Kiloparsec-Scale Simulations of Magnetised Molecular Clouds in Disk Galaxies

Abstract: We present simulations of the evolution of self-gravitating dense gas on kiloparsec-size scales in a galactic disk, designed to study dense clump formation from giant molecular clouds (GMCs). These dense clumps are expected to be the precursors to star clusters and this process may be the rate limiting step controling star formation rates in galactic systems as described by the Kennicutt-Schmidt relation. The evolution of these simulated GMCs and clumps is determined by self-gravity balanced by turbulent pressure support and the large scale galactic shear. While the cloud structures and densities significantly change during their evolution, they remain roughly in virial equilibrium for time scales exceeding the free-fall time of GMCs, indicating that energy from the galactic shear continuously cascades down. We implement star formation at a slow, inefficient rate of 2% per local free-fall time, but this yields global star formation rates that are more than ~two orders of magnitude larger than the observed Kennicutt-Schmidt relation due to the over-production of dense clump gas. To explain this discrepancy, we anticipate magnetic fields to provide additional support. Low-resolution simulations indeed show that the magnetic field reduces the star formation rate.