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 441 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 are just as common as or even outnumber negative color gradients. Varying stellar population, dust, and active galactic nuclei can contribute to the observed color gradient. We show that for the majority of our sample, the observed color gradients principally reflect radial variations in stellar population, without strong contribution from 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, are found preferentially in galaxies of lower stellar mass, smaller size, and bluer spectral energy distribution.