We present a census of Ly \alpha emission at z \gtrsim 7 utilizing deep near infrared HST 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 reaches 5 \times 10 ^ { -18 } erg/s/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 \sim 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 \sim 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 \langle z \rangle = 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 _ { \textrm { CIV } } / f _ { \textrm { Ly$ \alpha$ } } \lesssim 0.32 and f _ { \textrm { CIII ] } } / f _ { \textrm { Ly$ \alpha$ } } \lesssim 0.23 in good agreement with other values published in the literature .