Formation of disc galaxies in preheated media: a preventative feedback model

We develop a semi-analytic galaxy formation model that implements a self-consistent treatment for the hot halo gas and the assembly of central discs. We use the model to explore a preventative feedback scenario, in which the circumhalo medium is assumed to be preheated up to a certain entropy level by early starbursts or other processes, and contrast it with an ejective feedback scenario in which baryons are first accreted into dark matter haloes and subsequently ejected by feedback. When the preheating entropy is comparable to the halo virial entropy, baryon accretion can be reduced and delayed. The accreted medium establishes an extended gaseous halo, and the cooling gas that forms the central disc has a higher specific angular momentum than the halo material. The preventative feedback model can reproduce remarkably well a number of observational scaling relations, including the cold baryon mass fraction-halo mass relations, star formation histories, disc size-stellar mass relation and its evolution, and the evolution of the number density of low-mass galaxies. In contrast, the ejective feedback model fails to reproduce these observational trends. These results show that the properties of disc galaxies are closely tied to the thermal state of hot halo gas and perhaps also the circumhalo medium, which suggests that observations of the disc properties and circumgalactic hot/warm medium may jointly provide important constraints on galaxy formation.