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 Lyα 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 × 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 r_e ∼ 1 kpc in the H₁₆₀ band and a -Sti-eŕrsic 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.