用XMM刻画化石群的性质
Characterizing the nature of Fossil Groups with XMM
摘要：
我们提出了一个X射线的后续行动，XMM加上钱德拉，六个化石组（FG）的候选人在我们以前的工作中使用SDSS和RASS数据确定。四个候选者（六个）表现出扩展的X射线发射，确认它们是真正的FG。对于其他两组，RASS发射的起源要么是光学暗/X射线亮活动星系核，要么是不同X射线源的混合。使用SDSS-DR 7数据，我们确认，对于所有的群体，种子椭圆和第二级星系之间的r-波段星等差距的存在。然而，在0.5等以下，间隙值取决于如何估算种子星系的总通量，而这在使用SDSS（相对于Sersic）星等时被大大低估了。这意味着在使用SDSS数据时，许多FG实际上可能会被遗漏，这是一个在将观测到的FG数密度与宇宙学模拟的预期进行比较时应该仔细考虑的事实。在我们以前的研究中发现，种子-FG和非化石椭圆星系的性质的相似性，延伸到X射线确认的FG样本，表明明亮的椭圆星系在FG不代表一个独特的人口星系。对于一个系统，我们还发现暗星系的速度分布是双峰的，可能表明该系统是由两个星系团合并而成的。这破坏了所有被选中的化石群形成一个真正的化石群的想法。
英文摘要：
We present an X-ray follow-up, based on XMM plus Chandra, of six Fossil Group (FG) candidates identified in our previous work using SDSS and RASS data. Four candidates (out of six) exhibit extended X-ray emission, confirming them as true FGs. For the other two groups, the RASS emission has its origin as either an optically dull/X-ray bright AGN, or the blending of distinct X-ray sources. Using SDSS-DR7 data, we confirm, for all groups, the presence of an r-band magnitude gap between the seed elliptical and the second-rank galaxy. However, the gap value depends, up to 0.5mag, on how one estimates the seed galaxy total flux, which is greatly underestimated when using SDSS (relative to Sersic) magnitudes. This implies that many FGs may be actually missed when using SDSS data, a fact that should be carefully taken into account when comparing the observed number densities of FGs to the expectations from cosmological simulations. The similarity in the properties of seed--FG and non-fossil ellipticals, found in our previous study, extends to the sample of X-ray confirmed FGs, indicating that bright ellipticals in FGs do not represent a distinct population of galaxies. For one system, we also find that the velocity distribution of faint galaxies is bimodal, possibly showing that the system formed through the merging of two groups. This undermines the idea that all selected FGs form a population of true fossils.
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