title: A MUSE search for the kinematic signatures of gas inflows to the nuclei of galaxies
Creators: name: Kolcu, T
Publisher: name: Liverpool John Moores University
Year: 2024

description: Gas inflows contribute to the evolution of galaxies in a plethora of ways. For instance, inflows can enhance star formation, lead to the formation of stellar substructures like nuclear rings and trigger starbursts. Further, gas reaching the nucleus can feed the supermassive black holes (SMBH), power active galactic nucleus (AGN) and trigger outflows, consequently causing quenching. Understanding the mechanisms causing gas inflows to the nucleus is essential for understanding the processes that drive evolution of galaxies. Mechanisms causing gas inflows, particularly inflow to the nucleus of galaxies, have been a prominent focus of numerous observational and theoretical studies. One of the well- established mechanisms causing gas inflows is extended shocks driven by bars. These extended shocks persist over large regions and should be observable as coherent velocity jumps in the gas kinematic maps. However, these shocks weaken in the central kpc. Despite extensive research, it remains unclear whether the dominant mechanism for gas inflows to galactic nuclei is analogous to the extended shock mechanism observed along bars, or whether it has a different origin. In my thesis research, by studying the kinematic maps of barred and unbarred galaxies, I search for the signatures of extended shocks, similar to those seen along bars, within the innermost galactic regions to determine whether extended shocks are the dominant mechanism of inflow to the galactic nuclei. In this thesis, using data from the Multi Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, I study ionised gas kinematic maps of ∼21 galaxies from the Composite Bulges Survey (CBS) and the Time Inference with MUSE in Extragalactic Rings (TIMER) Survey of nearby galaxies and search for extended shocks reaching to their nuclei. I first present my pilot study using the galaxy NGC 1097 where I developed a methodol- ogy for identifying signatures of extended shocks. The coherent appearance of extended shocks in the kinematic maps can be distorted by inaccurate extraction of the velocity measurements and dominated by the global rotational flow and local perturbations like stellar outflows. If emission lines contain excess emission, associated with emission from additional sources, I include multiple components in the emission-line fits to correct the inaccurate measurements from wrong fits. Moreover, I show that removal of the global rotational flow by subtracting the circular velocity of a fitted flat disk can produce artefacts that obscure signatures of the extended shocks in the residual velocities if the inner part of the disk is warped or if gas is moving around the centre on non-circular trajectories. As an alternative, I propose a model-independent method which exam- ines differences in the line-of-sight velocity distribution (LOSVD) moments of Hα and [N II]λ6583 emission lines. This new method reveals the presence of continuous shocks in the regions inward from the nuclear ring of NGC 1097, in agreement with nuclear spiral models. However, differences between LOSVD moments of Hα and [N II]λ6583 might be disguised if star formation or AGN activity dominates the kinematics. To strengthen the argument for the presence of extended shocks, I also study the origin of ionisation through line flux ratios, and I look into the dust morphology. I conclude from my pilot study, which is further confirmed in the analysis of the sample studied in this thesis, that relying solely on one approach is insufficient to rule out the presence of extended shocks in galaxies. It is crucial to employ multiple methodologies when searching for extended shocks in gas. Following my pilot study, I provide a detailed kinematic analysis of each galaxy I studied in this thesis. This study led to classifying galaxies based on the presence and charac- teristics of extended shocks within their inner kpc. Moreover, I further explore how the presence and characteristics of extended shocks relate to the dominant nuclear activity in galaxies. I also search for correlations between characteristics of extended shock sig- natures and the presence of nuclear bars and nuclear rings obtained from CBS, as well as with the overall characteristics of the galaxies. I conclude that extended shocks to the nucleus are prominent in 52% of the galaxies in my sample. This figure is a lower limit since 15% of the galaxies in the sample exhibit extended shocks along bars, but their extension into the nucleus can not be traced due to obscuration from AGN outflows. The signatures of extended shocks exhibit deprojected residual velocity amplitudes in agreement with models and former observational studies. These results imply that the extended shocks may be the dominant mechanism of inflow to the nucleus of galaxies. I find a clear absence of extended shock signatures in unbarred galaxies, which highlights the critical role of bars in driving these shocks and facilitat- ing gas inflows to the galactic nuclei. Galaxies with nuclear bars only exhibit LINER or Seyfert type emissions within their inner 150pc, while galaxies without nuclear bars show a diverse range of emission mechanisms, including a significant contribution from star formation. This indicates that in the presence of nuclear bars, the inflows preferen- tially feed the AGNs while in the absence of nuclear bars inflows can also result in star formation. I also find that the majority of galaxies with nuclear rings present extended shock signatures, whereas galaxies lacking nuclear rings generally do not exhibit these signatures. This finding highlights the crucial role of extended shocks and gas inflow in the formation of nuclear rings. Finally, I find that galaxies exhibiting extended shocks with higher amplitudes tend to have higher average Hα velocity dispersions within their inner ∼2kpc, suggesting that these shocks contribute to overall gas turbulence.