Bringing together star formation and galaxy formation calculations to interpret the stellar populations of galaxies (Astronomy Theory)

Stars have a wide range of masses, from less than 1/10 the mass of our Sun, to more than 100 times the mass of the Sun. When groups and clusters of stars are formed in our Galaxy, most of the stars have low masses (the most common stars have ~1/4 of the Sun's mass) and only a few have large masses. This distribution of stellar masses is known as the initial mass function (IMF).

The radiation produced by stars, the elements that they produce via nuclear fusion, and their fate when they die, all depend on their mass. Massive stars produce intense, high-energy radiation. They also return much of their mass into space via strong stellar winds or in supernova explosions just a few million years after they are born.

They are responsible for producing most of the heavy elements in the Universe. Stars like the Sun end their lives as white dwarfs, but much of their mass is also lost into space, for example, when they become red giants. Low-mass stars have comparatively weak radiation and winds, and live longer than the current age of the Universe, so they have comparatively little effect on their surroundings.

So the IMF is important not only for stars themselves, but because it dictates how groups of stars impact their surroundings (e.g. the galaxies in which they form).

Throughout our Galaxy, no matter where we see stars being born, the IMF seems to be very similar. However, if the distribution of stellar masses produced when groups of stars form is different somewhere in the Universe this could have huge effects, for example, on how galaxies are formed. Observations of ensembles of stars in the centres of elliptical galaxies seem to indicate that the IMF may have been different when those stars formed, perhaps producing more low-mass stars for each high-mass star.

There are also other lines of evidence that suggest that the IMF may have been different when the Universe was young, perhaps due to the lack of heavy elements in the early Universe or when the cosmic microwave background radiation that was produced by the Big Bang was hotter. Recent James Webb Space Telescope (JWST) images of very young galaxies (<500 million years old) may also help us to determine whether the stars formed in these galaxies had different IMFs from those forming in our Galaxy today.

In this project, we will use detailed simulations of star cluster formation to determine quantitatively, for the first time, how the IMF depends on the age of the Universe when the stars formed, the amount of heavy elements in the gas that formed the stars, the local level of radiation, and the density of the gas clouds. For example, preliminary calculations published by Prof Bate show that the IMF is quite insensitive to the heavy element abundance in the present-day Universe, but that when the Universe was ~1 billion years old any gas that was rich in heavy elements produced a smaller fraction of low-mass stars.

We will then use the IMFs that we determine from these star formation simulations to determine the populations of stars that are produced as galaxies form in cosmological / galaxy formation simulations. We will then simulate the radiation emitted by these stellar populations in different types of galaxies and at different ages to compare with observations of real galaxies to determine whether or not the predicted IMF variation is consistent with the properties of observed galaxies, and whether the predicted variation of the IMF can explain why the stellar populations of some of these galaxies look like they have unusual stellar mass distributions.

In this way, we will expand our knowledge of how star formation varies throughout the Universe, and how galaxies form and evolve.