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Abstract:

In a ΛCDM cosmology, galaxy formation is a globally inefficient process: it is often the case that far fewer baryons are observed in galaxy discs than expected from the cosmic baryon fraction. The location of these ‘missing baryons’ is unclear. By fitting halo profiles to the rotation curves of galaxies in the SPARC data set, we measure the ‘missing baryon’ mass for individual late-type systems. Assuming that haloes initially accrete the cosmological baryon fraction, we show that the maximum energy available from supernovae is typically not enough to completely eject these ‘missing baryons’ from a halo, but it is often sufficient to heat them to the virial temperature. The energy available from supernovae has the same scaling with galaxy mass as the energy needed to heat or eject the ‘missing baryons’, indicating that the coupling efficiency of the feedback to the ISM may be constant with galaxy virial mass. We further find that the energy available from supernova feedback is always enough to convert a primordial cusp into a core and has magnitude consistent with what is required to heat the ‘missing baryons’ to the virial temperature. Taking a census of the baryon content of galaxies with 109 < Mvir/M⊙ < 1012 reveals that ∼86 per cent of baryons are likely to be in a hot phase surrounding the galaxies and possibly observable in the X-ray, ∼7 per cent are in the form of cold gas, and ∼7 per cent are in stars.