室女星系团多余场天体中i-退出的超密度
An Overdensity of i-dropouts Among A Population of Excess Field Objects in the Virgo Cluster
摘要：
Yan，Windhorst & Cohen（2003）利用ACS在HST部署后不久获得的一组深度成像数据，发现了大量的F775 W波段脱落（i-dropout），这与z~6的星系一致。然而，由此得出的i-脱落的表面密度比在其他深ACS领域（包括HUDF）中发现的后续研究高一个数量级。在这里，我们重新审视这个问题，使用现有的和新的数据。我们确认，在这个领域的i-辍学的大过密度确实存在，并且他们的光学到红外颜色是类似的HUDF。然而，我们已经发现，i-dropout过密度伴随着在该区域及其附近的更大的过量暗场物体。这一大量过剩的场天体很可能是由潮汐"碎片“和/或晕星引起的，它们与室女星系团M60/NGC 4647中的一个相互作用的星系对有关，该星系团距离过剩天体所在的区域有几弧分之遥。这一过剩的星群也是红色的，其颜色分布的红翼一直延伸到i-dropouts所在的区域。虽然我们仍然不能完全排除i-dropouts的过密度可能是z~6处星系的真正大尺度结构的可能性，但我们更倾向于这样的解释，即它们中的大多数是与M60/NGC 4647相关的过剩恒星星族的一部分。未来的光谱工作将需要确定这种i-dropout过密度的性质。（节录）
英文摘要：
Using a set of deep imaging data obtained by the ACS on the HST shortly after its deployment, Yan, Windhorst & Cohen (2003) found a large number of F775W-band dropouts (i-dropouts), which are consistent with being galaxies at z~6. The surface density of i-dropouts thus derived, however, is an order of magnitude higher than those subsequent studies found in other deep ACS fields, including the HUDF. Here we revisit this problem, using both the existing and the new data. We confirm that the large overdensity of i-dropouts does exist in this field, and that their optical-to-IR colors are similar to those in the HUDF. However, we have discovered that the i-dropout overdensity is accompanied with an even larger excess of faint field objects in this region and its vicinity. This large excess of field objects is most likely caused by the tidal ``debris'' and/or halo stars related to an interacting galaxy pair in the Virgo Cluster, M60/NGC4647, which lies several arcminutes away from the region where the excess is found. This excess population is also red in color, and the red wing of its color distribution continuously extends to the regime where the i-dropouts reside. While we still cannot completely rule out the possibility that the overdensity of i-dropouts might be a genuine large-scale structure of galaxies at z~6, we prefer the interpretation that most of them are part of the excess stellar population related to M60/NGC4647. Future spectroscopic work will be needed to identify the nature of this i-dropout overdensity. (Abridged)
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