The Lyman-alpha Forest as a Cosmological Tool

We review recent developments in the theory of the Lyman-alpha forest and their implications for the role of the forest as a test of cosmological models. Simulations predict a relatively tight correlation between the local Lya optical depth and the local gas or dark matter density. Statistical properties of the transmitted flux can constrain the amplitude and shape of the matter power spectrum at high redshift, test the assumption of Gaussian initial conditions, and probe the evolution of dark energy by measuring the Hubble parameter H(z). Simulations predict increased Lya absorption in the vicinity of galaxies, but observations show a Lya deficit within Delta_r ~ 0.5 Mpc/h (comoving). We investigate idealized models of "winds" and find that they must eliminate neutral hydrogen out to comoving radii ~1.5 Mpc/h to marginally explain the data. Winds of this magnitude suppress the flux power spectrum by \~0.1 dex but have little effect on the distribution function or threshold crossing frequency. In light of the stringent demands on winds, we consider the alternative possibility that extended Lya emission from target galaxies replaces absorbed flux, but we conclude that this explanation is unlikely. Taking full advantage of the data coming from large telescopes and from the Sloan Digital Sky Survey will require more complete understanding of the galaxy proximity effect, careful attention to continuum determination, and more accurate numerical predictions, with the goal of reaching 5-10% precision on key cosmological quantities.Comment: 13 pages, to appear in "The Emergence of Cosmic Structure," Proceedings of the 13th Annual Astrophysics Conference in Maryland, eds. S. Holt and C. Reynolds, AIP Press, 200

Searching for Star Formation Beyond Reionization

The goal of searching back in cosmic time to find star formation during the epoch of reionization will soon be within reach. We assess the detectability of high-redshift galaxies by combining cosmological hydrodynamic simulations of galaxy formation, stellar evolution models appropriate for the first generations of stars, and estimates of the efficiency for Lyman alpha to escape from forming galaxies into the intergalactic medium. Our simulated observations show that Lyman alpha emission at z ~ 8 may be observable in the near-infrared with 8-meter class telescopes and present-day technology. Not only is the detection of early star-forming objects vital to understanding the underlying cause of the reionization of the universe, but the timely discovery of a z > 7 star-forming population -- or even an interesting upper limit on the emergent flux from these objects -- will have implications for the design of the next generation of ground- and space-based facilities.Comment: 4 pages, submitted to ApJ Letter