2025 Theses Doctoral

Coupling and Collisions of Galaxies and Their Atmospheres

Carr, Christopher T.

The circumgalactic medium (CGM) is a vast, multiphase atmosphere of gas bound to the halos of galaxies, caught at the intersection of inflowing gas from the cosmic web, swarming satellites, and outflows from central galaxies. Precisely how these diverse processes couple to shape the thermal and kinematic properties of the CGM—and, in turn, how the CGM regulates galaxy evolution—remains an open question in our understanding of galaxy formation. Galaxies, and by extension their halos, are also shaped by their environment and the gravitational and hydrodynamical perturbations from nearby neighbors. This dissertation investigates how the interplay between galaxies and their surrounding gaseous halos regulates star formation in low-mass galaxies, and how interactions between galaxies and their satellites—focusing in particular on the Milky Way (MW) system—affect both the distribution of stellar populations in galactic disks and the physical and kinematic structure of the MW’s dark matter and stellar halo, and CGM.

Supernova-driven outflows are thought to be a powerful regulator of a galaxy's star-forming efficiency. Outflows of mass, energy, and metals—quantified by the loading factors 𝜂_𝑀, 𝜂_𝐸, and 𝜂_𝐸 (normalized by the star formation rate, and the rates of supernova energy and metal production, respectively)—can both eject gas and metals from galaxies and heat the CGM, suppressing future accretion. To explore how ejective (high mass-loading) versus preventative (high energy-loading) feedback mechanisms shape galaxy properties, we develop a simple gas-regulator model in which the stellar mass, interstellar medium (ISM), and CGM are treated as distinct reservoirs exchanging mass, energy, and metals. In halos with masses between 10¹⁰ and 10¹² 𝑀_⊙, we show that low mass-loading (𝜂_𝑀 ∼ 0.1–10) and high energy-loading (𝜂_𝐸 ∼ 0.1–1) outflows can reproduce key scaling relations such as the stellar-to-halo mass and ISM-to-stellar mass relations. We find that model predictions are robust to changes in 𝜂_𝑀 but highly sensitive to 𝜂_𝐸, favoring values of 𝜂_𝐸 ∼ 1 in low-mass halos and ∼ 0.1 in Milky Way-like halos, with self-regulation occurring primarily through heating and cooling of the CGM.

Shifting focus to Galactic structure, we investigate the influence of satellite interactions on stellar migration in disk galaxies, using the Sagittarius dwarf galaxy (Sgr) and its ongoing interaction with the MW. We begin by applying the impulse approximation to estimate how Sgr's disk passages perturb stellar orbits. These perturbations manifest as changes in guiding radius (𝚫𝑅_𝑔) and orbital eccentricity (as measured by the maximum radial excursion, 𝚫𝑅_max). These changes follow a quadrupole-like pattern across the face of the disk that intensify at larger Galactocentric radii. We next examine a collisionless N-body simulation of a Sgr-like satellite interacting with a MW-like galaxy and find that Sgr’s influence in the outer disk dominates over internal secular evolution of orbits between disk passages. By painting the simulation with stellar populations of different metallicities and ages, we explore the observational signatures of Sgr-induced orbital migration. We find that Sgr passages imprint a quadrupole-like pattern in azimuthal metallicity variations (𝛿_[Fe/H]) and systematic changes in 𝚫𝑅_max that persist over several rotational periods. These signatures may help to distinguish between internal and external migration mechanisms shaping the chemical structure of the MW disk.

Finally, we bridge satellite dynamics and CGM physics by exploring the first infall of the Large Magellanic Cloud (LMC) into the MW. We use idealized, hydrodynamical simulations of a MW-like CGM embedded in a dark matter halo with an infalling LMC-like satellite initialized with its own CGM to study how such an encounter affects the structure and kinematics of the MW halo. We find that the LMC drives order-unity enhancements in MW CGM density, temperature, and pressure due to a 𝙈 ≈ 2 shock from the supersonic CGM-CGM collision. The resulting shock front extends from the LMC to beyond ∼ R_ ₂₀₀,_MW, amplifying column densities, X-ray brightness, thermal Sunyaev-Zeldovich (tSZ) distortion, and potentially synchrotron emission from cosmic rays over large angular scales. The MW’s reflex motion relative to its outer halo induces a dipole in CGM radial velocities, with 𝜈_𝑅 ± 30-50 km/s at 𝑅 > 50 kpc in the northern and southern hemispheres respectively, consistent with measurements in the stellar halo. Moreover, ram pressure strips most of the LMC’s CGM, leaving behind ∼ 10⁸–10⁹ 𝑀_⊙ of warm ionized gas trailing the LMC’s orbit at distances of ∼ 50–100 kpc. These results suggest that massive satellites like the LMC leave their mark on the CGM structure of their host galaxies.

We conclude with new simulations of the MW–LMC interaction that incorporate radiative cooling, star formation, and feedback. After 3 Gyr of evolution, runaway cooling along the compressive interface between the MW and LMC CGMs forms a reservoir of dense, cold gas (𝑇 ∼ 10² K) embedded within the diffuse warm LMC CGM. This cold gas is traceable via its {H 𝖨} column density, producing typical values of log 𝑁(H 𝖨) ≈ 18.5 over a trailing distance of 20–250 kpc behind the LMC. High-ion absorbers, such as {C 𝖨𝖵} and {O 𝖵𝖨}, trace the stripped warm LMC CGM and its mixed interface with the MW CGM. The column densities of both ions increase with distance from the LMC, peaking at log 𝑁(C 𝖨𝖵) ≈ 13.9 and log 𝑁(O 𝖵𝖨) ≈ 13.5 at distances greater than 100 kpc. The warm phase of the LMC CGM and the cold gas formed through enhanced cooling both present novel formation pathways for the ionized and neutral cold gas in the trailing Magellanic Stream.