Doctoral defence in Physics - Tamar Meshveliani

Doctoral candidate: Tamar Meshveliani

Title of thesis: The impact of Self-Interacting Dark Matter and Warm Dark Matter in dwarf galaxies

Opponents:
Dr. Azadeh Fattahi Savadjani, Assistant Professor at Durham University, UK
Dr. Camila Correa, Researcher at CEA, Paris - Saclay, France

Advisor: Dr. Jesús Zavala Franco, Professor at the Faculty of Physical Sciences, University of Iceland.

Also in the doctoral committee:
Dr. Gunnlaugur Björnsson, Research Scientist at the Science Institute, University of Iceland
Dr. David James Edward Marsh, Ernest Rutherford Fellow, Proleptic Lecturer, King's College, London, UK

Chair of Ceremony: Dr. Einar Örn Sveinbjörnsson, Professor and Head of the Faculty of Physical Sciences, University of Iceland.

Abstract:
The ΛCDM model is the most widely accepted model of cosmological structure formation and evolution. It includes a form of Cold Dark Matter (CDM), which is non-quantum, non-relativistic and collisionless. It settles into extended and dense self-gravitating haloes as a result of cosmic structure formation. CDM haloes act as a stabilizing agent for galaxies, while the late-time accelerating expansion of the Universe is sourced by a cosmological constant Λ. Due to the disagreement between ΛCDM-based simulations and observations on galactic and sub-galactic scales and the fact that traditional dark matter candidates remain undetected, the theoretical space for alternatives has been widening, making the particle nature of dark matter a fundamental question in Physics. This thesis revolves around this question in the context of two proposed modifications to the CDM cosmology: Self-interacting dark matter (SIDM) and Warm Dark Matter (WDM). In the former, strong self-interactions modify the inner dynamics of dark matter haloes, while in the latter, non-negligible thermal velocities in the early Universe suppress the abundance and inner densities of low-mass haloes. These modifications have been proposed to solve outstanding inconsistencies between CDM and observations of dwarf galaxies. In the WDM project, we use a high-resolution hydrodynamical simulation that includes the EAGLE galaxy formation model to understand how the properties and statistics of the dwarf galaxy population are related to the ~Mpc-scale environment in our Local Group. We are interested in a cosmologically underdense region that has been relatively unexplored to uncover divergent predictions of CDM and WDM. We find that the cumulative stellar mass function is almost identical for high stellar mass systems (M_* > 10^7 Msun), while it is suppressed below this mass, where WDM predicts fewer dwarf galaxies than CDM. In the SIDM project, we use a semi-analytic framework calibrated to simulations to study the final stage of the SIDM halo evolution - the "gravothermal collapse" phase. We show that in certain region of the parameter space of SIDM models, dwarf-size SIDM haloes have a bimodal distribution, with some having central density cores and others being centrally cuspy; the latter being those that have collapsed and contain an intermediate-mass black hole. This offers a promising solution to the so-called "diversity problem" in Milky-Way satellites. Finally, we extend the analysis of the core-collapse phase in SIDM haloes towards including the impact on the baryonic component within. In particular, we discuss how the use of adiabatic invariants can be exploited to predict the response of stellar orbits to the collapsing SIDM core.

About the candidate:
Tamar Meshveliani was born in 1993. She completed her BS degree in Physics from the Free University of Tbilisi in 2015 and earned a joint master's degree in Astronomy and Astrophysics  in 2018 through the Erasmus Mundus Joint Masters Degree program, studying at the University of Innsbruck, the University of Rome — Tor Vergata, and the University of Göttingen. Tamar started her doctoral studies at the University of Iceland in 2020.