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: 7,006 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 ≤ log (M * /M ) ≤ 11.5. We test three methods for calculating half-mass radii from multi-band PSF-matched HST imaging: two based on spatially-resolved SED 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.

The Regulation of Galaxy Growth along the Size–Mass Relation by Star Formation, as Traced by Hα in KMOS3D Galaxies at 0.7 ≲ z ≲ 2.7*

“…If true, there should be no epoch at which age gradients should disappear without invoking complex age-dependent radial migration. This appears to contradict the lack of M/L gradients at z = 2 seen by Suess et al (2019). We do also find a greater difference between mass and light sizes in galaxies with higher sersic index, but only at z ∼ 1 (not z ∼ 2).…”

“…This would suggest that other physical processes for growth of star forming galaxies might be at play, and we will discuss candidates in the next section. However the results of Suess et al (2019) suggest that evolution might not be so steep if we assume there is also a role for evolution in the mass to light gradients (even if this is driven in part by bulge formation). Such milder evolution can be consistent with our upper limit of ∼ 26% for the size growth factor.…”

Half-mass Radii of Quiescent and Star-forming Galaxies Evolve Slowly from 0 ≲ z ≤ 2.5: Implications for Galaxy Assembly Histories*

“…In this Letter, we present the evolution of galaxy halfmass radii from z = 2.5 to z ∼ 0. From 1.0 ≤ z ≤ 2.5, we use the sample of galaxy half-mass radii presented in Suess et al (2019). This high-redshift sample consists of 7,006 galaxies selected from the ZFOURGE photometric -Top: color gradient strength as a function of stellar mass for two redshift intervals.…”

“…This low-redshift sample consists of all galaxies in the 3D-HST photometric catalog with z ≤ 1.0, log M * /M > 9.0, S/N F160W ≥ 10, a use flag equal to one, and a convergent GALFIT fit. These selection criteria are equivalent to those of the higher-redshift sample presented in Suess et al (2019), but using the 3D-HST catalogs as opposed to the ZFOURGE catalogs. This allows us to include galaxies from the AEGIS and GOODS-N fields (not included in ZFOURGE) and better sample the z 0.5 universe.…”

“…We calculate the half-mass radii of galaxies in the lowredshift sample following the methods of Suess et al (2019). In brief: we calculate aperture photometry in elliptical annuli for each galaxy in each band of PSF-convolved HST imaging (Skelton et al 2014), then use FAST (Kriek et al 2009) to model the resulting spatially-resolved spectral energy distributions (SEDs).…”