A Lyman Break Galaxy in the Epoch of Reionization from HST Grism Spectroscopy

We present observations of a luminous galaxy at 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 Å. 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-α emission line profile to be determined. We compare the line profile to composite line profiles at z=4.5. The Lyman-α line profile shows no signature of a damping wing attenuation, confirming that the intergalactic gas is ionized at 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-α is too attenuated by the neutral IGM to be detectable using traditional spectroscopy from the ground