来自Z-FOURGE的第一个结果：在COSMOS中发现z = 2.2的候选簇
First Results from Z-FOURGE: Discovery of a Candidate Cluster at z = 2.2 in COSMOS
摘要：
我们报告了Z-FOURGE巡天的第一个结果：发现了一个z = 2.2的候选星系团，它由两个致密的超密度组成，在平均表面密度以上20σ处探测到了红色星系。这一发现是通过一个新的深（Ks < 24.8 AB 5σ）麦哲伦/四星近红外成像调查与5个定制的中等带宽滤波器。滤波器精确地定位了演化中恒星星族在1.5 < z < 3.5处的巴耳末/4000赫的断裂位置，产生了比单独使用宽带成像更精确的光度红移。在COSMOS场中，这些超密度相互之间的距离在1“以内，并且似乎嵌入了一个更大的结构中，该结构至少包含一个额外的超密度（\sim 10σ）。考虑到超密度的全局性质，z = 2.2的系统似乎是拥有红色星系群的星系团中最遥远的例子。与大型ΛCDM模拟的比较表明，该系统可能由合并的子星系团组成，其性质介于z > 2的蓝色星系团和具有显著红色序列的低红移星系团之间。该结构在COSMOS的公共光学目录中完全不存在，在较浅的近红外巡天中只能微弱地看到。这一发现展示了使用中频滤波器进行深度近红外探测的潜力，以促进对z > 1.5的环境和星系演化的理解。
英文摘要：
We report the first results from the Z-FOURGE survey: the discovery of a candidate galaxy cluster at z = 2.2 consisting of two compact overdensities with red galaxies detected at >20σ above the mean surface density. The discovery was made possible by a new deep (Ks < 24.8 AB 5σ) Magellan/FourStar near-IR imaging survey with 5 custom medium-bandwidth filters. The filters pinpoint the location of the Balmer/4000Å break in evolved stellar populations at 1.5 < z < 3.5, yielding significantly more accurate photometric redshifts than possible with broadband imaging alone. The overdensities are within 1' of each other in the COSMOS field and appear to be embedded in a larger structure that contains at least one additional overdensity (\sim10σ). Considering the global properties of the overdensities, the z = 2.2 system appears to be the most distant example of a galaxy cluster with a population of red galaxies. A comparison to a large ΛCDM simulation suggests that the system may consist of merging subclusters, with properties in between those of z > 2 protoclusters with more diffuse distributions of blue galaxies and the lower-redshift galaxy clusters with prominent red sequences. The structure is completely absent in public optical catalogs in COSMOS and only weakly visible in a shallower near-IR survey. The discovery showcases the potential of deep near-IR surveys with medium-band filters to advance the understanding of environment and galaxy evolution at z > 1.5.
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