Thermal instabilities in cooling galactic coronae: fuelling star
  formation in galactic discs
  We investigate the means by which cold gas can accrete onto Milky Way mass
galaxies from a hot corona of gas, using a new smoothed particle hydrodynamics
code, 'SPHS'. We find that the 'cold clumps' seen in many classic SPH
simulations in the literature are not present in our SPHS simulations. Instead,
cold gas condenses from the halo along filaments that form at the intersection
of supernovae-driven bubbles from previous phases of star formation. This
positive feedback feeds cold gas to the galactic disc directly, fuelling
further star formation. The resulting galaxies in the SPH and SPHS simulations
differ greatly in their morphology, gas phase diagrams, and stellar content. We
show that the classic SPH cold clumps owe to a numerical thermal instability
caused by an inability for cold gas to mix in the hot halo. The improved
treatment of mixing in SPHS suppresses this instability leading to a
dramatically different physical outcome. In our highest resolution SPHS
simulation, we find that the cold filaments break up into bound clumps that
form stars. The filaments are overdense by a factor of 10-100 compared to the
surrounding gas, suggesting that the fragmentation results from a physical
non-linear instability driven by the overdensity. This 'fragmenting filament'
mode of disc growth has important implications for galaxy formation, in
particular the role of star formation in bringing cold gas into disc galaxies.
