The impact of strong and weak bars on galaxy evolution
Keywords: galaxy morphology
galaxy zoo
kinematics
strong and weak bars
galaxy evolution

Bars are common structures in disc galaxies that are thought to be long-lived and to have the ability to redistribute matter and angular momentum in their hosts. This implies that bars are important in the evolution of galaxies. Previous studies have traditionally been focussed on strong bars, as they are easier to identify and observe, while the role of weak bars remains relatively unclear. In this thesis, I study the impact that both strong and weak bars have on their hosts, especially in the context of galaxy evolution and quenching.
Bars are typically more common in quiescent galaxies, but it is uncertain whether it is easier to form a bar in a quenched galaxy or if a bar helps to quench its host. In order to answer this question, I use Galaxy Zoo DECaLS to identify a large volume- limited sample of 1,867 galaxies from a wide range of bar types. Using data from SDSS and ALFALFA, I find that the strong bar fraction is higher among quiescent galaxies, consistent with prior work. However, this was not found for weak bars, which seem to be distributed equally between star forming and quiescent galaxies. Additionally, I find that strongly barred star forming galaxies have lower gas masses, higher fibre star formation rates and shorter depletion timescales, compared to unbarred star forming galaxies. Interestingly, this was not found for weakly barred star forming galaxies. I conclude that star forming galaxies with a strong bar are typically rapidly evolving galaxies that quench more efficiently due to the presence of a strong bar. I also find that the differences between weakly and strongly barred galaxies disappear when accounting for bar length and suggest that weak and strong bars are therefore not fundamentally distinct physical phenomena. Instead, I propose that they are part of a continuum of bar types, which ranges from ‘weakest’ to ‘strongest’.
I also study the kinematics of bars, as this can provide insight into their formation and evolution. This is done by applying the Tremaine-Weinberg method on MaNGA IFU data of a sample of 122 strongly barred and 103 weakly barred galaxies identified using Galaxy Zoo DESI, which is the largest sample this method has been applied to so far. I found that strongly barred galaxies have significantly lower values for the bar pattern speed and R, the ratio of the corotation radius to the bar radius, than weakly barred galaxies. As simulations have shown that the bar pattern speed decreases as the bar evolves, these results suggest that strong bars are more evolved structures than weak bars. Additionally, simulations have shown that the ratio R is correlated with the bar formation mechanism. This implies that strong bars are more likely to be formed by global bar instabilities, while weak bars are more likely triggered by tidal interactions. Ultrafast bars (bars that have R < 1) should theoretically not exist, however, they are repeatedly found in observational studies. I find a much lower fraction of ultrafast bars in my sample compared to the literature: 11% are ultrafast, while only 2% of the sample are confidently within the ultrafast regime. The overall distribution of R in my sample is higher than what is typically found in other studies, which also decreases the recently claimed tension with ΛCDM.
I use resolved star formation indicators obtained from MaNGA to study the effect that strong and weak bars, as well as fast and slow bars, have on different regions of their hosts. I found that strongly barred star forming galaxies have significantly more star formation in their centre and region beyond the bar-end, while suppressing star formation in the arms of the bar, compared to weakly barred star forming galaxies. These results are consistent with strong bars facilitating quenching, while weak bars do not. I also found that, while the global star formation rate remains the same between fast and slow bars, slow bars concentrate the star formation along the barred regions more than fast bars. I hypothesise that this is due to the greater difference in velocity between the bar-end region and the gas in the disc for slow bars than for fast bars. I also find preliminary evidence for a synergistic effect between bar strength and kinematics: slow strong bars seem to affect their hosts the most. Based on these results, I suggest to add a second, kinematic, axis to the aforementioned bar continuum. However, an increased sample of barred galaxies with IFU data is needed to study this in more detail. In any case, these results highlight that it is important to study the kinematics of bars in order to fully understand galaxy evolution.