Bulgeless Evolution And the Rise of Discs (BEARD) I. Physical drivers of the mass-size relation for Milky Way-like galaxies

C. Marrero-de la Rosa; J. MÃ©ndez-Abreu; A. de Lorenzo-CÃ¡ceres; S. Cardona-Barrero; J. RomÃ¡n; E. Arjona-GÃ¡lvez; M. Chamorro-Cazorla; E. M. Corsini; L. Costantin; V. Cuomo; C. Dalla Vecchia; A. Di Cintio; D. FernÃ¡ndez; D. Gasparri; E. Iodice; D. Mayya; L. Morelli; F. Pinna; A. Pizzella; D. Rosa-GonzÃ¡lez; Y. Rosas-Guevara; O. Vega; S. Zarattini

doi:10.1051/0004-6361/202557193

paper

Bulgeless Evolution And the Rise of Discs (BEARD) I. Physical drivers of the mass-size relation for Milky Way-like galaxies

arXiv:2605.18976 · doi:10.1051/0004-6361/202557193

Abstract

In the standard $I^$ cold dark matter ( $I^$ CDM) cosmology, the existence of massive pure-disc galaxies remains challenging within the hierarchical framework and is key to understanding the evolutionary history of Milky Way-like systems. In this work, we investigate the physical origin of the scatter in the stellar mass-size relation of massive spiral galaxies, with a particular focus on bulgeless systems. We analyse 22 nearby bulgeless galaxies from the Bulgeless Evolution And the Rise of Discs (BEARD) survey using deep $g$ - and $r$ -band imaging obtained with the 2.5 m Isaac Newton Telescope Wide Field Camera. We derive surface-brightness, colour, and stellar-mass-density radial profiles to measure $R_{1}$ , the radius where $I^£_{*} = 1 M_{⊙} pc^{- 2}$ , adopted here as a physically motivated size proxy. Point spread function (PSF) effects are corrected through star subtraction and wavelet deconvolution. BEARD bulgeless galaxies follow the tight stellar mass- $R_{1}$ relation defined in previous studies, with a similar scatter of $\sim 0.1$ dex. Using galaxies from the IllustrisTNG50 simulation, we find that the scatter is linked to morphology, with bulgeless, BEARD-like analogues and bulge-dominated galaxies tracing the upper and lower envelopes of the relation, respectively. This trend correlates with the specific central stellar mass density, $I^£_{1, kpc}^{spec}$ , suggesting that differences between observations and simulations reflect the broader central-density range spanned by BEARD bulgeless galaxies. A deeper analysis of the physical driver of this morphological segregation reveals that the scatter in the mass-size relation is also related to the spatial configuration of merger events, rather than their frequency, with bulgeless systems tending to inhabit halos with a slightly higher spin. (abridged)

18 pages, 9 figures. Published in A&A