Ultra faint dwarf galaxies for probing the cosmological framework

Existence of low mass dark matter halos is one of the core predictions of the concordance cosmological model, Lambda Cold Dark Matter (LCDM). Reconciling these predictions with the observations of faint (dwarf) galaxies, which inhabit the centre of these halos, has proven to be challenging for more than a decade (Bullock & Boylan-Kolchin 2017). Addressing these challenges and understanding the formation and evolution of the low mass galaxies are therefore critical for testing the validity of the cosmological framework.

Thanks to the advancement of cosmological galaxy formation simulations in recent years, which follow the formation and evolution of galaxies self-consistently, some of the tensions concerning dwarf galaxies are addressed (Sales et al 2022). However these are mostly related to brighter (or "classical") dwarf galaxies. Understanding the formation and evolution of ultra faint ones still remain a major challenge in cosmological hydrodynamical simulations.

The importance of studying ultra faint dwarfs is manifold. Most of these extreme objects have been discovered only recently and many more can be found with new upcoming surveys such as the Rubin Observatory. The abundance of these objects are very sensitive to the properties of dark matter particles (i.e. cold vs warm dark matter), as well as the physics of galaxy formation.

Robust theoretical predictions for their abundance combined with the new observations are necessary to rule out or accept certain dark matter models. Moreover, dwarf galaxies are the most dark matter dominated galaxies in the universe and ideal places for studying the distribution of dark matter in the central parts of galaxies (aka core-cusp problem) and constrain various dark matter models, such as self-interacting dark matter.

This PhD thesis aims to probe the formation of the faintest galaxies in their low mass halos with the aim of understanding the galaxy-halo connection at the faintest end and putting constraints on the abundance of these objects. The project will exploit, and develop further, some of the highest resolution cosmological simulations of dwarf galaxy formation and evolution in the LCDM framework, as part of the LYRA project (Gutcke et al. 2021,2022).

In the first stage, the project will analyse the currently available simulations to investigate the galaxy and dark matter build up in these low mass systems, and in particular the dark matter distribution in the central regions. In the second stage, the project will further improve the galaxy formation model by including extra physical ingredients such as self-shielding.

Then, a sample of lower mass halos will be simulated and analysed in order to probe the regime where some halos remain dark for their entire lifetime. This transition mass scale, where all halos remain dark to where all of them host a faint galaxy, is poorly constrained but has significant impact on the predicted number of ultra faint dwarfs, hence dark matter models.