My group studies the properties of diffuse matter between galaxies in the early universe.  This intergalactic medium does not emit (as-yet detectable) light, but it is traced by very low density gas whose presence we infer by its absorption of the light from background sources – typically quasars.

Although intergalactic gas is extremely rarefied, it occupies a tremendous volume compared to those of stars or galaxies.  So much in fact that on the balance, intergalactic matter accounts for most of the total cosmic matter budget.  This is particularly true at the early times my group studies, where the IGM contains over 95% of all atoms.  At these early times, most atoms have simply not yet fallen into a galaxy from their diffuse primordial state.

My group’s particular focus is on the chemistry of the IGM.  Hydrogen and helium comprise the vast majority of the baryonic matter in the universe, but a trace amount (arguably for us, the interesting part!) consists of heavier elements such as oxygen, carbon, iron, and the rest of the periodic table.  These elements were fused almost exclusively in stars, either through stellar nucleosynthesis in their cores or in their explosive deaths as supernovae.

Using astronomical spectra of distant quasars, we search for the absorption fingerprint of different chemicals to trace where and when stars have existed throughout cosmic time.  I have been especially interested to learn how far back in time we can trace the existence of the heavy elements, a question which relates closely to the when the first stars in the universe were born.  To study this problem and essay, my team built a new instrument for the Magellan telescopes that enables chemical measurements of the universe at 95% lookback time to the Big Bang.

The study of heavy elements also offers a way to trace how gas moves into and out of nascent galaxies, since all galaxies possess chemically enriched haloes of gaseous matter extending 5-10 times past the boundary of their stellar disks.  Often times the absorption signature of Mg, O, or C can act as a convenient signpost for galaxies that are too faint or distant to be studied easily from their emission.