The Radio Sky as a Test of Galaxy Formation Models

A radio revolution is underway in astronomy. The Square Kilometre Array (SKA) project, headquartered in Manchester, is a large international collaboration to build the world's largest radio telescope across two continents. The main portion is being built in South Africa, where the SKA precursor telescope MeerKAT has already been operating since 2019, providing world-leading capabilities in resolution and sensitivity.

One such aspect is the observation of hydrogen, the most abundant cosmic element, via its 21cm transition when in atomic form (HI). So far, the study of HI in extragalactic objects has mostly been relegated to (cosmically) nearby systems, but with MeerKAT, we will be able to probe HI across more than half of cosmic time. This HI data will stringently test models of how we think galaxies form and evolve.

HI represents a key stage of gas as it falls into galaxies to form new stars, and is also sensitive to how energetic processes throw material back from galaxies into intergalactic space.

This proposal will take advantage of several large Key Projects using MeerKAT in order to carefully assess how well state-of-the-art galaxy formation simulations stack up against emerging HI observations. The PI leads the SIMBA project, which has shown to be one of the best simulations at reproducing nearby HI-21cm galaxy properties. Other simulations such as TNG and EAGLE do reasonably well locally, but predict qualitatively different evolution over cosmic time compared to SIMBA.

Thus even basic counting statistics of HI galaxies will be a discriminating test of galaxy formation models.

But these MeerKAT Key Projects contain much more than HI data. These deep fields also have excellent optical, infrared, X-ray, and other observations. This enables one to connect HI galaxies to galaxies seen via their stellar light or black hole properties, thereby situating HI within the wider context of galaxy evolution.

Again, this provides a unique test of models that, up till now, could only be done on very nearby samples. This proposal will further compare models using scaling relations of HI vs. other observable properties such as stellar mass, star formation rate, metallicity, and environment. This will provide an unprecedented set of constraints on such models, which can be used to narrow down the nature of key physical processes in galaxy evolution such as gas accretion and supernova feedback.

Finally, simulations offer a view that is not possible when observing the real Universe, by providing the entire history of the gas in and around galaxies on a particle-by-particle basis. A final aspect of this proposal takes advantage of the SIMBA simulation suite to explore the role of HI in key galactic processes, by tracking HI-rich particles backwards and forwards in time to understand where they end up, how much they drive star formation, and how they interact with large-scale structure.

This sort of particle tracking analysis will provide novel insights into the role of HI in galaxy evolution, and the physical processes that drive HI in and around galaxies.

Overall, this proposal will provide a major step forward in situating now-arriving HI observations within the context of hierarchical galaxy evolution in a Lambda-CDM universe. It will set the bar for future comparison projects as more data arrives, and establish a baseline approach for taking full advantage of the upcoming SKA era in terms of constraining the latest galaxy evolution models.