Comprehending star formation in nearby galaxies is essential for deciphering the core mechanisms behind stellar birth. Using high-resolution CO line emission data from the Atacama Large Millimeter/submillimeter Array (ALMA), this research examines the CO excitation characteristics in two nearby star-forming galaxies, NGC 2903 and NGC 3627. We processed raw data cubes with pystructure, creating 2D moment maps to visualize CO ratios. High-resolution CO data from different rotational transitions (CO(1-0), CO(2-1), CO(3-2)) were used to study CO excitation. The study also incorporated stellar mass surface density and star formation rate (SFR) surface density maps to explore correlations between these properties and CO ratios. Using the Dense Gas Toolbox, we outlined the density structures of molecular gas, offering deeper insights into the underlying physical conditions influencing observed CO excitation patterns. We detected changes in CO line ratios that emphasize areas with elevated excitation conditions, suggesting higher gas density or temperature, which are closely associated with star formation activities. These variations suggest that non-thermal processes, such as collisions and radiation from stars, significantly influence CO excitation, as evidenced by the non-local thermodynamic equilibrium (non-LTE) excitation observed. Our findings indicate that the CO(3-2)-to-lower-J ratios are significantly affected by the SFR surface density, underscoring the influence of local star-forming environments on molecular gas excitation. Moment ratio maps display higher CO(3-2)/CO(2-1) ratios in the central regions of both galaxies, implying increased radiation and elevated temperatures in these zones. The connection between elevated CO ratios and regions with high SFR surface density further supports this relationship.x In contrast, stellar mass surface density appears to have a less pronounced effect on CO excitation, suggesting that local star formation processes, rather than large-scale galactic structures, predominantly drive the excitation conditions. This study highlights the importance of CO line ratios as diagnostic tools for understanding the excitation conditions of molecular gas in star-forming galaxies.
| Identifer | oai:union.ndltd.org:UPSALLA1/oai:DiVA.org:uu-531340 |
| Date | January 2024 |
| Creators | Roos, Linn |
| Publisher | Uppsala universitet, Institutionen för fysik och astronomi |
| Source Sets | DiVA Archive at Upsalla University |
| Language | English |
| Detected Language | English |
| Type | Student thesis, info:eu-repo/semantics/bachelorThesis, text |
| Format | application/pdf |
| Rights | info:eu-repo/semantics/openAccess |
| Relation | FYSAST ; FYSPROJ1344 |
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