The baryonic mass–size relation of galaxies

II. Implications for the evolutionary paths between star-forming and passive galaxies

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

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Received: 20 October 2025
Accepted: 15 January 2026

Abstract

The baryonic mass–size relation of galaxies links the total baryonic mass (stars plus gas) to the baryonic half-mass radius. In the first paper of this series, we showed that star-forming galaxies from the SPARC sample follow two distinct relations in the baryonic mass–size plane: one defined by high-surface-density (HSD), star-dominated, Sa-to-Sc galaxies, and one defined by low-surface-density (LSD), gas-dominated, Sd-to-dI galaxies. In this second paper, we study the structural relations between baryonic mass, half-mass radius, and mean surface density to constrain possible morphological transformations between star-forming and passive galaxies. We complemented the SPARC sample with ∼1200 passive galaxies that are nearly devoid of gas: ellipticals (Es), lenticulars (S0s), dwarf ellipticals (dEs) or dwarf spheroidals (dSphs), and the so-called ‘ultra-diffuse galaxies’ (UDGs). Our results can be summarised as follows: (1) passive stellar components follow four distinct relations at high statistical significance, namely (i) Es plus bulges, (ii) S0 disks, (iii) non-nucleated dwarfs (dEs, dSphs, and UDGs), and (iv) nucleated dEs. (2) Star-forming HSD disks (mostly Sa to Sc) overlap with S0 disks within 2σ in the baryonic relations and within 1σ in the stellar ones, so present-day spirals may simply evolve into S0s as they run out of gas. (3) Star-forming LSD disks (mostly Sd to dI) are offset from non-nucleated passive dwarfs by more than 3σ in the baryonic relations, but the two galaxy populations overlap within 1σ in the stellar relations, suggesting that non-nucleated passive dwarfs can form from star-forming dwarfs only after gas removal. (4) UDGs extend the sequence of non-nucleated dEs/dSphs and may originate from the most diffuse star-forming LSD galaxies with no need for a substantial expansion of the stellar component.