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 some 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. We are also working to characterize the Lyman
alpha selected population physically. I will show near-infrared
spectra of such galaxies, which we are using to study rest-optical
wavelength emission lines of low mass galaxies at redshift z=3.
The combination of [OIII] and Lyman alpha lines provides evidence
for galactic winds, which provides further evidence of active ongoing
star formation, while establishing a key input parameter for modeling
Lyman alpha galaxy populations in a neutral IGM. I will close by
discussing prospects for definitive applications of Lyman-alpha
reionization tests using future space missions.
