VLT/XSHOOTER & Subaru/MOIRCS Spectroscopy of HUDF-YD3: No Evidence for Lyman-alpha Emission at z=8.55

TL;DR

This study conducted deep spectroscopic observations of a high-redshift galaxy and found no evidence of Lyman-alpha emission at z=8.55, challenging previous claims and suggesting possible suppression of such emission at very high redshifts.

Contribution

The paper provides the first independent, deep spectroscopic verification that questions earlier reports of Lyman-alpha emission at z=8.55, indicating the need for further observations of early galaxies.

Findings

No Lyman-alpha emission detected at z=8.55 in independent spectroscopy.

Previous Lyman-alpha detection at z=8.55 is likely spurious.

Lyman-alpha emission may be suppressed at z>7 due to cosmic reionization.

Abstract

We present spectroscopic observations with VLT/XSHOOTER and Subaru/MOIRCS of a relatively bright Y-band drop-out galaxy in the Hubble Ultra Deep Field, first selected by Bunker et al. (2010), McLure et al. (2010) and Bouwens et al. (2010) to be a likely z~8-9 galaxy on the basis of its colours in the HST ACS and WFC3 images. This galaxy, HUDF.YD3 (also known as UDFy-38135539) has been targetted for VLT/SINFONI integral field spectroscopy by Lehnert et al. (2010), who published a candidate Lyman-alpha emission line at z=8.55 from this source. In our independent spectroscopy using two different infrared spectrographs (5 hours with VLT/XSHOOTER and 11 hours with Subaru/MOIRCS) we are unable to reproduce this line. We do not detect any emission line at the spectral and spatial location reported in Lehnert et al. (2010), despite the expected signal in our combined MOIRCS & XSHOOTER data being 5-sigma. The line emission also seems to be ruled out by the faintness of this object in recently extremely deep F105W (Y-band) HST/WFC3 imaging from HUDF12; the line would fall within this filter and such a galaxy should have been detected at Y(AB)=28.6mag (~20 sigma) rather than the marginal Y(AB)~30mag observed in the Y-band image, >3 times fainter than would be expected if the emission lie was real. Hence it appears highly unlikely that the reported Lyman-alpha line emission at z>8 is real, meaning that the highest-redshift sources for which Lyman-alpha emission has been seen are at z=6.9-7.2. It is conceivable that Lyman-alpha does not escape galaxies at higher redshifts, where the Gunn-Peterson absorption renders the Universe optically thick to this line. However, deeper spectroscopy on a larger sample of candidate z>7 galaxies will be needed to test this.