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

Abstract

Following our previous spectroscopic observations of z > 7 galaxies with Gemini/Gemini Near Infra-Red Spectrograph (GNIRS) and Very Large Telescope (VLT)/XSHOOTER, which targeted a total of eight objects,we present here our results froma deeper and largerVLT/FOcal Reducer and Spectrograph (FORS2) spectroscopic sample of Wide Field Camera 3 selected z>7 candidate galaxies.With our FORS2 setup we cover the 737-1070 nm wavelength range, enabling a search for Lyman α in the redshift range spanning 5.06-7.80. We target 22 z-band dropouts and nd no evidence of Lyman α emission, with the exception of a tentative detection (<5σ, which is our adopted criterion for a secure detection) for one object. The upper limits on Lyman α ux 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 = 3-6.5is inconsistent with our non-detections at z ~ 7.8 at the 96 per cent condence level. This may indicate that a signicant neutral HI fraction in the intergalactic medium suppresses Lyman α, with an estimated neutral fraction χHI ~ 0.5, in agreement with other estimates. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.