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

Abstract

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

Accepted by Astronomy & Astrophysics