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arXiv 0807.2760astro-ph
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BCGs中最近恒星形成的证据：蓝核和UV过剩之间的对应关系

Evidence for recent star formation in BCGs: a correspondence between blue cores and UV excess

A. Pipino, S. Kaviraj, C. Bildfell, A. Babul, H. Hoekstra, J. Silk

备注：
11 pages, 5 figures, added references, corrected typos, matches MNRAS accepted version
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摘要：

我们提出了一个联合分析的近紫外（NUV）数据从GALEX任务和（光学）颜色配置文件的样本7 Brilliant集群Galerie（BCGs）在加拿大集群比较项目。我们发现，每一个BCG的休息帧的紫外颜色是蓝色的，也显示了一个蓝色的核心，在其光学颜色配置文件。相反，缺乏蓝色核心并表现出典型的古老椭圆星系的单调颜色梯度的BCGs在紫外线下是红色的。我们认为这是蓝色BCGs的NUV增强是由最近的恒星形成驱动的证据，而不是来自旧的进化恒星星族，如水平分支恒星。蓝色BCGs中最近形成的恒星通常年龄小于2亿年，质量分数小于1%。虽然这里研究的样本是小的，我们证明，第一次，在椭圆星系（特别是BCGs）和NUV增强观测使用GALEX的蓝核之间的一一对应。这种一一对应的组合和最近恒星形成的持续年轻年龄，再加上与主星团X射线特性的额外相关性，强烈表明恒星形成是由星团内介质的气体冷却提供燃料的。反过来，这意味着任何大质量星系团中星系团内介质的活动星系核加热只会减少冷却流的大小，而且一旦冷却流开始，它几乎总是活跃的。（节略）

英文摘要：

We present a joint analysis of near-ultraviolet (NUV) data from the GALEX mission and (optical) colour profiles for a sample of 7 Brightest Cluster Galaxies (BCGs) in the Canadian Cluster Comparison Project. We find that every BCG which has a blue rest-frame UV colour also shows a blue-core in its optical colour profile. Conversely, BCGs that lack blue cores and show monotonic colour gradients typical of old elliptical galaxies, are red in the UV. We interpret this as evidence that the NUV enhancement in the blue BCGs is driven by recent star formation and not from old evolved stellar populations such as horizontal branch stars. The recent star formation in the blue BCGs typically has an age less than 200 Myrs and contributes mass fractions of less than a percent. Although the sample studied here is small, we demonstrate, for the first time, a one-to-one correspondence between blue cores in elliptical galaxies (in particular BCGs) and a NUV-enhancement observed using GALEX. The combination of this one-to-one correspondence and the consistently young age of recent star formation, coupled with additional correlations with the host cluster's X-ray properties, strongly suggests that the star formation is fueled by gas cooling out of the intracluster medium. In turn, this implies that any AGN heating of the intracluster medium in massive clusters only acts to reduce the magnitude of the cooling flow and that once this flow starts, it is nearly always active. (abridged)

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