Bulgeless Evolution And the Rise of Discs (BEARD)

I. Physical drivers of the mass–size relation for Milky Way-like galaxies

¹ Universidad de La Laguna, Dpto. de Astrofísica Avda. Astrofísico Francisco Sánchez S/N E-38200 S.C de La Laguna, Spain
² Instituto de Astrofísica de Canarias C/ Vía Láctea S/N E-38205 San Cristóbal de La Laguna, Spain
³ Departamento de Física de la Tierra y Astrofísica, Universidad Complutense de Madrid 28040 Madrid, Spain
⁴ Instituto de Física de Partículas y del Cosmos (IPARCOS), Facultad de Ciencias Físicas, Universidad Complutense de Madrid E-28040 Madrid, Spain
⁵ Dipartimento di Fisica e Astronomia “G. Galilei”, Università di Padova vicolo dell’Osservatorio 3 I-35122 Padova, Italy
⁶ INAF-Osservatorio Astronomico di Padova vicolo dell’Osservatorio 2 I-35122 Padova, Italy
⁷ Centro de Astrobiología, CSIC-INTA Ctra. de Ajalvir km 4 Torrejón de Ardoz E-28850 Madrid, Spain
⁸ Departamento de Astronomía, Universidad de La Serena Av. Raúl Bitrán 1305 La Serena, Chile
⁹ Instituto Nacional de Astrofísica, Óptica y Electrónica Tonantzintla 72840 Puebla, México
¹⁰ Planetarium La Enseñanza Medellín Antioquia CP. 050022, Colombia
¹¹ Canada-France-Hawaii Telescope Kamuela HI 96743, USA
¹² Instituto de Astronomía y Ciencias Planetarias, Universidad de Atacama Copayapu 485 Copiapó, Chile
¹³ INAF–Astronomical Observatory of Capodimonte Salita Moiariello 16 80131 Naples, Italy
¹⁴ Donostia International Physics Centre (DIPC) Paseo Manuel de Lardizabal 4 20018 Donostia-San Sebastian, Spain
¹⁵ Departamento de Física, Universidad de Córdoba, Campus Universitario de Rabanales Ctra. N-IV Km. 396 E-14071 Córdoba, Spain
¹⁶ Centro de Estudios de Física del Cosmos de Aragón (CEFCA) Plaza San Juan 1 44001 Teruel, Spain

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Received: 10 September 2025
Accepted: 20 November 2025

Abstract

In the standard Λ cold dark matter (ΛCDM) cosmology, galaxies grow through smooth accretion and hierarchical mergers. While this framework explains many large-scale structures, the existence of massive disc galaxies without prominent bulges-pure discs-remains a challenge. 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. Studying these systems is also key to understanding the evolutionary history of our own Galaxy, the Milky Way, which is known to host a low-mass bulge. We performed a structural analysis of 22 nearby bulgeless galaxies from the Bulgeless Evolution And the Rise of Discs (BEARD) survey. To minimise the scatter in the stellar mass–size relation, we adopted a proxy for the physically motivated definition for the galaxy size, based on the radius R₁, where the stellar mass surface density reaches Σ_* = 1 M_⊙ pc⁻². For this purpose, we used deep g- and r-band imaging obtained with the 2.5 m Isaac Newton Telescope-Wide Field Camera. We derived surface brightness, colour, and stellar mass density radial profiles, which allowed us to obtain precise measurements of R₁. Point spread function (PSF) effects were corrected through star subtraction and wavelet deconvolution. BEARD bulgeless galaxies follow the tight stellar mass–R₁ relation defined in previous studies with a similar scatter (∼0.1 dex). We also constructed the same relation using galaxies from the IllustrisTNG50 simulation. We find a morphological segregation contributing to the scatter of the relation, with bulgeless (BEARD-like analogues) and bulge-dominated galaxies defining the upper and lower envelope, respectively. We find that this morphological trend shown by the simulations is strongly correlated with the specific central stellar mass density, Σ^spec_1,kpc, defined as the stellar mass surface density enclosed within the central kiloparsec, normalised using the total galaxy mass. The observed discrepancy between observations and simulations can be attributed to the broader Σ^spec_1,kpc distribution covered by our observed 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.