2023MNRAS.523.3991Z

Giant low surface brightness (GLSB) galaxies, such as Malin 1 and UGC 1382, contain the largest stellar discs known. GLSB galaxies also often contain large masses of neutral hydrogen (H I). However, these extreme galaxies' origin and properties remain poorly understood. Using the cosmological simulation IllustrisTNG 100, we identify and select a sample of ∼200 galaxies with extended ($R_{\rm {\rm H\,{\small I}}}\,\gt\, 50$ kpc) and well-defined H I discs, ∼6 per cent of the total galaxies in the same stellar mass range (10.2 < log (M_*/M_⊙) < 11.6). This GLSB sample is heterogeneous, with mixed galaxy morphologies ranging from the most disc-dominated systems to massive ellipticals. These simulated GLSB galaxies are located in massive haloes ($V_{\max }\, \gt \, 150\ \rm {km\ s^{-1}}$) and their properties, such as total H I content, stellar disc parameters, star formation rate, and rotation curves, agree with observed GLSB galaxies. We construct a paired control sample to contrast with the GLSB galaxies. The GLSB galaxies tend to have large galaxy spin parameters ($40{{\ \rm per\ cent}}$ larger) and larger ex situ stellar mass fractions than the paired control. We find evidence that aligned mergers promote the formation of extended discs and that isolated environments help the survival of those discs across cosmic time.