Hogarth, Lucy M; (2024) Going with the Radial Flow: How molecular gas flows affect the large-scale evolution of galaxies. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The properties of galaxies which drive variations in the position of galaxies along the Kennicutt-Schmidt relation continue to be elusive to both observation and model based studies. In this work, we consider the role of radial molecular gas flows along the plane of galactic discs in creating scatter in not only the star-forming efficiency (SFE) of the host galaxy, but also its secular evolution. The first two sections of this work focus on a sample of eight edge-on galaxies on the star-forming main sequence, which are identified as hosting galactic-scale outflows of ionised gas. We perform a direct comparison between molecular gas maps created with the Atacama Large Millimeter/submillimeter Array (ALMA) and ionised gas integral field spectroscopy (IFS) from the Sydney Australian Astronomical Observatory Multi-object Integral Field Spectrograph (SAMI) Galaxy Survey, both resolved on scales of ≈ 1 kpc. Our most notable finding in this analysis is that molecular gas is more centralised in our sample of galaxies with large-scale ionised outflows compared to a corresponding control sample. Following this work directly, we create full three-dimensional molecular gas kinematic models of these objects and the control sample. This is with the motivation of detecting signatures of non-circular/disturbed kinematics, which could potentially be powering the centralisation of molecular gas. We find possible signatures of radial gas flow in a subset of our sample harbouring ionised outflows, suggesting one potential mechanism for driving molecular gas towards the centre of these galaxies. Radial gas flows are often triggered by axi-asymmetric features, such as a bar or morphological asymmetries created during a merger. We theorise that the radial gas flow we detect in a subset of our objects may be caused by the presence of a bar or minor-merger, but are unable to confirm this due to their high inclination. In the final part of this body of work, we use a larger sample of galaxies from the ALMA MaNGA Quenching and Star-Formation (ALMaQUEST) Survey to study the statistical relationship between the presence of optical bars and radial molecular gas flows. Radial gas flow is detected with the same method of kinematic modelling used on our smaller edge-on sample referred to previously. We find that galaxies hosting bars are more likely to have radial gas flows than galaxies classed as un-barred or edge-on in the ALMaQUEST. Moreover, these classes also reveal variation in the measured SFE relative to the expected SFE given the galaxies' places on the star-forming main sequence (ΔSFE). Galaxies classed as hosting an optical bar with radial molecular gas flow appear suppressed in ΔSFE relative to both barred objects with no detectable radial flow and un-barred objects. It is possible, therefore, that a component of the scatter we observe in the Kennicutt-Schmidt relation between galaxies is driven by the internal dynamics of their molecular gas. We interpret this in the context of the ``Compaction Scenario'', where the motion of molecular gas regulates a galaxy's movement above and below the main sequence as gas cycles through phases of compaction and subsequent dispersal following a central starburst.

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