A New View of the Size–Mass Distribution of Galaxies: Using r 20 and r 80 Instead of r 50

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 Sérsic 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 Sérsic 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.