March 12, 1998 CfA Colloquium

Title: Cosmology from Studies of Quasar Absorption Lines

Speaker: Lars Hernquist

Abstract: Cosmological simulations with gasdynamics have revealed a new interpretation of the absorption features seen in the spectra of distant quasars. According to this picture, the absorbing gas is cosmologically distributed and traces the large scale structure produced in the matter distribution by hierarchical clustering. The physical state of the low density gas responsible for the Lyman alpha forest is particularly simple and is determined by an interplay between photoionization heating and adiabatic cooling. This fact, coupled with the intimate relation between the gas and the matter distribution, transforms the study of quasar absorption lines into a powerful tool which can be used to infer basic cosmological parameters and the nature of the intergalactic medium at high redshifts. In this talk, I review the evidence supporting this model for the absorbing gas and then illustrate its use as a cosmological probe. I describe ongoing efforts to derive fundamental properties of the Universe from the Lyman alpha forest, such as the mass density of baryons. I also discuss absorption by elements such as carbon and oxygen and show how these spectral features can be used to constrain the metallicity of the intergalactic medium at high redshifts.

Reference for students: ApJ 457, L51 (1996) and ApJ 489, 7 (1997).