The cool circumgalactic medium of massive galaxies

This thesis investigates the distribution of cool gas in the circumgalactic medium (CGM) of massive galaxies and the relationships with galaxy properties using Mg II absorption measured through quasar spectroscopy. The analysis is based on large datasets of galaxy-quasar pairs from the Sloan Digital Sky Survey (SDSS) and the Dark Energy Spectroscopic Instrument (DESI), allowing a statistical approach through spectral stacking. A methodology is developed to construct galaxy-quasar pairs, remove quasar continua and produce stacked spectra as a function of projected separation (impact parameter). Mg II equivalent widths are measured using integration and uncertainties are estimated using bootstrap resampling. This method is validated using shuffled redshift tests. First, the framework is applied to SDSS data, showing a strong anti-correlation between Mg II equivalent width and impact parameter and a slight change in behaviour at ∼ 1 Mpc. Trends with the star formation rate and specific star formation rate are identified, while stellar mass is shown to have a limited effect. The CGM of post-starburst galaxies is examined using Mean Field Independent Component Analysis (MFICA) classifications by Krishna et al. (2025), revealing enhanced absorption in the CGM compared to star-forming and quiescent galaxies. The analysis is extended to DESI DR2 data, which significantly increases the available sample size. The same fundamental trends with impact parameter and galaxy properties are found, with additional investigation showing that environmment and starburst activity produce differences in the CGM. These results demonstrate that galaxy evolutionary state is crucial in determining the amount of cool gas in the CGM, as well as highlighting the power of large spectroscopic surveys in probing that cool gas.