Investigating the role of ionised outflows in galactic evolution through spatially resolved spectroscopy

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Gallagher, Robert Michael 

How feedback mechanisms trigger, regulate and suppress star formation in galaxies remains one of the main questions in astronomy today. Theoretical models and cosmological simulations of galaxy formation and evolution frequently employ feedback mechanisms from active galactic nuclei (AGN) and supernovae (SNe) to quench star formation in the early stages of massive galaxy formation and to explain the low efficiency of star formation in disk galaxies. A majority of this feedback is likely due outflows, ejecting large reservoirs of star forming gas from the galaxy and quenching star formation, powered by either AGN or SNe. However, recent theoretical and numerical models suggest that feedback due to outflows may also be able to promote star formation in gas in both galactic discs and entrained within outflows themselves. This work focus upon studying the ionised component of outflows in the local Universe, with a particular focus on ’positive’ feedback, where outflow feedback may trigger star formation in gas both in the galactic disk and inside the outflow itself, but also investigating the physical properties of galaxies and their negative feedback effect. To achieve this, I have developed a complex software specifically to explore multiple gaseous kinematic components in galaxies. I have applied this software to investigate outflows in the SDSS-IV MaNGA dataset, as well as data from other instruments. In the first part of this thesis I discuss the first confirmed detection of star formation within a galactic outflow. By investigating the optical and near-IR emission line ratios of the outflowing gas, I unambiguously prove star formation is occurring within the gas. This is supported by the ionisation parameter of the gas, showing the gas excitation must come from UV radiation due to stars in the outflow and not stars in the galactic disk. Finally, I trace kinematical fingerprints of young stars blueshifted relative to those in the galactic disk. The second part of this thesis focuses upon expanding the study of star formation in outflows in an unbiased sample of local galaxies. Through emission line ratios I find ∼30% of our outflow sample show signs of in-situ star formation. This is supported again by the ionisation parameter, showing the gas excitation must be due UV radiation of young stars in the outflow. Investigating the relationship between star formation rates within the outflow and ionised outflow mass rates, I find a relationship implying large amounts of star formation should occur in high-z outflows. This new channel of star formation has numerous implications for galactic evolution. The final part of this thesis focuses on general properties of ionised outflows and their effect on their host galaxies. I classify 168 outflows in local galaxies. Emission line ratios of the galactic disk show our outflow-host galaxies form a representative sample of local galaxies, v indicating outflow prevalence in all types of galaxies. I find a large fraction of the outflows are AGN-driven, though most are star formation-driven. Most star formation-driven outflows exist on or above the ’Main Sequence’ in the SFR-M∗ plane, suggesting the importance of starbursts for star formation-driven outflows. AGN-driven outflows exist on and below the main sequence, suggesting their efficiency at driving outflows in all types of galaxy. By studying the gas density through the [SII] doublet, I find outflowing gas is denser than gas in the galactic disk by a factor ∼3, likely as a consequence of gas compression. Finally, by studying a number of properties of the outflowing gas, I find that some outflow properties (e.g. dynamical timescale) may be influenced by the properties of their host , whilst other properties (e.g outflow velocity) are more affected by the driving source of the outflow .

Maiolino, Roberto
Galactic Evolution, Outlfows, Galaxies
Doctor of Philosophy (PhD)
Awarding Institution
University of Cambridge
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