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

Abstract

When investigating the sizes of galaxies it is standard practice to use the half-light radius, r 50 . Here we explore the effects of the size definition on the distribution of galaxies in the size–stellar mass plane. Specifically, we consider r 20 and r 80 , 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 r 20 and r 80 are markedly different from each other and also from the canonical r 50 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 r 50 , this difference nearly vanishes for r 80 . By contrast, the distance between the two populations increases for r 20 . Considering all galaxies in a given stellar mass and redshift bin we detect a significant bimodality in the distribution of r 20 , 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; r 20 is closely related to processes controlling the star formation rate of galaxies and r 80 may be sensitive to accretion processes and the relation of galaxies with their halos.