Abstract
Cosmology requires at least half of the baryons in the Universe to be in the intergalactic medium, much of which is believed to form hot coronae around galaxies. Star-forming galaxies must be accreting from their coronae. HI observations of external galaxies show that they have HI halos associated with star formation. These halos are naturally modelled as ensembles of clouds driven up by supernova bubbles. These models can fit the data successfully only if clouds exchange mass and momentum with the corona. As a cloud orbits, it is ablated and forms a turbulent wake where cold high-metallicity gas mixes with hot coronal gas causing the prompt cooling of the latter. As a consequence the total mass of HI increases. This model has recently been used to model the Leiden-Argentina-Bonn survey of Galactic HI. The values of the model's parameters that are required to model NGC 891, NGC 2403 and our Galaxy show a remarkable degree of consistency, despite the very different natures of the two external galaxies and the dramatic difference in the nature of the data for our Galaxy and the external galaxies. The parameter values are also consistent with hydrodynamical simulations of the ablation of individual clouds. The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf. Thus the model explains how major mergers can make galaxies "red and dead."
Highlights
The model predicts that a galaxy that loses its cool-gas disc for instance through a major merger cannot reform it from its corona; it can return to steady star formation only if it can capture a large body of cool gas, for example by accreting a gas-rich dwarf
The Milky Way is typical of the galaxies that dominate the cosmic star-formation rate (SFR): its luminosity lies extremely close to the characteristic luminosity L∗ of the Schechter galaxy luminosity function, so it is one of the most massive galaxies that are still actively forming stars
Marinacci et al (2011) presented more elaborate simulations of clouds moving through coronal gas and showed that there is a net transfer of momentum from the cloud to the corona if their relative velocity exceeds a threshold ∼50 − 85 km s−1 that increases with the coronal density
Summary
The Milky Way is typical of the galaxies that dominate the cosmic star-formation rate (SFR): its luminosity lies extremely close to the characteristic luminosity L∗ of the Schechter galaxy luminosity function, so it is one of the most massive galaxies that are still actively forming stars. The model is still rather crude, and a number of aspects need to be worked out in greater detail, it ties together disparate data in a remarkably coherent manner, which suggests that the underlying physical picture is correct.
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