2024ApJ...974..145S

We report the spectroscopic confirmation of a massive ($\mathrm{log}({M}_{\star }/{M}_{\odot })=10.34{\pm }_{0.07}^{0.06}$), Hubble Space Telescope-dark (m_F150W - m_F444W = 3.6) quiescent galaxy at z_spec = 3.97 in the UNCOVER survey. NIRSpec/PRISM spectroscopy and a nondetection in deep Atacama Large Millimeter/submillimeter Array imaging surprisingly reveals that the galaxy is consistent with a low (<10 M_⊙ yr^–1) star formation rate (SFR) despite evidence for moderate dust attenuation. The F444W image is well modeled with a two-component Sérsic fit that favors a compact, r_e ∼ 200 pc, n ∼ 2.9 component and a more extended, r_e ∼ 1.6 kpc, n ∼ 1.7 component. The galaxy exhibits strong color gradients: the inner regions are significantly redder than the outskirts. Spectral energy distribution models that reproduce both the red colors and low SFR in the center of UNCOVER 18407 require both significant (A_v ∼ 1.4 mag) dust attenuation and a stellar mass-weighted age of 900 Myr, implying 50% of the stars in the core already formed by z = 7.5. Using spatially resolved annular mass-to-light measurements enabled by the galaxy's moderate magnification ($\mu =2.12{\pm }_{0.01}^{0.05}$) to reconstruct a radial mass profile from the best-fitting two-component Sérsic model, we infer a total mass-weighted ${r}_{\mathrm{eff}}=0.74{\pm }_{0.17}^{0.22}$ kpc and log $({{\rm{\Sigma }}}_{1\ \mathrm{kpc}}\ [{M}_{\odot }\,{\mathrm{kpc}}^{-2}])=9.65{\pm }_{0.15}^{0.12}$. The early formation of a dense, low SFR, and dusty core embedded in a less attenuated stellar envelope suggests an evolutionary link between the earliest-forming massive galaxies and their elliptical descendants. Furthermore, the disparity between the global, integrated dust properties and the spatially resolved gradients highlights the importance of accounting for radially varying stellar populations when characterizing the early growth of galaxy structure.