The Origin of Filamentary Star Forming Clouds in Magnetised Galaxies
  Observations show that galaxies and their interstellar media are pervaded by
strong magnetic fields with energies in the diffuse component being at least
comparable to the thermal and even as large or larger than the turbulent
energy. Such strong magnetic fields prevent the formation of stars because
patches of the interstellar medium are magnetically subcritical. Here we
present the results from global numerical simulations of strongly magnetised
and self-gravitating galactic discs, which show that the buoyancy of the
magnetic field due to the Parker instability leads at first to the formation of
giant filamentary regions. These filamentary structures become gravitationally
unstable and fragment into $\sim10^5 M_{\odot}$ clouds that attract kpc long,
coherent filamentary flows that build them into GMCs. Our results thus provide
a solution to the long-standing problem of how the transition from sub- to
supercritical regions in the interstellar medium proceeds.
