Environmental invariance of the galaxy size–mass relation

¹ Institute of Space Sciences (ICE, CSIC), Campus UAB, Carrer de Can Magrans, s/n, 08193 Barcelona, Spain
² Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain
³ Institute of Astronomy and Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
⁴ Center for Informatics and Computation in Astronomy, National Tsing Hua University, Hsinchu 30013, Taiwan

^★ Corresponding authors: This email address is being protected from spambots. You need JavaScript enabled to view it. , This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 5 August 2025
Accepted: 17 November 2025

Abstract

Context. The galaxy size-luminosity and size-stellar mass relations are important constraints on the galactic baryon cycle of gas accretion, star formation, and feedback. There are conflicting claims in the literature regarding how an environment influences size, and both “direct” transformative effects and “assembly bias” may contribute to observed variations with environment.

Aims. We constructed a large homogeneous sample of size measurements to M_r ∼ −14 (M_★ ∼ 10⁷ M_⊙). Our sample fills a gap in field galaxy size measurements around ∼10⁷ − 10⁸ M_⊙; the literature at these masses is biased toward satellites of L^★ galaxies and members of galaxy clusters.

Methods. We used sizes from the DESI Legacy Survey (DESI-LS; which is significantly larger and deeper than SDSS) together with a published catalog that contains stellar masses and cluster positions derived from DESI-LS photometry. Our sample extends to z < 0.3 and comprises 540 228 galaxies with spectroscopic redshifts and 9 513 732 galaxies with photometric redshifts. We explored the environmental dependence of size for a mass-limited subset of our sample at z < 0.05 based on the distance to the nearest cluster center.

Results. We obtained size-luminosity and size-mass relations in good agreement with previous studies. By separating galaxies according to color and morphology, we show that the environmental variation of the overall size-mass relation on megaparsec scales can be understood as the consequence of a changing mixture of subpopulations, rather than “direct” size transformation. For example, at a fixed mass, quiescent (red) late-type galaxies within 2 Mpc of a cluster have the same size as quiescent late-type galaxies 30 Mpc from the nearest cluster.

Conclusions. Our results support individual galaxy assembly histories as the primary determinant of galaxy size. The existence of significantly different environment-insensitive size-mass relations for subpopulations separated by color (star formation rate) and Sérsic index (morphology) provides a clear target for calibration of the baryon cycle in cosmological simulations.