Spectroscopic Confirmation of an Ultra Massive and Compact Galaxy at
  z=3.35: A Detailed Look at an Early Progenitor of Local Most Massive
  Ellipticals
  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 $M_{*}=3.1^{+0.6}_{-0.7}\times10^{11} M_{\odot}$, a
star-formation rate of <7 $M_{\odot} yr^{-1}$, 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 ~$10^{46}erg s^{-1}$. Potential
contamination to the observed SED from the AGN continuum is constrained,
placing a lower limit on the stellar mass of $2\times10^{11} M_{\odot}$.
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.
