Structure Formation with Ultralight Axion Dark Matter

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Ultralight axions (ULAs) are dark matter candidates that are very well motivated from particle physics and give rise to new, previously unexplored astrophysical phenomena. With respect to cosmological structure formation, their properties are indistinguishable from the canonical cold dark matter paradigm on large scales. On small scales, in contrast, the gravitational growth of perturbations is suppressed below the so-called ``quantum Jeans length'' given by the de Broglie wavelength of particles with virial velocities. The wave-like nature of ULA dark matter on small scales and the existence of ground-state solutions with solitonic properties have stimulated the exploration of a rich phenomenology, including a low-mass cutoff of the halo and subhalo mass functions, solitonic halo cores consistent with observed dwarf galaxy rotation curves, as well as significantly reduced relaxation times for dark matter and stellar components. Crucially, it is impossible at the present time to make quantitative predictions based on cosmological simulations reaching the standards of N-body simulations. There is a strong demand for new computational methods to fill this gap. We propose to use a novel hybrid computational scheme to simulate cosmological structure formation with ULA dark matter that is currently being developed and tested by our group. Using adaptive mesh refinement of the existing Enzo code, it combines a particle-based semi-classical method for solving the Schrödinger-Poisson equation on coarse mesh levels with a grid-based solver on the finest level.We intend to study, for the first time, the formation and evolution of galaxies with ULA dark matter, using zoom-in simulations with the new hybrid method combined with Enzo's modules for baryonic physics and stellar components. Furthermore, we plan to investigate the substructure evolution of halos including the subhalo mass function as well as relaxation effects and dynamical friction in ULA halos.