Abstract
abbreviated We present a census of Ly\alpha\ 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\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
$5\times10^-18$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 $z\sim7$ 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 $z\sim6$. 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 $z\rangle=7.2$ 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 $f_CIV /
f_Ly$\alpha$ 0.32$ and $f_CIII /
f_Ly$\alpha$ 0.23$ in good agreement with other values
published in the literature.
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