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

TL;DR

This study investigates the physical factors influencing the mass-size relation of bulgeless, Milky Way-like galaxies, highlighting the role of central density and merger history in shaping their morphology and size.

Contribution

It provides new insights into the physical drivers of the mass-size relation for bulgeless galaxies, combining observational data with simulation analysis.

Findings

Bulgeless galaxies follow a tight mass-R1 relation with ~0.1 dex scatter.

Scatter correlates with central stellar density and merger configurations.

Bulgeless systems tend to reside in higher-spin halos.

Abstract

In the standard $\Lambda$ cold dark matter ($\Lambda$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 $\Sigma_* = 1\,\mathrm{M}_\odot\,\mathrm{pc}^{-2}$, 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, $\Sigma^{\mathrm{spec}}_{1,\mathrm{kpc}}$, 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)