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We present the results of 0.6"-resolution observations of CO J=3-2 lineemission in 10 massive star-forming galaxies at z=2.2-2.5 with the AtacamaLarge Millimeter/submillimeter Array (ALMA). We compare the spatial extent ofmolecular gas with those of dust and stars, traced by the 870 $\mu$m and 4.4$\mu$m continuum emissions, respectively. The average effective radius of theCO emission is 1.75$\pm$0.34 kpc, which is about 60 percent larger than that ofthe 870 $\mu$m emission and is comparable with that of the 4.4 $\mu$m emission.Utilizing the best-fit parametric models, we derive the radial gradients of thespecific star-formation rate (sSFR), gas depletion timescale, and gas-massfraction within the observed galaxies. We find a more intense star-formationactivity with a higher sSFR and a shorter depletion timescale in the innerregion than in the outer region. The central starburst may be the primaryprocess for massive galaxies to build up a core. Furthermore, the gas-massfraction is high, independent of the galactocentric radius in the observedgalaxies, suggesting that the galaxies have not begun to quench star formation.Given the shorter gas depletion timescale in the center compared to the outerregion, quenching is expected to occur in the center first and then propagateoutward. We may be witnessing the observed galaxies in the formation phase of acore prior to the forthcoming phase of star formation propagating outward.

Spatial extent of molecular gas, dust, and stars in massive galaxies at z=2-2.5 determined with ALMA and JWST