The baryonic mass–size relation of galaxies

The mass–size relations of galaxies are generally studied considering only stars or only gas separately. Here we study the baryonic mass–size relation of galaxies from the SPARC database, using the total baryonic mass (Mbar) and the baryonic half-mass radius (R50, bar). We find that SPARC galaxies define two distinct sequences in the Mbar − R50, bar plane: one that formed by high-surface-density (HSD), star-dominated, Sa-to-Sc galaxies, and one by low-surface-density (LSD), gas-dominated, Sd-to-dI galaxies. The Mbar − R50, bar relation of LSD galaxies has a slope close to 2, pointing to a constant average surface density, whereas that of HSD galaxies has a slope close to 1, indicating that less massive spirals are progressively more compact. Our results point to the existence of two types of star-forming galaxies that follow different evolutionary paths: HSD disks are very efficient in converting gas into stars, perhaps thanks to the efficient formation of non-axisymmetric structures (bars and spiral arms), whereas LSD disks are not. The HSD-LSD dichotomy is absent in the baryonic Tully-Fisher relation (Mbar versus flat circular velocity Vf) but moderately seen in the angular–momentum relation (approximately Mbar versus Vf × R50, bar), so it is driven by variations in R50, bar at fixed Mbar. This fact suggests that the baryonic mass–size relation is the most effective empirical tool to distinguish different galaxy types and study their evolution.