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

TL;DR

This study uses HST grism spectroscopy to conduct a census of Ly extalpha\, emission at redshifts greater than 7, confirming a decline in Ly extalpha\, emission compared to lower redshifts and identifying promising high-redshift candidates.

Contribution

First comprehensive Ly extalpha\, emission survey at $z extgreater7$ using HST grism data, providing new insights into early galaxy properties and reionization.

Findings

Detected Ly extalpha\, in 24 out of 159 galaxies.

Confirmed decline of Ly extalpha\, emission from $z extgreater6$.

Identified a promising $z=8.1$ Ly extalpha\, emitter.

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 $\langle z\rangle=7.2$ 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.