Spectroscopic Observation of Lyα Emitters at z ~ 7.7 and Implications on Re-ionization
We present spectroscopic follow-up observations on two bright Lyα emitter (LAE) candidates originally found by Krug et al. at a redshift of z ~ 7.7 using the Multi-Object Spectrometer for Infra-Red Exploration at Keck. We rule out any line emission at the >5σ level for both objects, putting on solid ground a previous null result for one of the objects. The limits inferred from the non-detections rule out the previous claim of no or even reversed evolution between 5.7 < z < 7.7 in the Lyα luminosity function (LF) and suggest a drop in the Lyα LF consistent with that seen in Lyman break galaxy (LBG) samples. We model the redshift evolution of the LAE LF using the LBG UV-continuum LF and the observed rest-frame equivalent width distribution. From the comparison of our empirical model with the observed LAE distribution, we estimate lower limits of the neutral hydrogen fraction to be 50%–70% at z ~ 7.7. Together with this, we find a strong evolution in the Lyα optical depth characterized by (1 + z)^(2.2 ± 0.5) beyond z = 6, indicative of a strong evolution of the intergalactic medium. Finally, we extrapolate the LAE LF to z ~ 9 using our model and show that it is unlikely that large area surveys, like UltraVISTA or Euclid, pick up LAEs at this redshift assuming the current depths and area.
© 2014 The American Astronomical Society. Received 2014 February 12; accepted 2014 April 23; published 2014 May 23. We would like to acknowledge the support of the Keck Observatory staff who made these observations possible as well as B. Trakhtenbrot and W. Hartley for valuable discussions. We also thank Nick Konidaris for providing and supporting the MOSFIRE reduction pipeline and Gwen Rudie for providing the MOSFIRE exposure time calculator. A.F. acknowledges support from the Swiss National Science Foundation. A.F. also thanks Caltech for hospitality while working on this article. The authors wish to recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community. We are most fortunate to have the opportunity to conduct observations from this mountain. Facility: Keck:I (MOSFIRE)
Published - 0004-637X_788_1_87.pdf