title: Evolution of Bars in Galaxies: Effects on Star Formation and Stellar Dynamics
Creators: name: Donohoe-Keyes, C
Publisher: name: Liverpool John Moores University
Year: 2021

description: The majority of disc galaxies have a bar, and bars play a major role in the evolution of galaxies and their properties. Given the cumulative influence that bars can have over the properties of their host, determining the epoch of their formation becomes a fundamental step in understanding disc galaxy evolution. However, this is not a straightforward task. The stars that make up the bar are not necessarily formed there and bars can radially move both gas and stars within a galaxy which makes determining a bar's age from the properties of its stellar population unreliable. Additionally, while bars grow as they age, this is not a linear process and bar growth progresses differently for different galaxies. In this thesis I have explored how the effects of bars on the star formation and stellar dynamics of galaxies can be used to recover the ages of bars using a sample of cosmological zoom-in re-simulations of galaxies in isolated environments. I first explored the effect of the bar on the star formation desert (SFD) in 6 of the isolated zoom-in cosmological re-simulations. The SFD is a region within the inner ring, lying either side of the bar in the area that the bar sweeps out. James and Percival (2016) found these regions had very little to no star formation and theorised that if star formation is suppressed by the bar the youngest stars in these regions should correspond to the age of the bar. I found that the removal of gas within the SFD occurs within 1-2 Gyr after the formation of the bar indicating there is little to no in-situ star formation after that time. We would, therefore, expect to see a sharp truncation in the star formation history. However, I found a gradual downturn in the star formation history of the SFD region in comparison to that of the bar, so all stars 1-2 Gyr younger than the bar must radially migrate into the SFD region. I propose that the onset of this downturn could still be used to recover the age of the bar, although the interpretation is more difficult than anticipated. However, I also present the discovery that the SFD is a region where any young stars must be radial migrators. By combining this with a bar age it would allow us to probe the timescales and efficiency of radial migration and thus gain unparalleled insight into the chemo-dynamical evolution of the SFD region. I also explored the effect of bars on galaxy stellar dynamics. As bars evolve they vertically thicken. Therefore, younger bars have a velocity dispersion similar to that of the disc while in older bars the difference is greater. I built on this by looking at features in the vertical velocity dispersion of the bar with a sample of 15 zoom-in cosmological re-simulations and 3 simulations of isolated galaxies. I uncovered a special feature in the vertical velocity dispersion of the bar. The location of this feature is remarkably stable with time and on average is 1.5 kpc shorter than the initial length of the bar. By taking the difference between the σ_z of this feature and the bar ends I calculated a value I call Δσ_z. I was able to recover Δσ_z in both cosmological and isolated simulations and found this value increases monotonically with the age of the bar at the same rate for all the bars in the sample. The growth of Δσ_z is influenced by two factors: the lengthening of the bar, and the vertical thickening of the bar. At early times after bar formation the lengthening of the bar is the main contributor to the increase seen in Δσ_z. However, after the bar buckles, the vertical thickening becomes the main contributor to the increase of Δσ_z. Therefore Δσ_z is a powerful tracer of bar growth as it is entirely constrained by the evolution of the bar. Thus I present a new bar dating method which uses Δσ_z to infer both the formation time of the bar and an estimate of the initial length of the bar. I have tested this new method on MUSE data of IC1438 and have found good agreement with literature data. This confirms that it is both possible to apply this method to current observational data and that the bar ages recovered are reasonable. This new method presents an exciting avenue for the reliable recovery of quantitative bar ages. By applying these methods and findings to large statistical surveys we can begin to explore the time of disc settling and the onset of secular processes. I conclude that this presents us with an exciting opportunity to explore how the formation of the bar can impact galaxy evolution.