Abstract
The main way galaxies grow is through hierarchical merging with other galaxies, during which gas is funnelled from the outskirts into the galaxy centres and can increase star formation (SF). This way, mergers can ignite active galactic nuclei (AGN) and/or starbursts (SBs), which can impart feedback into the interstellar medium (ISM) of the host galaxy.
In the local universe, ultra-luminous infrared galaxies (ULIRGs) are thought to exclusively be the result of galaxy mergers. These galaxies are powered by a combination of AGN and/or SB, and are very luminous at infrared (IR) wavelengths (λ = 8−1000 μm). Feedback from AGN and SB influences the SF inside galaxies and is therefore a key factor for understanding the evolution of galaxies as a whole. One possible manifestation of feedback is galactic outflows, which can contain atomic, ionised, and molecular gas phases. Molecular outflows embed dense gas that can directly be used as fuel for SF and could therefore be a direct link to the suppression of SF in its host. ULIRGs host these powerful feedback mechanisms and are therefore perfect observational targets for obtaining information about these molecular outflows and how they impact their host.
In this thesis, we perform a spectral analysis of four local (U)LIRGs to investigate the presence and excitation of the dense molecular gas in possible outflow components of these galaxies. We utilise Atacama Large Millimetre/submillimetre Array (ALMA) Band 3 data to trace multiple molecular gas tracers. We compare the line luminosity ratios of CN/CO (1-0), HCN/CO (1-0) and HCO+/CO (1-0) of different spectral components in each of our four targets. This gives us insight into the relative content of high-density gas traced by CN, HCN and HCO+ compared to that of CO, which traces the majority of the molecular gas. Only IRAS 13120 – 5453 out of our four targets show a high CN/CO (1-0) luminosity ratio of 1.09 ± 0.17 in its red wing (200 < v < 300 km s−1), where the emission might arise from outflowing gas. This luminosity ratio is significantly higher than for all galaxies in the literature, but is similar to that found in the outflow in Mrk 231 by a previous study. We argue that this enhancement could be due to the outflow either being exposed to a strong external UV-radiation field or from young stars in the outflow itself due to in-situ SF.
Supervisors:
- Claudia Cicone, Professor at Institute of Theoretical Astrophysics, UiO
- Stephen John Molyneux, Postdoctoral Fellow at Institute of Theoretical Astrophysics, UiO
Intern. assessor: Øystein Elgarøy, Professor at Institute of Theoretical Astrophysics, UiO
Extern. assessor: Shenli Tang, Senior Research Assistant at University of Southampton