A Lyman Break Galaxy in the Epoch of Reionization from HST Grism
  Spectroscopy
  We present observations of a luminous galaxy at redshift z=6.573 --- the end
of the reioinization epoch --- which has been spectroscopically confirmed
twice. The first spectroscopic confirmation comes from slitless HST ACS grism
spectra from the PEARS survey (Probing Evolution And Reionization
Spectroscopically), which show a dramatic continuum break in the spectrum at
restframe 1216 A wavelength. The second confirmation is done with Keck +
DEIMOS. The continuum is not clearly detected with ground-based spectra, but
high wavelength resolution enables the Lyman alpha emission line profile to be
determined. We compare the line profile to composite line profiles at redshift
z=4.5. The Lyman alpha line profile shows no signature of a damping wing
attenuation, confirming that the intergalactic gas is ionized at redshift
z=6.57. Spectra of Lyman breaks at yet higher redshifts will be possible using
comparably deep observations with IR-sensitive grisms, even at redshifts where
Lyman alpha is too attenuated by the neutral IGM to be detectable using
traditional spectroscopy from the ground.
