Unravelling the stellar Initial Mass Function of early-type galaxies with hierarchical Bayesian modelling

Stars form with a distribution of masses, called the initial mass function (IMF). In a general sense, one can say that there are much more low-mass than high-mass stars. In recent years, continuing evidence has shown that the ratio between low-mass and high-mass stars in the heaviest galaxies of the universe is not the same as in the Milky Way. If confirmed, this variable mass distribution can help to shed light on theories of star formation and also affect other results that assume a so-called ‘universal IMF’. A careful determination of the stellar mass distribution and possible variations of it is therefore of great importance for astronomy.

In distant galaxies, we cannot observe individual stars. Therefore the stellar mass distribution of these galaxies can only be determined indirectly. In this thesis, we have developed an advanced statistical model for determining the stellar mass distribution of unresolved galaxies with spectroscopy. The results of this thesis confirm that the mass distribution of the heaviest galaxies in the universe is not the same as in the Milky Way. Our results show that the (relative) number of high-mass stars is approximately constant as function of galaxy mass. However, the (relative) number of low-mass stars changes as function of galaxy mass, such that more massive galaxies contain more low-mass stars.