Deep LBT/LUCI Spectroscopy of a Lyman-alpha Emitter Candidate at z ~ 7.7

TL;DR

This study used deep infrared spectroscopy to investigate a bright Lyman-alpha emitter candidate at z ~ 7.7, but found no convincing emission line, providing important constraints on the evolution of the Lyman-alpha luminosity function and the ionization state of the early universe.

Contribution

First deep spectroscopic attempt to confirm a bright z ~ 7.7 Lyman-alpha emitter candidate, constraining the luminosity function evolution at high redshift.

Findings

No convincing Lyman-alpha detection at candidate position.

Strong upper limits on the bright-end Lyman-alpha luminosity function at z ~ 7.7.

Evidence for rapid evolution of the Lyman-alpha LF from z ~ 6.5 to 7.7.

Abstract

We present deep spectroscopic observations of a Lyman-alpha emitter (LAE) candidate at z ~ 7.7 using the infrared spectrograph LUCI on the 2 x 8.4m Large Binocular Telescope (LBT). The candidate is the brightest among the four z ~ 7.7 LAE candidates found in a narrow-band imaging survey by Krug et al. 2012. Our spectroscopic data include a total of 7.5 hours of integration with LBT/LUCI and are deep enough to significantly (3.2-4.9 sigma) detect the Lyman-alpha emission line of this candidate, based on its Lyman-alpha flux 1.2 x 10^{-17} erg s^{-1} cm^{-2} estimated from the narrow-band photometry. However, we do not find any convincing signal at the expected position of its Lyman-alpha emission line, suggesting that this source is not an LAE at z ~ 7.7. The non-detection in this work, together with the previous studies of z ~ 7.7 LAEs, puts a strong constraint on the bright-end Lyman-alpha luminosity function (LF) at z ~ 7.7. We find a rapid evolution of the Lyman-alpha LF from z ~ 6.5 to 7.7: the upper limit of the z ~ 7.7 LF is more than 5 times lower than the z ~ 6.5 LF at the bright end (f > 1.0 x 10^{-17} erg s^{-1} cm^{-2}, or L > 6.9 x 10^{42} erg s^{-1}). This is likely caused by an increasing neutral fraction in the IGM that substantially attenuates Lyman-alpha emission at z ~ 7.7.