Merging galaxies and clusters

Mergers have long been understood to be a driver of galaxy and galaxy cluster evolution. They release tremendous amounts of gravitational potential energy -- $\mathcal{O}(10^{59})$ and $\mathcal{O}(10^{64})$ ergs in galaxies and clusters, respectively -- which is dissipated in powerful shock waves. In galaxies especially, the strong tidal effects can have profound effects on the remnant morphology. Although the modelling of mergers has a long history, it is only recently that it has been fully appreciated just how sensitive they are to a range of factors, including the existence of circumgalactic media (CGM), accretion along filaments, and pre-existing magnetic fields. Modelling these aspects in a cosmologically-consistent manner necessitates the use of high-resolution cosmological magnetohydrodynamic (MHD) simulations. In this work, we use such simulations to investigate two distinct merger-related phenomena: i) magnetic fields in galaxy mergers, and ii) the origin of \textit{radio relics} in galaxy clusters. For the first topic, we isolate the impact of magnetic fields by running a series of hydrodynamic and MHD ``zoom-in'' simulations of disc galaxy mergers. We show that magnetic fields are able to have a major impact on the morphology of the remnant galaxy: remnants in MHD simulations form extended discs with flocculent spiral structures, whilst hydrodynamic simulations form compact remnants with bar-and-ring morphologies uncommon in observations. We show that a small-scale dynamo can only develop given sufficient resolution, explaining why this effect has not been seen before. Furthermore, we present a mechanism that explains \textit{how} the magnetic fields affect mergers: we find that the amplified magnetic field alters the transport of angular momentum during the merger, with subsequent resonances, stellar feedback, and accretion ultimately causing the diverging morphologies. Finally, we show that the impact of such mergers can be felt even in galaxies that have more quiescent merging histories. We conclude that magnetic fields are thus essential for the accurate simulation of disc galaxies. For the second topic, we start by outlining the seven major problems that currently challenge our understanding of radio relics. To solve these, we use a hybrid strategy; first running zoom-in simulations of cluster mergers to identify typical shock conditions at relic distances, before using the results to inform a series of idealised shock-tube simulations. We find that, upon colliding with an accretion shock, merger shocks produce a thin, dense sheet, which propagates into upstream density fluctuations. This scenario leads to a) the formation of a Mach number distribution, b) shock corrugation, and c) a Rayleigh-Taylor instability at the contact discontinuity. These effects flatten cosmic ray electron spectra, biasing radio-derived Mach numbers high, thereby explaining the observed discrepancy with X-ray derived Mach numbers. They also lead to additional compression, which produces the observed $\upmu$G-strength magnetic fields, and turbulence behind the shock front, which invalidates laminar-flow based cooling models. Furthermore, we find that the tail of the Mach number distribution dominates the radio emission. This explains the observation of radio relics in shocks with $\mathcal{M}_\rmn{X-ray} \lesssim 2$, despite the increasing evidence for the existence of a critical Mach number of $\mathcal{M}_\rmn{crit} \approx 2.3$, below which cosmic ray electron acceleration is ineffective. Finally, we find that upstream density turbulence can explain many aspects of radio relic morphology. We hence solve four of the seven outstanding problems, with partial answers provided to a fifth.