Mr Andrew Sullivan

I am a first-year PhD student at ICRAR-UWA and I am studying theoretical dark matter halo idealisations and investigating their implications for observationally constraining the halo mass. 

I completed my undergraduate Bachelor of Philosophy Honours, with a double major in Physics and Mathematics at the University of Western Australia in Perth, with one full year spent studying abroad at the University of Canterbury in Christchurch, New Zealand.

My passion is and always has been science; in particular the exciting prospect that there remains so much that we do not know about the world and the Universe around us.

As it stands, the Universe is thought to be made from about 70% dark energy, 25% dark matter, and just 5% of ordinary matter. As we do not know the physics of 95% of our Universe, research in astrophysics is an exciting and wide-open scientific frontier, with implications as colossal as the future of our Universe and its distant past. 

Whilst we may not know the details of these mysterious dark components, scientists have been able to study their properties on large scales, allowing us to begin to understand their importance in shaping the Universe we find ourselves today.

Over the past few decades astrophysicists have developed compelling models for the structure of dark matter, and its role in the formation of the first galaxies. Current theories of structure formation suggest that all galaxies (including our Milky Way) are embedded within massive dark matter ‘halos’, which are thought to be the fundamental building block of our Universe. Whilst not able to be visibly seen, these halos can be detected from their gravitational influence on stars and other luminous tracers embedded inside their potential, as well as from their distortion of space-time due to their large mass. This allows us to infer the size and mass of dark matter halos observationally; typically from the velocity dispersion of luminous tracers, the X-ray temperature of gas embedded within, and from the gravitational lensing of more distant light, as predicted from General Relativity.

Fundamentally, the particle nature of dark matter places constraints on the size and abundance of dark matter halos that could have formed in the early Universe. At present, the most favoured model for dark matter is Cold Dark Matter; with ‘cold’ describing particles that were travelling at low, non-relativistic speeds in the early Universe. Whilst the Cold Dark Matter model appears to describe our Universe incredibly well, alternative dark matter models such as Warm Dark Matter (‘warm’ meaning travelling at faster, relativistic speeds at early times) or Interacting Dark Matter (as opposed to being collisionless) cannot be entirely ruled out.

Due to the dependence of dark matter halos on these models, in a perfect Universe where the halo mass of every halo could be calculated with pin-point accuracy, it would be possible to rule out all but one of these dark matter models. Unfortunately, this remains a faraway dream, and often only the largest halos can be observationally studied, and even then only measured with large uncertainties. Due to this inherent problem, developing more precise and applicable halo mass estimators is an exciting and fast-moving frontier of modern astrophysics and cosmology. 

Today, dark matter is typically studied in large cosmological simulations using state of the art computational methods. These models allow observers to compare their results to those predicted in simulations, allowing constraints to be placed on the halo mass of real halos from the scaling relations expected in the computational models. Despite the power of these computational tools, there are often a plethora of underlying assumptions, leading to results that are model-dependent. This motivates the need for a strong theoretical footing to model the structure and dynamics of dark matter halos; to allow observers to estimate the halo mass more accurately, without relying on any significant assumptions or simulation calibrations. 

During my PhD, I plan to develop a rigorous mathematical framework for dark matter halos with an idealised, scale-independent, minimal parameter model that is widely applicable to equilibrium halos. This model will assume the halo is in a stable, theoretical state: perfectly spherically symmetric, in complete virial and hydrostatic equilibrium, with a relaxed outer configuration and with only simple baryon physics. Despite these inherent simplicities, this idealised model is motivated by its ability to approximate on average the expected macroscopic structure of dark matter halos that are in a stable configuration, without recent collisions or mergers.

In this way, I plan to investigate the dependence of these halos on a fixed parameter space, and derive mathematical relations for the halo mass in terms of observational properties whilst remaining agnostic to the parameter values. With theoretical models for observational proxies such as the gravitational lensing profiles, the velocity dispersion profiles and the X-ray temperature profile fully-determined from this framework, the hope is that these mathematical models can help guide future and upcoming surveys for the halo mass function.