Half-mass Radii for ∼7000 Galaxies at 1.0 ≤ z ≤ 2.5: Most of the Evolution in the Mass–Size Relation Is Due to Color Gradients

Radial mass-to-light ratio gradients cause the half-mass and half-light radii of galaxies to differ, potentially biasing studies that use half-light radii. Here we present the largest catalog to date of galaxy half-mass radii at z > 1: 7006 galaxies in the CANDELS fields at 1.0 ≤ z ≤ 2.5. The sample includes both star-forming and quiescent galaxies with stellar masses . We test three methods for calculating half-mass radii from multiband PSF-matched Hubble Space Telescope (HST) imaging: two based on spatially resolved spectral energy distribution modeling, and one that uses a rest-frame color profile. All three methods agree, with scatter ≲0.3 dex. In agreement with previous studies, most galaxies in our sample have negative color gradients (the centers are redder than the outskirts, and ). We find that color gradient strength has significant trends with increasing stellar mass, half-light radius, U − V color, and stellar mass surface density. These trends have not been seen before at z > 1. Furthermore, color gradients of star-forming and quiescent galaxies show a similar redshift evolution: they are flat at z ≳ 2, then steeply decrease as redshift decreases. This affects the galaxy mass–size relation. The normalizations of the star-forming and quiescent relations are 10%–40% smaller than the corresponding relations; the slopes are ∼0.1–0.3 dex shallower. Finally, the half-mass radii of star-forming and quiescent galaxies at M* = 1010.5 M⊙ only grow by ∼1% and ∼8% between z ∼ 2.25 and z ∼ 1.25. This is significantly less than the ∼37% and ∼47% size increases found when using the half-light radius.