Ruxin Barré

Topic

The Impact of Sub-Resolution Modelling on Brightest Group Galaxies in Cosmological Simulations

Department of Physics and Astronomy

Date & location

• Tuesday, January 20, 2026
• 9:00 A.M.
• Clearihue Building, Room B007

Examining Committee

Supervisory Committee

• Dr. Arif Babul, Department of Physics and Astronomy, University of Victoria (Supervisor)
• Dr. Sara Ellison, Department of Physics and Astronomy, UVic (Member)

External Examiner

• Prof. Ilani Loubser, Centre for Space Research, North-West University

Chair of Oral Examination

• Dr. Yu-Ting Chen, Department of Mathematics and Statistics, UVic

Abstract

Deep in the dense central regions of galaxy groups reside some of the most massive galaxies in the universe: brightest group galaxies (BGGs). Galaxy groups occupy a crucial position in the cosmic structural hierarchy, bridging large isolated galaxies and massive galaxy clusters, and collectively contain the bulk of all galaxies and baryonic matter in the local universe. The evolution of BGGs is intimately tied to their environments through interactions with neighbouring galaxies and the hot diffuse atmosphere of gas that permeates the halo. In turn, BGGs shape their surroundings through energetic outbursts from their central black holes known as active galactic nucleus (AGN) feedback. This complex interplay of internal and environmental factors makes BGGs ideal laboratories for testing how accurately cosmological simulations model the diverse, multi-scale physics driving galaxy evolution.

This thesis investigates how BGGs are represented across four different galaxy formation models used by the Romulus, Simba, Simba-C, and Obsidian cosmological simulations. My core analysis involves comparing stellar properties of the simulated BGGs to those of real BGGs observed in the universe. Through this, my work isolates how specific choices made in modelling unresolved physical processes influence the properties and evolution of the BGG populations produced by simulations.

The simulated BGGs are profoundly impacted by the strength and mechanism of the AGN feedback model. The thermal energy injection used by Romulus is inefficient at suppressing cooling flows, leading to unregulated star formation in BGGs. Simba and Simba-C use a two-mode kinetic outflow model that is heavily dependent on the activation and calibration of powerful jet feedback, which abruptly quenches BGG star formation. The Obsidian simulation incorporates a more physically realistic three-regime kinetic AGN feedback model that directly links the physics of the black hole accretion flow to properties of the resulting outflows. Through this model, Obsidian achieves the highest overall agreement with observations, producing a BGG population that quenches through a gradual decline in star formation that is the most representative of real massive galaxy evolution.

My findings reinforce the critical role of AGN feedback in regulating the assembly and growth of massive galaxies within group environments. The success of Obsidian demonstrates the importance of developing physically motivated models that can capture the nuanced evolutionary processes underlying global galaxy properties. Improving the realism of galaxy formation models will not only enhance the predictive power of cosmological simulations but also advance our understanding of the complex physical processes governing the formation and evolution of galaxies.