VLT/XSHOOTER和HUDF-YD3的Subaru/MOIRCS光谱：在z=8.55处没有莱曼α发射的证据
VLT/XSHOOTER & Subaru/MOIRCS Spectroscopy of HUDF-YD3: No Evidence for Lyman-alpha Emission at z=8.55
摘要：
我们利用VLT/XSHOOTER和Subaru/MOIRCS对哈勃超深场中一个相对明亮的Y波段脱落星系进行了光谱观测，Bunker et al.（2010），McLure et al.（2010）和Bouwens et al.（2010）首先根据HST ACS和WFC 3图像中的颜色将其选为可能的z~8-9星系。这个星系，HUDF.YD3（也被称为UDFy-38135539）已经被Lehnert等人（2010）作为VLT/SINFONI积分场光谱的目标，他们发表了来自这个源的z=8.55的候选莱曼α发射线。在我们使用两种不同的红外光谱仪（VLT/XSHOOTER 5小时和Subaru/MOIRCS 11小时）的独立光谱中，我们无法重现这条线。我们没有在Lehnert等人（2010）报告的光谱和空间位置检测到任何发射线，尽管我们的组合MOIRCS和XSHOOTER数据中的预期信号是5 σ。在最近来自HUDF 12的极深F105 W（Y波段）HST/WFC 3成像中，该物体的模糊似乎也排除了线发射;这条线会落在这个过滤器内，这样的星系应该在Y（AB）= 28.6等时被探测到。（~20 sigma），而不是在Y波段图像中观察到的边缘Y（AB）~ 30 mag，比如果发射线是真实的预期暗3倍。因此，在z>8处的莱曼α线发射极不可能是真实的，这意味着已经看到的莱曼α发射的最高红移源是在z=6.9-7.2处。可以想象的是，莱曼α不会在更高的红移下逃离星系，在那里，冈恩-彼得森吸收使宇宙在光学上厚到这条线。然而，需要在更大的候选z>7星系样本上进行更深的光谱学测试。
英文摘要：
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.
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