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We investigate the means by which cold gas can accrete onto Milky Way massgalaxies from a hot corona of gas, using a new smoothed particle hydrodynamicscode, 'SPHS'. We find that the 'cold clumps' seen in many classic SPHsimulations in the literature are not present in our SPHS simulations. Instead,cold gas condenses from the halo along filaments that form at the intersectionof supernovae-driven bubbles from previous phases of star formation. Thispositive feedback feeds cold gas to the galactic disc directly, fuellingfurther star formation. The resulting galaxies in the SPH and SPHS simulationsdiffer greatly in their morphology, gas phase diagrams, and stellar content. Weshow that the classic SPH cold clumps owe to a numerical thermal instabilitycaused by an inability for cold gas to mix in the hot halo. The improvedtreatment of mixing in SPHS suppresses this instability leading to adramatically different physical outcome. In our highest resolution SPHSsimulation, we find that the cold filaments break up into bound clumps thatform stars. The filaments are overdense by a factor of 10-100 compared to thesurrounding gas, suggesting that the fragmentation results from a physicalnon-linear instability driven by the overdensity. This 'fragmenting filament'mode of disc growth has important implications for galaxy formation, inparticular the role of star formation in bringing cold gas into disc galaxies.

Thermal instabilities in cooling galactic coronae: fuelling star formation in galactic discs