The Grism Lens-Amplified Survey from Space (GLASS). III. A census of Ly�� Emission at $z\gtrsim$7 from HST Spectroscopy

[abbreviated] We present a census of Ly�� emission at $z\gtrsim7$ 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�� 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$��$ flux limits reaches $5\times10^{-18}$erg/s/cm$^{2}$ per position angle over the full wavelength range of GLASS (0.8-1.7$��$m). Based on the conditional probability of Ly�� emission measured from the ground at $z\sim7$ we would have expected 12-18 Ly�� emitters. This is consistent with the number of detections, within the uncertainties, confirming the drop in Ly�� emission with respect to $z\sim6$. These candidates include a promising source at $z=8.1$. The spatial extent of Ly�� in a deep stack of the most convincing Ly�� emitters with $\langle z\rangle=7.2$ is consistent with that of the rest-frame UV continuum. Extended Ly$��$ 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 $f_\textrm{CIV} / f_\textrm{Ly$��$} \lesssim 0.32$ and $f_\textrm{CIII]} / f_\textrm{Ly$��$} \lesssim 0.23$ in good agreement with other values published in the literature.

Accepted for publication in ApJ, 23 pages, 11 figures, 6 tables