UNCOVER NIRSpec/PRISM Spectroscopy Unveils Evidence of Early Core Formation in a Massive, Centrally Dusty Quiescent Galaxy at z_spec=3.97

We report the spectroscopic confirmation of a massive (=10.38 ±_0.09^0.08), HST-dark (m_F150W - m_F444W = 3.6) quiescent galaxy at z_spec=3.97 in the UNCOVER survey. NIRSpec/PRISM spectroscopy surprisingly reveals that the galaxy core is consistent with a low (≲3 M_⊙ yr^-1) star formation rate 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 star formation rate in the center of UNCOVER 18407 require both significant (A_v∼1.4 mag) dust attenuation and a stellar mass-weighted age of 1 Gyr, implying 50% of the stars in the core already formed by z=8. Using spatially resolved annular mass-to-light measurements enabled by the galaxy's moderate magnification (μ=2.12±_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_eff = 0.74 ±_0.17^0.22 kpc and log(Σ_1 kpc[M_⊙/kpc^2]) = 9.65 ±_0.15^0.12. The early formation of a dense, quiescent, 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.