  The epoch of reionization (6 < z < 10) marks the period in our universe when
the first large galaxies grew to fruition, and began to affect the universe
around them. Massive stars, and potentially accreting supermassive black holes,
filled the universe with ionizing radiation, burning off the haze of neutral
gas that had filled the intergalactic medium (IGM) since recombination
(z~1000). The evolution of this process constrains key properties of these
earliest luminous sources, thus observationally constraining reionization is a
key science goal for the next decade. The measurement of Lyman-alpha emission
from photometrically-identified galaxies is a highly constraining probe of
reionization, as a neutral IGM will resonantly scatter these photons, reducing
detectability. While significant work has been done with 8-10m telescopes,
these observations require extremely large telescopes (ELTs); the flux limits
available from today's 10m class telescopes are sufficient for only the
brightest known galaxies (m < 26). Ultra-deep surveys with the Giant Magellan
Telescope (GMT) and Thirty Meter Telescope (TMT) will be capable of detecting
Lyman-alpha emission from galaxies 2-3 magnitudes fainter than today's deepest
surveys. Wide-field fiber spectroscopy on the GMT combined with narrow-field
AO-assisted slit spectroscopy on the TMT will be able to probe the expected
size of ionized bubbles throughout the epoch of reionization, following up
degree scale deep imaging surveys with the Wide Field Infrared Space Telescope.
These data will provide the first resolved Lyman-alpha-based maps of the
ionized intergalactic medium throughout the epoch of reionization, constraining
models of both the temporal and spatial evolution of this phase change.
