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We present a spatially resolved study of stellar populations in 6 galaxieswith stellar masses $M_*\sim10^{10}M_\odot$ at $z\sim3.7$ using 14-filterJWST/NIRCam imaging from the JADES and JEMS surveys. The 6 galaxies arevisually selected to have clumpy substructures with distinct colors overrest-frame $3600-4100$\r{A}, including a bright dominant stellar core that isclose to their stellar-light centroids. With 23-filter photometry from HST toJWST, we measure the stellar-population properties of individual structuralcomponents via SED fitting using Prospector. We find that the central stellarcores are $\gtrsim2$ times more massive than the Toomre mass, indicating theymay not form via in-situ fragmentation. The stellar cores have stellar ages of$0.4-0.7$ Gyr that are similar to the timescale of clump inward migration dueto dynamical friction, suggesting that they likely instead formed through thecoalescence of giant stellar clumps. While they have not yet quenched, the 6galaxies are below the star-forming main sequence by $0.2-0.7$ dex. Within eachgalaxy, we find that the specific star formation rate is lower in the centralstellar core, and the stellar-mass surface density of the core is alreadysimilar to quenched galaxies of the same masses and redshifts. Meanwhile, thestellar ages of the cores are either comparable to or younger than theextended, smooth parts of the galaxies. Our findings are consistent with modelpredictions of the gas-rich compaction scenario for the buildup of galaxies'central regions at high redshifts. We are likely witnessing the coevalformation of dense central cores, along with the onset of galaxy-wide quenchingat $z>3$.

JADES + JEMS: A Detailed Look at the Buildup of Central Stellar Cores and Suppression of Star Formation in Galaxies at Redshifts 3 < z < 4.5