The formation of galaxies like our Milky Way began soon after the Big Bang, as pristine gas poured into clumps of dark matter to form the first groups of stars. However, the lives and deaths of these stars soon made galaxy growth much more complex. Through their intense ultraviolet radiation, explosive supernovae, and exotic black hole descendants, these stars turned the one-way flow of gas into galaxies into a dance of light and color.
One specific color of light is the Lyman-alpha emission line of hydrogen. Lyman-alpha emission is produced whenever gas is excited by stars or black holes, so it is a powerful tracer of galaxy formation. However, Lyman-alpha emission is also resonantly scattered by the plentiful gas in and around galaxies, so interpreting this light can be as complex as galaxy formation itself. Nonetheless, the scattered nature of Lyman-alpha emission can be a tool as well as a challenge, and I will describe how we can use scattered Lyman-alpha emission to probe the rich interactions between stars, gas, and black holes that regulate galaxy formation in the early Universe.