The Formation of Exponential Disk Galaxies in MOND

Abstract The formation and evolution of galaxies are highly dependent on the dynamics of stars and gas, which is governed by the underlying law of gravity. To investigate how the formation and evolution of galaxies take place in Milgromian gravity (MOND), we present full hydrodynamical simulations with the Phantom of Ramses code. These are the first-ever galaxy formation simulations done in MOND with detailed hydrodynamics, including star formation, stellar feedback, radiative transfer, and supernovae. These models start from simplified initial conditions, in the form of isolated, rotating gas spheres in the early universe. These collapse and form late-type galaxies obeying several scaling relations, which was not a priori expected. The formed galaxies have a compact bulge and a disk with exponentially decreasing surface mass density profiles and scale lengths consistent with observed galaxies, as well as vertical stellar mass distributions with distinct exponential profiles (thin and thick disk). This work thus shows for the first time that disk galaxies with exponential profiles in both gas and stars are a generic outcome of collapsing gas clouds in MOND. These models have a slight lack of stellar angular momentum because of their somewhat compact stellar bulge, which is connected to the simple initial conditions and the negligible later gas accretion. We also analyze how the addition of more complex baryonic physics changes the resulting main properties of the models and find this to be negligibly so in the Milgromian framework.