2019ApJ...872L..14M

When investigating the sizes of galaxies it is standard practice to use the half-light radius, r₅₀. Here we explore the effects of the size definition on the distribution of galaxies in the size-stellar mass plane. Specifically, we consider r₂₀ and r₈₀, the radii that contain 20% and 80% of a galaxy's total luminosity, as determined from a Sersic profile fit, for galaxies in the 3D-HST/CANDELS and COSMOS-DASH surveys. These radii are calculated from size catalogs based on a simple calculation assuming a Sersic profile. We find that the size-mass distributions for r₂₀ and r₈₀ are markedly different from each other and also from the canonical r₅₀ distribution. The most striking difference is in the relative sizes of star-forming and quiescent galaxies at fixed stellar mass. Whereas quiescent galaxies are smaller than star-forming galaxies in r₅₀, this difference nearly vanishes for r₈₀. By contrast, the distance between the two populations increases for r₂₀. Considering all galaxies in a given stellar mass and redshift bin we detect a significant bimodality in the distribution of r₂₀, with one peak corresponding to star-forming galaxies and the other to quiescent galaxies. We suggest that different measures of the size are tracing different physical processes within galaxies; r₂₀ is closely related to processes controlling the star formation rate of galaxies and r₈₀ may be sensitive to accretion processes and the relation of galaxies with their halos.