In this thesis I investigate three key questions about the interstellar medium (ISM) and star formation in nearby galaxies. The first question is, “how do bars and galaxy interactions affect the distribution of cold gas and the level of central star formation in galaxies?” I use publicly-available spatially-resolved images of CO(1-0) emission in a sample of 126 nearby galaxies from the Extragalactic Database for Galaxy Evolution (EDGE) survey to measure molecular gas concentrations, and I use spatially-resolved optical spectroscopy from the Calar Alto Legacy Integral Field Area (CALIFA) survey to measure the level of central star formation enhancement. I find that gas concentration and the level of central star formation enhancement are positively correlated in barred galaxies but not in unbarred galaxies, and that interacting galaxies show signs of a correlation but not in all cases. These results indicate that central star formation enhancement occurs only in barred galaxies and interacting galaxies with high gas concentrations, which supports theories of bar- and interaction-driven galaxy evolution.
The second question is, “what is the relationship between mid-infrared (MIR) emission and molecular gas at spatially-resolved scales in galaxies?” I extend previous work, which found a tight correlation between global MIR emission in the Wide-field Infrared Survey Explorer (WISE) 12 micron band and CO emission from single-dish radio telescopes, to spatially-resolved scales using EDGE CO data smoothed to WISE 12 micron resolution. I find that these quantities are tightly correlated at ~kiloparsec scales, and that the correlation shows offsets from galaxy to galaxy. I find that these offsets are explained best by differences in the level of global near- and far-ultraviolet emission, and that the 12 micron-CO correlation is the strongest of all the resolved correlations that I considered. These results suggest that there is a tight physical link between WISE 12 micron emission and CO emission on kiloparsec scales, possibly due to a connection between polycyclic aromatic hydrocarbons (PAHs, which dominate the 12 micron emission) and molecular gas. My findings can be used to estimate resolved CO emission based on (easily obtained) WISE 12 micron images and a small number of global multi-wavelength measurements. These results also motivate further work exploring the CO-PAH connection in more diverse conditions and at higher resolution.
Finally, the third question is, “what is the ISM content of red star-forming galaxies?” In comparison to blue star-forming galaxies (“blue actives”) which lie on or above the star-forming main sequence (SFMS), these “red misfits” tend to lie on or slightly below the SFMS. I find that the main property other than colour that differentiates red misfits from blue actives is their low gas mass fractions. The gas depletion times and gas-to-dust ratios are similar between these populations. My results indicate that the star formation of red misfits is in the act of quenching.
The unifying theme of each of these projects is the approach: studying key questions in nearby galaxies based on their molecular gas content along with other multi-wavelength data, at a variety of resolutions. This approach is enabled by large publicly available multi-wavelength data sets at a variety of physical resolutions. Surveys of the global gas content of galaxies with accompanying multi-wavelength data will always be larger, and will continue to be an important reference for smaller resolved surveys. I hope that this thesis serves as a useful comparison between the science that can be done on both global and resolved scales, and will motivate future work on the connection between the ISM and star formation in nearby galaxies. / Thesis / Doctor of Science (PhD)
| Identifer | oai:union.ndltd.org:mcmaster.ca/oai:macsphere.mcmaster.ca:11375/27071 |
| Date | January 2021 |
| Creators | Chown, Ryan |
| Contributors | Wilson, Christine, Parker, Laura, Physics and Astronomy |
| Source Sets | McMaster University |
| Language | English |
| Detected Language | English |
| Type | Thesis |
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