Charlotte Christensen

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How did galaxies form and evolve into their current state? What physical processes determined their composition and structure? These are the types of questions I, along with my collaborators and students, are endeavoring to answer. In order to do so, I create and analyze massive, high-resolution computer simulations that follow one or more galaxies from soon after the Big Bang to the present day. Simulations like these enable me to track the history of a given galaxy across cosmic time and to experiment with changing the physical models at play — two things I'm not allowed to do with the actual Universe!

Below are descriptions of some of my current research interests. If you are a Grinnell student curious about this line of research, please get in touch!

Movie showing the formation of a simulated low-mass spiral galaxy. The grey-scale represents gas, the red and blue represents stars, colored according to their age.

The Quenching of Star Formation in Dwarf Satellites

Dwarf galaxies, galaxies with masses about 10% that of the Milky Way or smaller, such as the Magellanic Clouds, are perfect laboratories for studying galaxy evolution. The small gravitational potentials of dwarf galaxies make them uniquely sensitive environments for understanding the physics of galaxy formation, including the processes that drive gas accretion, gas loss, and star formation. Dwarf satellites of the Milky Way or similar nearby galaxies may help constrain these processes, but only if the effect of the large halo environment the dwarf galaxies exist in can be well understood. For my NSF CAREER grant, I am using extremely high-resolution cosmological simulations of Milky Way-mass galaxies to examine how the halo environment affects satellite evolution by modifying the rates of gas accretion and expulsion. Specifically, I am determining the relative importance of ram pressure stripping, strangulation, and stellar feedback in quenching star formation and how these processes may conspire together. I am also examining the baryons and metals retained in satellites compared to field dwarfs and measuring observational signatures of satellite quenching mechanisms. Finally, I am studying how satellites enrich the circumgalactic medium (the gas surrounding galaxies) and the material accreted onto the host galaxy.

Image colored according to gas surface density showing several dwarf galaxies

In this project, I am analyzing one of the largest and highest-resolved samples of simulated dwarf galaxies in collaboration with researchers at Rutgers University, the University of Oklahoma, Queensborough Community College, and the University of Washington. With these simulations we are answering questions like: How is gas driven from dwarf galaxies? What is the role of galactic environment in determining dwarf galaxy structure and star formation? How do black holes form in dwarf galaxies? And what is the correlation between the mass of stars in a dwarf galaxy and its dark matter?


A Reservoir of Gas around Galaxies

What we tend to think of as a "galaxy" — the stars and the cold gas that lies among them — is actually embedded in a much larger structure consisting mainly of dark matter but also including hot, low-density gas. This hot, low-density gas surrounding galaxies is known as the circumgalactic media, and it plays an important role in the continued growth of galaxies. The circumgalactic media both provides a staging ground for gas prior to accretion onto the galaxy and a repository for gas that used to exist within it. As such, it both preserves the galaxy's history and determines its future.

Because of the low densities of the circumgalactic medium, it is very difficult to observe. Nevertheless, observers, in concert with computational astronomers, are making progress in characterizing the extent, structure, thermodynamic properties, and chemical composition of the circumgalactic media. I am currently analyzing the history, content, and morphology of the circumgalactic medium around the previously mentioned large sample of simulated isolated dwarf galaxies in order to better understand the accretion and outflow properties by which dwarf galaxies grow. In order to bridge the gap between theory and observation, I also generate mock observations of the circumgalactic media around these galaxies. These comparisons are important for determining the validity of computational models and in interpreting observational data.


Analyzing Galactic Outflows

Galaxies evolve through a balance of gas accretion and gas loss. These two processes are, in fact, intrinsically connected: the accretion of gas provides new material for star formation while the resulting new stars provide energy (for example, through supernovae) capable of expelling gas from the galaxy. In order to follow both of these processes, I track the gas within my galaxy formation simulations. Through this tracking, I have been able to determine the efficiency at which galaxies of a given mass expel their gas and heavy elements, where this material is distributed to, and the likelihood of it being reaccreted onto the galaxy.


Simulating the Star Forming Gas

Through observations, astronomers have found that star formation is strongly associated with the presence of cold molecular hydrogen gas. For my thesis with Tom Quinn, I analyzed the effect of tying star formation to the presence of molecular hydrogen, including implementing a method for following the non-equilibrium abundances of molecular hydrogen in smoothed particle hydrodynamic galaxy formation simulations. More recently, astronomers have wondered whether the correlation between molecular hydrogen and star formation is not causal, but rather both require the gas be shielded by dust grains from radiation. Lindsey Byrne (Grinnell, 2018) compared both molecular hydrogen-based star formation and dust shielding-based star formation. She found that while the shielding-based model resulted in star formation in lower density gas, both models produced reasonable fits to observations. Future projects in this area will involve analyzing the cold interstellar media within galaxies at different redshifts using simulated observations.