NIHAO XXI: the emergence of low surface brightness galaxies

The existence of galaxies with a surface brightness μ lower than the night sky has been known since three decades. Yet, their formation mechanism and emergence within a Lambda cold dark matter universe has remained largely undetermined. For the first time, we investigated the origin of low surface brightness (LSB) galaxies with M_⋆ ∼ 10^9.5-10 M_⊙, which we are able to reproduce within hydrodynamical cosmological simulations from the NIHAO suite. The simulated and observed LSB galaxies share similar properties, having large H I reservoir, extended star formation histories and effective radii, low Sérsic index, and slowly rising rotation curves. The formation mechanism of these objects is explored: simulated LSB galaxies form as a result of coplanar co-rotating mergers and aligned accretion of gas at early times, while perpendicular mergers and misaligned gas accretion result in higher μ galaxies by z = 0. The larger the merger, the stronger the correlation between merger orbital configuration and final μ. While the halo spin parameter is consistently high in simulated LSB galaxies, the impact of halo concentration, feedback-driven gas outflows, and merger time only plays a minor-to-no role in determining μ. Interestingly, the formation scenario of such `classical' LSB galaxies differs from the one of less massive, M_⋆ ∼ 10^7-9 M_⊙, ultra-diffuse galaxies, the latter resulting from the effects of SNae-driven gas outflows: an M_⋆ of ∼10⁹ M_⊙ thus represents the transition regime between a feedback-dominated to an angular-momentum-dominated formation scenario in the LSB realm. Observational predictions are offered regarding spatially resolved star formation rates through LSB discs: these, together with upcoming surveys, can be used to verify the proposed emergence scenario of LSB galaxies.