first spectroscopic confirmation of an ultramassive galaxy at redshift z > 3 using data from Keck- NIRSPEC, VLT-X-shooter, and GTC-Osiris. We detect strong [O III] and Lya emission, and weak [O II], C IV, and He II, placing C1-23152 at a spectroscopic redshift of zspec = 3.351. The modeling of the emission-line-corrected spectral energy distribution (SED) results in a best-fit stellar mass of M* = 3.1-0.7+0.6 × 1011 M?, a star formation rate of <7 M?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 µm, we confirm the presence of a luminous hidden active galactic nucleus (AGN), with bolometric luminosity of ~1046 erg s-1. Potential contamination of the observed SED from the AGN continuum is constrained, placing a lower limit on the stellar mass of 2 × 1011 M?. HST/WFC3 H160 and ACS I814 images are modeled, resulting in an effective radius of re ~ 1 kpc in the H160 band and a -Sti-errsic 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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