The scaling relationship between baryonic mass and stellar disc size in morphologically late-type galaxies

Abstract

Abstract Here I report the scaling relationship between the baryonic mass and scale-length of stellar discs for ∼1000 morphologically late-type galaxies. The baryonic mass–size relationship is a single power law $R_\ast \propto M_{\rm b}^{0.38}$ across ∼3 orders of magnitude in baryonic mass. The scatter in size at fixed baryonic mass is nearly constant and there are no outliers. The baryonic mass–size relationship provides a more fundamental description of the structure of the disc than the stellar mass–size relationship. The slope and the scatter of the stellar mass–size relationship can be understood in the context of the baryonic mass–size relationship. For gas-rich galaxies, the stars are no longer a good tracer for the baryons. High-baryonic-mass, gas-rich galaxies appear to be much larger at fixed stellar mass because most of the baryonic content is gas. The stellar mass–size relationship thus deviates from the power-law baryonic relationship, and the scatter increases at the low-stellar-mass end. These extremely gas-rich low-mass galaxies can be classified as ultra-diffuse galaxies based on the structure.