Abstract
We present a census of Ly alpha emission at z greater than or similar to 7, utilizing deep near-infrared Hubble Space Telescope grism spectroscopy from the first six completed clusters of the Grism Lens-Amplified Survey from Space (GLASS). In 24/159 photometrically selected galaxies we detect emission lines consistent with Ly alpha in the GLASS spectra. Based on the distribution of signal-to-noise ratios and on simulations, we expect the completeness and the purity of the sample to be 40%-100% and 60%-90%, respectively. For the objects without detected emission lines we show that the observed (not corrected for lensing magnification) 1 sigma flux limits reach 5 x 10(-18) erg s(-1) cm(-2) per position angle over the full wavelength range of GLASS (0.8-1.7 mu m). Based on the conditional probability of Ly alpha emission measured from the ground at z similar to 7, we would have expected 12-18 Ly alpha emitters. This is consistent with the number of detections, within the uncertainties, confirming the drop in Ly alpha emission with respect to z similar to 6. Deeper follow-up spectroscopy, here exemplified by Keck spectroscopy, is necessary to improve our estimates of completeness and purity. and to confirm individual candidates as true Ly alpha emitters. These candidates include a promising source at z = 8.1. The spatial extent of Ly alpha in a deep stack of the most convincing Ly alpha emitters with < z > = 7.2 is consistent with that of the rest-frame UV continuum. Extended Ly alpha emission, if present, has a surface brightness below our detection limit, consistent with the properties of lower-redshift comparison samples. From the stack we estimate upper limits on rest-frame UV emission line ratios and find f(C IV)/f(Ly alpha) less than or similar to 0.32 and f(C III])/f(Ly alpha) less than or similar to 0.23, in good agreement with other values published in the literature.