A High Incidence of Central Star Formation Inferred from the Color Gradients of Galaxies at $z>4$

Abstract: We study the rest-frame ultraviolet-optical color gradients of 669 galaxies at $4<z<8$ by characterizing the wavelength dependence of their structural parameters derived from simultaneously fitting the seven-band NIRCam images acquired with the James Webb Space Telescope. Distinct from trends observed at lower redshifts, where most galaxies exhibit negative color gradients whereby galaxy centers are redder than their outskirts, in high-redshift galaxies positive color gradients match or even outnumber negative color gradients. The color gradients principally reflect radial variations in stellar population instead of dust reddening or contamination from active galactic nuclei. The sign and magnitude of the color profile depend systematically on the global properties of the galaxy: positive color gradients, characteristic of centrally concentrated star formation or outside-in growth, preferentially inhabit galaxies of lower stellar mass, smaller size, and bluer spectral energy distribution.