Massive Coronae of Galaxies

There is reason to suspect that about half of the baryons in the universe are in pressure-supported plasma in the halos of normal galaxies, drawn in by gravity along with about half of the dark matter. We present a model for this substantial baryonic component, the galactic coronae, that fits the available observational constraints. This phenomenological approach requires departures from state-of-the-art numerical models of galaxy formation, but the adjustments are not so large as to seem unreasonable. In particular, massive coronae would have to be hotter than the kinetic temperature of the halo dark matter so as to produce acceptable central electron densities. This higher temperature might result from the difference of the fluid dynamics of the baryons and the collisionless dynamics of the dark matter during the assembly of the protogalaxy, in an analogy to what seems to happen in cluster formation. The cooling time of a massive corona would be longer than the gravitational collapse time but, in the inner parts, shorter than the Hubble time, making the corona thermally unstable to the formation of a cloudy structure that is settling and adding to the mass in interstellar matter and stars. Since in this picture the mass in the corona of a spiral galaxy is much larger than the mass in condensed baryons, the corona would be a substantial reservoir that could supply matter for star formation in isolated spirals continuing well past the present epoch.