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

TL;DR

This paper reports the spectroscopic confirmation of a luminous galaxy at redshift 6.573 during the epoch of reionization, using HST grism and Keck observations, revealing an ionized intergalactic medium at that time.

Contribution

It demonstrates the effectiveness of HST grism spectroscopy in confirming high-redshift galaxies and analyzing their Lyman alpha profiles during reionization.

Findings

Confirmed a galaxy at z=6.573 spectroscopically

Lyman alpha profile shows no damping wing attenuation

Intergalactic gas is ionized at z=6.57

Abstract

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.