Spectroscopy of z ~ 7 candidate galaxies: Using Lyman-alpha to constrain the neutral fraction of hydrogen in the high-redshift universe

Following our previous spectroscopic observations of $z>7$ galaxies with Gemini/GNIRS and VLT/XSHOOTER, which targeted a total of 8 objects, we present here our results from a deeper and larger VLT/FORS2 spectroscopic sample of Wide Field Camera 3 selected $z>7$ candidate galaxies. With our FORS2 setup we cover the 737-1070nm wavelength range, enabling a search for Lyman-$\alpha$ in the redshift range spanning 5.06 - 7.80. We target 22 $z$-band dropouts and find no evidence of Lyman-$\alpha$ emission, with the exception of a tentative detection ($<5\sigma$, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman-$\alpha$ flux and the broad-band magnitudes are used to constrain the rest-frame Equivalent Widths for this line emission. We analyse our FORS2 observations in combination with our previous GNIRS and XSHOOTER observations, and suggest that a simple model where the fraction of high rest-frame Equivalent Width emitters follows the trend seen at $z=3-6.5$ is inconsistent with our non-detections at $z\sim7.8$ at the 96\% confidence level. This may indicate that a significant neutral HI fraction in the intergalactic medium suppresses Lyman-$\alpha$, with an estimated neutral fraction $\chi_{HI}\sim0.5$, in agreement with other estimates.