Redistributing hot gas around galaxies: do cool clouds signal a solution   to the overcooling problem?
  We present a pair of high-resolution smoothed particle hydrodynamics (SPH) simulations that explore the evolution and cooling behavior of hot gas around Milky-Way size galaxies. The simulations contain the same total baryonic mass and are identical other than their initial gas density distributions. The first is initialised with a low entropy hot gas halo that traces the cuspy profile of the dark matter, and the second is initialised with a high-entropy hot halo with a cored density profile as might be expected in models with pre-heating feedback. Galaxy formation proceeds in dramatically different fashion depending on the initial setup. While the low-entropy halo cools rapidly, primarily from the central region, the high-entropy halo is quasi-stable for ~4 Gyr and eventually cools via the fragmentation and infall of clouds from ~100 kpc distances. The low-entropy halo's X-ray surface brightness is ~100 times brighter than current limits and the resultant disc galaxy contains more than half of the system's baryons. The high-entropy halo has an X-ray brightness that is in line with observations, an extended distribution of pressure-confined clouds reminiscent of observed populations, and a final disc galaxy that has half the mass and ~50% more specific angular momentum than the disc formed in the low-entropy simulation. The final high-entropy system retains the majority of its baryons in a low-density hot halo. The hot halo harbours a trace population of cool, mostly ionised, pressure-confined clouds that contain ~10% of the halo's baryons after 10 Gyr of cooling. The covering fraction for HI and MgII absorption clouds in the high-entropy halo is ~0.4 and ~0.6, respectively, although most of the mass that fuels disc growth is ionised, and hence would be under counted in HI surveys. 
Tobias Kaufmann (1), James S. Bullock (1), Ariyeh H. Maller (2),   Taotao Fang (1) and James Wadsley (3), ((1) University of...