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 lik
ely 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.
We have searched for star-forming galaxies at z~7 by applying the Lyman-break technique to newly-released 1.1micron Y-band images from WFC3 on HST. By comparing these images of the Hubble Ultra Deep Field with the ACS z-band (0.85micron), we identify
objects with red colours, (z-Y)_AB>1.3), consistent with the Ly-alpha forest absorption at z~6.7-8.8. We identify 12 of these z-drops down to a limiting magnitude Y_AB<28.5 (equivalent to a star formation rate of 1.3M_sun/yr at z=7.1), which are undetected in the other ACS filters. We use the WFC3 J-band image to eliminate contaminant low mass Galactic stars, which typically have redder colours than z~7 galaxies. One of our z-drops is a probably a T-dwarf star. The z~7 z-drops have much bluer spectral slopes than Lyman-break galaxies at lower redshift. Our brightest z-drop is not present in the NICMOS J-band image of the same field taken 5 years before, and is a possible transient object. From the 10 remaining z~7 candidates we determine a lower limit on the star formation rate density of 0.0017M_sun/yr/Mpc^3 for a Salpeter initial mass function, which rises to 0.0025-0.0034M_sun/yr/Mpc^3 after correction for luminosity bias. The star formation rate density is a factor of ~10 less than that at z=3-4, and is about half the value at z~6. While based on a single deep field, our results suggest that this star formation rate density would produce insufficient Lyman continuum photons to reionize the Universe unless the escape fraction of these photons is extremely high (f_esc>0.5), and the clumping factor of the Universe is low. Even then, we need to invoke a large contribution from galaxies below our detection limit. The apparent shortfall in ionizing photons might be alleviated if stellar populations at high redshift are low metallicity or have a top-heavy IMF.
