The lack of intense Lyman~alpha in ultradeep spectra of z=7 candidates in GOODS-S: imprint of reionization?

TL;DR

This study uses ultradeep spectroscopy to investigate Lyman-alpha emission in z>6.5 galaxy candidates, finding a lack of intense emission possibly due to reionization effects, which challenges previous trends observed at lower redshifts.

Contribution

It provides the first ultradeep spectroscopic analysis of high-redshift galaxy candidates, suggesting a decline in Lyman-alpha emission potentially caused by reionization.

Findings

Detected a marginal Lyman-alpha emission in one galaxy at z=6.972

Estimated low probability of observing such emission if the trend continued from lower redshifts

Proposed reionization as a possible cause for suppressed Lyman-alpha emission at z>6.5

Abstract

We present ultradeep optical spectroscopy obtained with FORS2 on VLT of seven Lyman-break galaxy (LBG) candidates at z>6.5 selected in the GOODS-S field from Hawk-I/VLT and WFC3/HST imaging. For one galaxy we detect a low significance emission line (S/N< 7), located at 9691.5 +/- 0.5A and with flux 3.4 x 10^(-18)erg/cm^2/s. If identified as Lyman alpha, it places the LBG at redshift z=6.972+/- 0.002, with a rest-frame equivalent width EW}=13A. Using Monte Carlo simulations and conservative EW distribution functions at 2<z<6, we estimate that the probability of observing no galaxies in our data with S/N>10 is ~ 2%, and that of observing only one galaxy out of seven with S/N=5 is ~4%, but these can be as small as ~1E-3, depending on the details of the EW distribution. We conclude that either a significant fraction of the candidates is not at high redshift or that some physical mechanism quenches the Lyman alpha emission emerging from the galaxies at z>6.5, abruptly reversing the trend of the increasing fraction of strong emitters with increasing redshift observed up to z~ 6.5. We discuss the possibility that an increasingly neutral intergalactic medium is responsible for such quenching.