标题： 在z~1.67处发现一个大的Lyman-alpha+HeII星云：低金属量区域的候选者？ 显示英文标题

标题： The Discovery of a Large Lyman-alpha+HeII Nebula at z~1.67: A Candidate Low Metallicity Region?

摘要： 我们发现了一个~45 kpc的Lya星云（或Lya“blob”），在z~1.67处，表现出强烈的、空间扩展的HeII发射和非常弱的CIV和CIII]发射。 这是首次观测到的空间扩展的Lya+HeII发射体，也是迄今为止发现的最低红移的Lya blob。 在没有金属线的情况下，强Lya和HeII-1640发射被认为是一种原始星系形成的独特观测特征（例如，来自引力冷却辐射或第三星族恒星形成），但在高红移（z > 4）的Lya发射星系调查或低红移Lya星云研究中，尚未出现确凿的空间扩展Lya+HeII发射体的例子。 通过与光致电离模型的比较，我们发现该星云中的观测线比与低金属度气体（Z < 10^-2 - 10^-3 Z_sun）一致，但这一结论取决于系统未知的离化参数。 HeII等值宽度（~37+/-10A）和HeII/Lya比率（0.12+/-0.04）较大，表明该云团正被一个硬电离连续谱照射，可能是AGN或非常低金属度的恒星，或者可能由引力冷却辐射驱动。 到目前为止，系统中没有明显的强大AGN迹象，因此为了在中红外中仍然保持隐藏状态，AGN需要被严重遮挡。 尽管有强烈的Lya+HeII发射，但目前尚不清楚该星云的主要能量来源是什么。 因此，该系统为如何利用强Lya+HeII特征作为原始星系形成的唯一示踪物时，真实天体物理源的复杂性所带来的挑战提供了一个有启发性的例子。 显示英文摘要

摘要： We have discovered a ~45 kpc Lya nebula (or Lya ``blob'') at z~1.67 which exhibits strong, spatially-extended HeII emission and very weak CIV and CIII] emission. This is the first spatially-extended Lya+HeII emitter observed and the lowest redshift Lya blob yet found. Strong Lya and HeII-1640 emission in the absence of metal lines has been proposed as a unique observational signature of primordial galaxy formation (e.g., from gravitational cooling radiation or Population III star formation), but no convincing examples of spatially-extended Lya+HeII emitters have surfaced either in Lya-emitting galaxy surveys at high redshifts (z > 4) or in studies of Lya nebulae at lower redshifts. From comparisons with photoionization models, we find that the observed line ratios in this nebula are consistent with low metallicity gas (Z < 10^-2 - 10^-3 Z_sun), but that this conclusion depends on the unknown ionization parameter of the system. The large HeII equivalent width (~37+/-10A) and the large HeII/Lya ratio (0.12+/-0.04) suggest that the cloud is being illuminated by a hard ionizing continuum, either an AGN or very low metallicity stars, or perhaps powered by gravitational cooling radiation. Thus far there is no obvious sign of a powerful AGN in or near the system, so in order to power the nebula while remaining hidden from view even in the mid-infrared, the AGN would need to be heavily obscured. Despite the strong Lya+HeII emission, it is not yet clear what is the dominant power source for this nebula. The system therefore serves as an instructive example of how the complexities of true astrophysical sources will complicate matters when attempting to use a strong Lya+HeII signature as a unique tracer of primordial galaxy formation.