How the present-day distribution of dwarf galaxies encodes the physics of reionisation

The reionisation of hydrogen is one of the most important phase transitions in the early Universe: by raising the gas temperature and pressure, it prevents gas from cooling into small haloes, thus having a profound effect on the formation of the first galaxies and, by extension, the present-day small galaxies. Using a semi-analytic model of galaxy formation, I will show that two key aspects of the reionisation process — when reionisation takes place, and the characteristic scale below which it suppresses galaxy formation — are imprinted in the luminosity function of dwarf galaxies. In particular, I will demonstrate that the details of these two characteristic properties of reionisation determine the shape of the luminosity distribution of satellites in a unique way, and is largely independent of the other details of the galaxy formation model — and may have already been detected in the satellite luminosity function of the Milky Way and M31. With the increasing number of observational programmes targeting the faint galaxy population, the statistical significance of the features we identify may be confirmed.

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