Sean D. Johnson
I am an LSA Collegiate Fellow and Assistant Professor at the University of Michigan where I work on observations of galaxy evolution, AGN/quasars, and the intergalactic medium. Prior to joining UMich, I was a Hubble and Carnegie-Princeton Fellow at Princeton. I obtained my PhD at U. Chicago where I worked with Prof. Hsiao-Wen Chen.
Galaxies are thought to form and evolve through a combination of accretion of gas from intergalactic space and mergers. The gas cools and condenses under the influence of gravity to form stars. However, in simulations of galaxies the gas cooling and star-formation rates are far higher than observed resulting in unrealistically luminous simulated galaxies. This “over-cooling” problem reveals a fundamental gap in our understanding of the formation of galaxies like our own Milky Way.
The over-cooling problem can be solved by a variety of “feedback” mechanisms that cause star-formation to self-regulate (e.g. radiation pressure from hot stars or supernova explosions). Simulations that include feedback are better able to reproduce the observed properties of galaxies. However, a variety of possible feedback processes can produce more realistic galaxies. These degeneracies can be broken through combined studies of galaxies and the gas that surrounds them.
The gas that surrounds galaxies is nearly always too low in density to be observed in emission, but it can be observed with ultraviolet absorption-line spectroscopy when the light from a background quasar passes near foreground galaxies (illustrated below). Ions contained in the gas surrounding the galaxies leave unique absorption patterns in the spectra of background quasars, providing detailed constraints on the ionization state and composition of the gas. In 2009, astronauts installed the Cosmic Origins Spectrograph aboard the Hubble Space Telescope, enabling precise measurements of gas properties in the distant Universe. To connect these gas measurements with the properties of galaxies, I am conducting galaxy surveys in the vicinity of the highest quality Hubble sightlines with the Magellan Telescopes (pictured above). These surveys will provide a detailed characterization of the relationships between galaxy and gas properties and form a significant test-bed for galaxy simulations.