Disentangling the Effects of Mergers and Interactions on Bar Formation and Evolution

Cooke, A (2026) Disentangling the Effects of Mergers and Interactions on Bar Formation and Evolution. Doctoral thesis, Liverpool John Moores University.

Preview

Text 2026cookephd.pdf - Published Version Download (26MB) | Preview

Abstract

Bars in disc galaxies are drivers of secular evolution. They are greatly affected in turn by their surroundings, from the immediate influences of the stars, gas and dark matter within their radius to the tidal forces introduced by nearby external galaxies. In this thesis, I investigate the relative importance of these scales of influence on the bar, disentangling the effects of galaxy mergers and interactions from the effects of the host galaxy on its bar. I use the Auriga suite of Milky Way-mass magneto-hydrodynamical cosmological zoom-in simulations to do so, including high temporal resolution reruns for a detailed view of changes in the bar.
I first examine bar formation in the Milky Way using a case study from the Auriga suite with similar present-day chemo-dynamical properties, a consistent bar formation time, and a similar merger history including an analogue for the Gaia-Enceladus/Sausage (GES) merger. I also rerun this simulation from redshift 4 with the GES analogue removed to highlight the effects of the merger. In the original simulation, the GES-like merger event triggers tidal forces on the disc, gas inflows and a burst of star formation, with the formation of a bar occurring within 1 Gyr of the first pericentre. In contrast, the rerun delays bar formation by over 2 Gyr, indicating a strong possibility of the real Milky Way’s bar forming around the epoch of the GES merger due to tidal forces on its first pericentre. The rerun also indicates that the GES merger may have been responsible for continued radial growth in the Milky Way disc. I additionally find that, in other Auriga galaxies with GES-like merger events, those with stellar mass ratios below 10% form bars within 1 Gyr of the merger, while bar formation is delayed in the more massive merger scenarios. These include the 4 oldest bars in the simulation suite, reinforcing the importance of GES to the Milky Way bar.

I next investigate the influences on the evolution of bar pattern speed in Milky Way-mass galaxies across the full Auriga suite. I specifically calculate the bar pattern speed and the bar rotation rate – the ratio of corotation radius to bar length – at the time of bar formation and at ʐ = 0, and compare these to bar age, bar strength, baryon dominance, galaxy stellar mass, and the history of external galaxy interactions. I find that galaxies which are more baryon dominated at ʐ = 0 – and which lie above the observed stellar mass-halo mass abundance matching relation – host faster bars, while more dark matter dominated galaxies host slower bars. Baryon-dominated galaxies also form their bars earlier and their rotation rates stay constant or even decrease over time. This leads to older bars being faster than their younger counterparts – in contrast to the expectation of bar slow-down from dynamical friction imparted by the dark matter halo. I also find a trend in stellar mass, with ‘faster’ bars being hosted in more massive galaxies, which could be driven by the underlying higher baryon-dominance of more massive galaxies. Furthermore, I find that external interactions correlate with lower bar rotation rates, particularly for strong interactions that occur around bar formation time. However, this correlation is relatively weak, leaving internal baryon dominance as the main driver of fast bar rotation rates.
Lastly, I evaluate the applicability of the Tremaine-Weinberg (TW) method – the primary model-independent method for finding pattern speeds from observations – to the fast, cosmologically motivated bars in the Auriga suite. I find that the method is generally accurate to within 10% for the majority of systems. Nevertheless, with the exception of one double-barred system with large error, I find that the greatest errors in the TW method arise from warps in the disc. I determine that the effect of these warps on errors in the TW method is greatest at angles where the warp is most visible in the plane of the sky. These warps reach the inner disc, so I also reanalyse these results as errors in inclination and position angle of the disc. I find that, while inclination errors have little effect, the errors in the TW method produced by position angle errors are greater than those previously found in idealised simulations.

Item Type:	Thesis (Doctoral)
Uncontrolled Keywords:	Astrophysics, Galactic dynamics, Methods - simulation
Subjects:	Q Science > QB Astronomy
Divisions:	Astrophysics Research Institute
Date of acceptance:	17 September 2026
Date of first compliant Open Access:	15 October 2026
Date Deposited:	15 Oct 2026 14:47
Last Modified:	15 Oct 2026 14:47
Supervisors:	Grand, RJJ, Fragkoudi, F and Martig, M
URI:	https://researchonline.ljmu.ac.uk/id/eprint/29674

View Item