明亮z~7 HUDF星系的52小时VLT/FORS2光谱：无Ly-alpha发射
A 52 hours VLT/FORS2 spectrum of a bright z~7 HUDF galaxy: no Ly-alpha emission
摘要：
我们的目标是确定GDS-1408的红移，它是位于哈勃超深场的最坚实的z~7星系候选者。我们使用了我们和另外两个小组在过去五年中在VLT上用FORS 2收集的GDS-1408的所有VLT光谱，总积分时间为52小时。合并后的光谱是在再电离时期获得的星系最深的光谱。我们没有检测到任何发射线或连续的整个波长范围内，高达10100 A。基于一组精确的模拟，我们能够在所探索的波长范围内设定严格的上限，即在3-9 σ处f（Lya）<3 × 10 ^（-18）erg/s/cm 2，对应于静止帧等效宽度EW<9 A。结合这一限制与SED模型，我们细化红移为z=6.82+/- 0.1（1-sigma）。同样的SED拟合表明GDS-1408相对灭绝（A1600~1），其尘埃校正的恒星形成率约为20 Msol/yr。通过对B类复合理论预测的未衰减等效宽度与实验测量的上限值的比较，给出了Lya有效逃逸分数f_（esc）^（eff）（Lya）<8%的限制。尽管我们不能排除星系间/环星系介质对衰减线的主要贡献，但一个合理的解释是，GDS-1408是适度进化的，在到达星系间介质之前，它含有足够的气体和尘埃来衰减Lya发射。即使是最好的z~7候选者的红移确认也很难用当前一代的8- 10米级望远镜实现（除非Lya或异常强的静止紫外星云发射线存在）。我们表明，JWST和E-ELT将是必要的，以取得决定性的进展。目前，通过这种分析获得的增加的红移精度使ALMA成为红移确认的有趣选择。
英文摘要：
We aim to determine the redshift of GDS-1408, the most solid z~7 galaxy candidate lying in the Hubble Ultra Deep Field. We have used all the VLT spectra of GDS-1408 collected by us and two other groups with FORS2 at VLT in the last five years, for a total integration time of 52hr. The combined spectrum is the deepest ever obtained of a galaxy in the Reionization epoch. We do not detect any emission line or continuum over the whole wavelength range, up to 10100A. Based on an accurate set of simulations, we are able to put a stringent upper limit of f(Lya)<3x10^(-18) erg/s/cm2 at 3-9 sigma in the explored wavelength range, corresponding to a rest-frame equivalent width EW<9A. Combining this limit with the SED modelling we refine the redshift to be z=6.82+/- 0.1 (1-sigma). The same SED fitting indicates that GDS-1408 is relatively extinct (A1600~1) with a dust corrected star formation rate of ~ 20 Msol/yr. The comparison between the un-attenuated equivalent width predicted by the case-B recombination theory and the observed upper limit, provides a limit on the effective Lya escape fraction of f_(esc)^(eff)(Lya)<8%. Even though we cannot rule out a major contribution of the inter/circum galactic medium in damping the line, a plausible interpretation is that GDS-1408 is moderately evolved and contains sufficient gas and dust to attenuate the Lya emission, before it reaches the intergalactic medium. The redshift confirmation of even the best z~7 candidates is very hard to achieve (unless the Lya or unusually strong rest-UV nebular emission lines are present) with the current generation of 8-10m class telescopes. We show that both JWST and E-ELT will be necessary to make decisive progresses. Currently, the increased redshift accuracy obtained with this kind of analysis makes ALMA an interesting option for the redshift confirmation.
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