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Galaxies are surrounded by sizeable gas reservoirs which host a significantamount of metals: the circum-galactic medium (CGM). The CGM acts as a mediatorbetween the galaxy and the extra-galactic medium. However, our understanding ofhow galaxy mergers, a major evolutionary transformation, impact the CGM remainsdeficient. We present a theoretical study of the effect of galaxy mergers onthe CGM. We use hydrodynamical cosmological zoom-in simulations of a majormerger selected from the Illustris project such that the z=0 descendant has ahalo mass and stellar mass comparable to the Milky Way. To study the CGM wethen re-simulated this system at a 40 times better mass resolution, andincluded detailed post-processing ionization modelling. Our work demonstratesthe effect the merger has on the characteristic size of the CGM, itsmetallicity, and the predicted covering fraction of various commonly observedgas-phase species, such as H I, C IV, and O VI. We show that merger-inducedoutflows can increase the CGM metallicity by 0.2-0.3 dex within 0.5 Gyrpost-merger. These effects last up to 6 Gyr post-merger. While the mergerincreases the total metal covering fractions by factors of 2-3, the coveringfractions of commonly observed UV ions decrease due to the hard ionizingradiation from the active galactic nucleus, which we model explicitly. Ourstudy of the single simulated major merger presented in this work demonstratesthe significant impact that a galaxy interaction can have on the size,metallicity, and observed column densities of the CGM.

Galaxy mergers moulding the circum-galactic medium - I. The impact of a major merger