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

Abstract

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 9.0≤slant {log}({M}_* /{M}_⊙ )≤slant 11.5. 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 {r}_{{e},{mass}}< {r}_{{e},{light}}). 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 {r}_mass}{--}{M}_* relations are 10%-40% smaller than the corresponding {r}_light}{--}{M}_* relations; the slopes are ∼0.1-0.3 dex shallower. Finally, the half-mass radii of star-forming and quiescent galaxies at M _* = 10^10.5 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.