The Carbon Cycle of Galactic Atmospheres: Connecting Star-Formation and Cosmic Gas Flows with the Physical Conditions of the Circumgalactic Medium

Abstract

The diffuse gaseous atmosphere surrounding the star-filled inner region of a galaxy is known as its circumgalactic medium (CGM). Galaxies draw gas from the CGM to fuel star formation; internal processes within the galaxies, in turn produce feedback that ejects gas and material back out into the CGM. This constant cycling of gas allows the CGM to maintain a record of how a galaxy builds its star population and holds key information on how galaxies begin to quench. By investigating this record, we can tell a richer story of a galaxy's past and make strides in predicting its future. In this dissertation I describe the work I have done using observations and analytical models focused on the metal ion C IV to answer two main questions. First, is there a correlation between the pc-scale physics of black hole growth and the global, kpc-scale gas flows of the CGM in observations, and how does this compare to simulations? Garza et al. (2024) suggest that C iv column density does not vary with SMBH mass; however, sSFR is highly correlated with the ionization content of the CGM. This raises the question: what do C IV observations reveal about the ionization state, temperature, and processes responsible for this ionization mechanism in the CGM? Garza et al. (2025) investigates whether a dichotomy exists for C IV between star-forming and passive galaxies (9.5 ≤ log10 M*/M⊙ ≤ 11.2). Results are statistically consistent (≥ 99%) with dichotomy findings for C iv in sub-L* galaxies (Bordoloi et al., 2014) and O VI in L* galaxies (Tumlinson et al., 2011). This suggests that C IV is more O VI-like than "low-ion-like," indicating it traces gas maintained by star formation and/or feedback, unlike other low-ionization state gas (e.g., H I, Si II, C III). In a follow-up paper, we find that C IV and O VI are kinematically coincident but the relationship between these intermediate to high ions and lower ions is not straightforward. To explore the origins of C IV and O VI, we employed a simple cooling flow model under collisional ionization (PI) and find that O VI is very consistent with the predictions for an expected temperature range, while C IV is not consistent and has column densities on average 2.5 times higher than the predictions. Since C IV has a lower ionization energy than O vi, it is possible that C vi has contributions from both the warm/hot phase and from the cool photoionized (PI) phase. Any successful model of the CGM which incorporates multiphase conditions, must result in ionized gas containing C IV and O VI that is kinematically coexist, but "avoids" existing at the same temperature and density. Ultimately, these analyses help us understand the complex relationships governing galaxy star formation rates, feedback from their supermassive black holes, and the contents of their diffuse gaseous halos.