Replenishment of the cold gas reservoirs of early-type galaxies in astrophysical simulations

Baker, Maximilian 2026. Replenishment of the cold gas reservoirs of early-type galaxies in astrophysical simulations. PhD Thesis, Cardiff university. Item availability restricted.

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Abstract

Cold gas forms one of the key ingredients in galaxy evolution where it is consumed as fuel in star formation and other galactic processes. However, the processes that dominate gas replenishment, especially in massive galaxies, are unresolved. Observational results have found significant molecular gas reservoirs in ≈23% of local early-type galaxies (ETGs), with ≈40% hosting sizeable HI reservoirs. This has challenged the notion that ETGs join and remain on the tight red sequence due to lack of cold gas. Furthermore, observational probes of gas replenishment, such a stellar-gas kinematic misalignments, conflict with current models of gas replenishment in ETGs. In this thesis I use the eagle and colibre cosmological hydrodynamical simulations to investigate the origins of cold gas reservoirs of ETGs. First, I use the eagle simulation to investigate the lifetimes of stellar-gas kinematic misalignments across the galaxy population which are thought to trace recent gas replenishment. I find that, at least in eagle, kinematic misalignments are not predominantly driven by gas-rich minor mergers. Processes that significantly extend relaxation times are not dominant in the galaxy population, and instead, misalignments are formed (and prolonged) through diverse pathways which include hot halo cooling. Next, I investigate the degree to which the presence of a stellar-gas kinematic misalignment enhances the growth of the central black hole (BH) in eagle. In agreement with observations, I find that BH activity is enhanced in misaligned systems in eagle and suggest that the presence of overmassive BHs may be indicative of a past stellar-gas kinematic misalignment. Finally, I exploit the inclusion of a cold interstellar gas phase model in the colibre simulations to investigate cold gas reservoirs of a sample of ETGs. I find that colibre is fairly successful at reproducing the cold gas mass functions of ETGs compared to observations. However, unlike observations, H2 reservoirs are found too frequently in ETGs and are more common in systems that have less rotational support at a given mass. I suggest that insufficient AGN feedback in massive ETGs enables the formation of cooling flows at the high-mass end of ETGs in colibre. In summary, the results I present suggest that cold gas replenishment in ETGs occurs through diverse pathways and underscores the need for future observations to probe halo cooling and kinematics of massive galaxies

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Schools > Physics and Astronomy
Subjects:	Q Science > QB Astronomy
Uncontrolled Keywords:	galaxies, galaxy evolution, cosmological simulations, cold gas
Funders:	STFC
Date of First Compliant Deposit:	13 October 2026
Last Modified:	14 Oct 2026 08:59
URI:	https://orca.cardiff.ac.uk/id/eprint/190228

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