Institute for Advanced Study Informal Astrophysics Seminar

ABSTRACT: Reionization was the last major phase transition for most of the normal matter in the universe, and a landmark event in the early phases of galaxy formation. Lyman alpha emission from galaxies provides a good tool for probing reionization, because Lyman alpha is strongly scattered in a neutral intergalactic medium. Applications of this tool at redshift z=6.5 have shown that the IGM is largely ionized by then. Recent work at z=7 has provided some evidence for neutral gas--- a result that is in tension with the results from microwave background polarization, and that would imply a very rapid evolution in the intergalactic medium between the two redshifts. More definite signatures should in any case be expected from higher-redshift searches, which we are now pursuing at redshifts 8 and 9. I will close by discussing prospects for definitive applications of Lyman-alpha reionization tests, both with wide-field ground based telescopes and with future space missions.