Multi-wavelength analysis to constrain the role of AGN in galaxy evolution

Abstract

It is widely accepted that the formation of stars in galaxies must somehow be regulated to achieve the observed galaxy demographics. In massive galaxies in particular, the supermassive black holes that are known to reside at their centres are the most likely source of this regulation. The growth of these black holes through the accretion of gas releases considerable amounts of energy in the form of radiation and / or bipolar jets of charged particles. During these growth phases they are referred to as `active galactic nuclei' (AGN). This energy has the potential to regulate star formation in the host galaxies, by removing gas through outflows, halting the accretion of new gas onto the galaxy, and/or rendering the gas incapable of forming stars (e.g.\ through heating / turbulence). This is collectively referred to as `AGN feedback'. Around 10~percent of AGN have extreme radio luminosities due to powerful jets which are capable of preventing gas accretion onto the galaxy. However, for the remaining $\sim$90~percent of the AGN population, the so called `radio-quiet' AGN, the physical processes at play are less clear. This thesis targets a sample representative of the majority of AGN, and uses multi-wavelength observations to resolve some of the outstanding questions about how `radio-quiet' AGN transfer energy into their host galaxies, impact the surrounding multi-phase gas and, consequently, impact the evolution of their host galaxies. In the first part of this thesis I study ten local ($z~10^45 erg/s), 'radio-quiet' AGN selected to have powerful ionised gas outflows, combining radio observations (with spatial resolutions up to 20 times better than the pre-existing data), and optical integral field spectroscopy. Jet-like radio emission on ~kpc scales was discovered in 70-80 percent of the sample, and it was revealed that these jets are driving ionised gas outflows in their host galaxies. This challenges the expectation that photon pressure should dominate feedback in these systems, and establishes that jets are playing an important role in the evolution of galaxies even in this 'radio-quiet' regime. I then used observations of the total carbon monoxide emission in nine of the sources studied in the radio to demonstrate that the outflows and jets do not have an immediate, global impact on the total reservoir of molecular, star-forming gas in their host galaxies. Specifically, at least seven of the AGN observed reside in gas-rich, highly star-forming galaxies. Finally, I present the first results on the full Quasar Feedback Survey, expanding the sample studied to 42 targets and removing the pre-selection for known outflows. In this ~four times larger, unbiased sample I showed that radio jet like features are still present in the majority (66 percent) of the sample and found a correlation between radio size and the ionised gas outflow properties, indicative of jet-gas interactions. This work has confirmed a connection between outflows and radio emission, and determined that jets are an important mechanism for driving outflows even in 'radio-quiet' AGN. I also discovered that even in systems with outflows and jets there is no immediate, appreciable impact on the global molecular gas content. These results will require simulations of feedback to be updated, and represent a significant step forward in the quest to form a complete understanding of how galaxies evolve