Multiphase Galactic Outflows: Physical Properties and Feedback Effects
Feedback mechanisms are invoked in theoretical models and cosmological simulations to regulate or even suppress star formation in galaxies. One feedback route is through galactic outflows, which eject large amounts of gas and thereby remove the fuel for future star formation. Such galactic outflows can be driven by active galactic nuclei (AGN) or processes associated with star formation such as supernovae or radiation pressure from young stellar populations. The goal of this thesis is to study outflows from an observational perspective in different phases to quantify their impact on their host galaxies comprehensively, primarily by using optical and millimetre band data from the Multi Unit Spectroscopic Explorer (MUSE) and the Atacama Large Millimeter/submillimeter Array (ALMA). In the first part of this thesis, I study the physical mechanisms driving outflows as well as whether outflows are capable of suppressing star formation using a study of molecular gas outflows with ALMA and archival data. In a sample of 50 local galaxies with outflow signatures in low-J carbon monoxide (CO) transitions, I find most outflows are consistent with the radiation pressure-driven or the energy-driven (but with a low coupling efficiency) models. Furthermore, most outflows are unable to escape the galaxy’s gravitational potential (with 5 % or less of outflowing gas exceeding the escape velocity), and the depletion time-scales of the galaxies’ total gas reservoirs are typically of the order of 10^8 years or more. Based on these findings, I conclude that outflows are unlikely to quench their entire host galaxy but might affect the central regions of the galaxy. Second, I investigate the physical conditions of the outflowing gas and the link between different gas phases in the galactic outflows. While there is observational evidence of outflows in many different phases, the connection between them and their relative importance is poorly understood. My results based on MUSE observations of 26 local (ultra)luminous infrared galaxies ((U)LIRGs) suggest that outflows have about three times higher electron density than the galaxy’s disc (< n_e,disc > 150 cm^-3 and < n_e,outflow > 500 cm^-3 ), but they are less dust obscured than the galaxy’s disc. Furthermore, I establish that the molecular phase is dominant in terms of mass and energy at all AGN luminosities, while the ionized gas phase is negligible in AGN host galaxies and at most a few per cent of the total mass or energy budget in star forming galaxies. The atomic neutral phase is slightly less massive and has lower kinetic power than the molecular phase. Third, I analyse galaxies which exhibit positive feedback, which are instances where outflows trigger or enhance star formation. My results suggest that about 30 % of outflows show signs of star formation inside them, as the gas in these outflows is characterised by SF-like BPT diagnostics. The star formation rate inside these outflows is correlated with their ionized mass outflow rate. These findings suggest this mode of star formation might be more prominent at higher redshifts, and may contribute to the formation of the spheroidal component of galaxies.
Galaxy formation, AGN, Outflows, quasars, negative feedbacck, positive feedback
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