SPECTROSCOPIC CONFIRMATION OF AN ULTRAMASSIVE AND COMPACT GALAXY AT Z=3.35: A DETAILED LOOK AT AN EARLY PROGENITOR OF LOCAL GIANT ELLIPTICALS

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 Ly alpha emission, and weak [O II], C IV, and He II, placing C1-23152 at a spectroscopic redshift of z(spec) = 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) x 10(11) M-circle dot, a star formation rate of <7 M-circle dot yr(-1), and negligible dust extinction. The stars appear to have formed in a short intense burst similar to 300-500 Myr prior to the observation epoch, setting the formation redshift of this galaxy at z similar to 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 similar to 10(46) 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 x 1011 M.. HST/WFC3 H-160 and ACS I-814 images are modeled, resulting in an effective radius of r(e) similar to 1 kpc in the H-160 band and a Sersic index n similar to 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.