Hidden figures in the sky

Context. Low-surface-brightness galaxies (LSBGs) are defined as galaxies with central surface brightness levels fainter than the night sky, making them challenging to observe. A key open question is whether their faint appearance arises from intrinsic properties or from stochastic events in their formation histories. Aims. We aim to trace the formation histories of LSBGs to assess whether their evolutionary paths differ from those of high-surface brightness galaxies (HSBGs) and to identify the key physical drivers behind these differences. Methods. We present a fast and efficient method to estimate stellar surface brightness densities in hydrodynamical simulations and a statistically robust exploration of over 150 properties in the reference run REF-L0100N1504 of the EAGLE simulation. To minimise any biases, we carefully matched the stellar and halo mass distributions of the selected LSB and HSB samples. Results. At ɀ = 0, LSBGs are typically extended, rotation-supported systems with lower stellar densities, older stellar populations, reduced star formation activity, and higher specific stellar angular momenta (j*) than their HSBG counterparts. They also exhibit larger radii of maximum circular velocity Rυmax). We identified key transition redshifts that mark the divergence of LSBG and HSBG properties: j* diverges at ɀ ~ 5–7 and Rυmax at ɀ ~ 2–3. Star formation activity and large-scale environment appear to play only a minimal role in the development of LSB features. Conclusions. LSBGs follow mass-dependent evolutionary pathways, where early rapid formation and later slowdowns, combined with their distinct structural properties, influence their response to external factors (e.g. mergers and gas accretion). Their LSB nature emerges from intrinsic dynamical and structural factors rather than environmental influences, with angular momentum as a key driver of divergence at high redshifts.