The baryonic mass-size relation of galaxies. I. A dichotomy in star-forming galaxy disks

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 (M_ bar) and the baryonic half-mass radius (R_ 50, bar). We find that SPARC galaxies define two distinct sequences in the M_ bar - R_ 50, 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 M_ bar - R_ 50, 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 (M_ bar versus flat circular velocity V_ f) but moderately seen in the angular-momentum relation (approximately M_ bar versus V_ f 50, bar ), so it is driven by variations in R_ 50, bar at fixed M_ bar. This fact suggests that the baryonic mass-size relation is the most effective empirical tool to distinguish different galaxy types and study their evolution.