Bar-Driven AGN Growth

Garland, Isobelle and Simmons, Brooke (2024) Bar-Driven AGN Growth. PhD thesis, Lancaster University.

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Abstract

The co-evolution observed between supermassive black holes (SMBHs) and the galaxies that host them is poorly understood, and there are many unanswered questions. Until very recently, it was assumed that this co-evolution was driven by two or more galaxies merging, growing both the stellar mass of the galaxy and the SMBH mass. Whilst certainly a contributor to the co-evolution relationships, this theory does not account for the rapidly accreting SMBHs, also known as active galactic nuclei (AGN), observed in galaxies with merger-free histories. The majority of black hole growth has occurred in the absence of mergers, but determining the exact pathways remains elusive. In this thesis, we examine the link between the presence of large-scale, galactic bars and AGN. We first use a sample of luminous, unobscured AGN in unambiguously disk-dominated galaxies, and we describe the spectroscopic data reduction. After carefully controlling for the confounding parameters of stellar mass (M∗) and star formation rate, we look at the bar fraction in our sample and compare it to a controlled sample of inactive disk-dominated galaxies. We find that the bar fraction in the AGN hosts, f = 0.59+0.08-0.09, is slightly higher than the bar inactive sample, f = 0.44+0.08-0.09, however the two values are in agreement to within 2σ, giving us nothing more than a tantalising hint at a correlation, and preventing us from drawing conclusions with any real certainty. It is worth noting that this small sample was able to reproduce similar levels of uncertainty as previous works, despite being a factor of 20 smaller, highlighting the progress that can be made with careful controlling and sample selection. We then examine this potential correlation over a larger subset of the population of disk-dominated galaxies using GZ DESI. We identify a sample of disk galaxies, and separate them into strongly barred, weakly barred and unbarred, and we examine the AGN fraction in each. After controlling for M∗ and (g − r)0 colour, we find that the AGN fraction in strongly barred galaxies, fAGN,Sbar = 0.316 ± 0.009, is greater than than in weakly barred galaxies, fAGN,Wbar = 0.233 ± 0.008, which is in turn greater than that in unbarred galaxies, fAGN,Ubar = 0.142±0.006. These results are highly statistically significant, and resolve the decade-long debate on the correlation between large-scale galactic bars and AGN. In summary, the work presented here demonstrates not only the existence of this correlation, but emphasises the importance of large-scale surveys combined with highly controlled, extreme samples to tease out robust results. This paves the way for future work to further investigate this correlation, and understand the physics behind it in greater detail.