The baryonic mass–size relation of galaxies

I. A dichotomy in star-forming galaxy disks

¹ Arcetri Astrophysical Observatory, INAF, Largo Enrico Fermi 5, 50125 Florence, Italy
² Department of Astronomy, University of Science and Technology of China, Hefei 230026, China
³ School of Astronomy and Space Sciences, University of Science and Technology of China, Hefei 230026, China
⁴ Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, 50019 Sesto Fiorentino, Italy
⁵ Department of Astronomy, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, OH 44106, USA
⁶ Department of Physics, University of Oregon, Eugene, OR 97403, USA

^⋆ Corresponding author: komsmosbild@gmail.com

Received: 29 May 2025
Accepted: 1 October 2025

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_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 × R_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.