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arXiv 0908.2974astro-ph.GAastro-ph.SR
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研究星团形成和破裂的基本工具:光度函数

Basic Tools for Studies on the Formation and Disruption of Star Clusters: the Luminosity Function

M. Gieles

摘要:

年轻星团的光度函数(LF)通常近似于幂律函数。对于大范围银河系环境中的星团,这导致拟合指数接近-2,但平均略陡。-2幂律函数的一个基本性质是,最亮天体的光度(L_max)与星团总数成线性比例,这与观测结果接近。这表明年轻大质量星系团(YMC)的形成是样本大小的结果,即当SFR高时,在统计上更有可能形成YMC,但不需要特定的物理条件。在这篇文章中,我们提供的证据表明,年轻星系团的LF不是一个-2的幂律,而是弯曲的,显示出系统的(对数)斜率从低光度的大约-1.8下降到高光度的大约-2.8。经验LF可以通过模型LF来再现,模型LF使用在M*=2x10^5 M_sun附近具有Schechter类型截断的基础集群IMF。这个M* 值不可能是普适的,因为在合并星系和合并残留物中,YMC远远超过这个M*。因此,超大质量星系团(>10^6 M_sun)的形成可能需要与(静止)螺旋星系不同的条件,因此不仅仅是样本大小效应的结果。从垂直偏移导出约10%的团簇形成效率。我们发现这种效率的迹象时,SFR较高。

英文摘要:

The luminosity function (LF) of young star clusters is often approximated by a power law function. For clusters in a wide range of galactic environments this has resulted in fit indices near -2, but on average slightly steeper. A fundamental property of the -2 power law function is that the luminosity of the brightest object (L_max) scales linearly with the total number of clusters, which is close to what is observed. This suggests that the formation of Young Massive Clusters (YMCs) is a result of the size of the sample, i.e. when the SFR is high it is statistically more likely to form YMCs, but no particular physical conditions are required. In this contribution we provide evidence that the LF of young clusters is not a -2 power law, but instead is curved, showing a systematic decrease of the (logarithmic) slope from roughly -1.8 at low luminosities to roughly -2.8 at high luminosities. The empirical LFs can be reproduced by model LFs using an underlying cluster IMF with a Schechter type truncation around M*=2x10^5 M_sun. This value of M* can not be universal since YMCs well in excess of this M* are known in merging galaxies and merger remnants. Therefore, forming super massive clusters (>10^6 M_sun) probably requires conditions different from those in (quiescent) spiral galaxies and hence is not only the result of a size-of-sample effect. From the vertical offset a cluster formation efficiency of ~10% is derived. We find indications for this efficiency to be higher when the SFR is higher.

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