LOW-MASS X-RAY BINARIES INDICATE A TOP-HEAVY STELLAR INITIAL MASS FUNCTION IN ULTRACOMPACT DWARF GALAXIES
LOW-MASS X-RAY BINARIES INDICATE A TOP-HEAVY STELLAR INITIAL MASS FUNCTION IN ULTRACOMPACT DWARF GALAXIES
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DOI：
10.1088/0004-637x/747/1/72
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发表时间：
2011-10
期刊：
影响因子：
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通讯作者：
J. Dabringhausen;P. Kroupa;J. Pflamm-Altenburg;S. Mieske
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作者：
J. Dabringhausen;P. Kroupa;J. Pflamm-Altenburg;S. Mieske
以前已经表明，超紧凑矮星系（UCD）的高质轻比可以解释，如果它们的恒星初始质量函数（IMF）是头重脚轻的，即，国际货币基金组织倾向于大质量恒星。在这种情况下，中子星（NS）和黑洞将在UCD中提供不可见的质量。为了用一种独立的方法来测试这种情况，我们使用了一部分UCD具有明亮X射线源的数据。这些X射线源被解释为低质量X射线双星（LMXB），即，NS从演化中的低质量星星吸积物质的双星。我们发现，LMXB确实是高达10倍以上的频率比预期的UCD，如果IMF是不变的。解释这种过剩所需的头重脚轻的IMF与解释UCD异常高的质轻比所需的相同，头重脚轻的IMF似乎是对这两个发现的唯一同时解释。此外，我们表明，在星爆星系阿普220中的II型超新星的高比率表明，在该系统中的头重脚轻的IMF。这一发现与星暴星系是UCD可能形成的地方以及UCD的IMF是头重脚轻的概念是一致的。据估计，当每体积的星星形成率在pc尺度区域超过0.1M yr−1 pc−3时，IMF就会变得头重脚轻。
It has been shown before that the high mass-to-light ratios of ultracompact dwarf galaxies (UCDs) can be explained if their stellar initial mass function (IMF) was top-heavy, i.e., that the IMF was skewed toward high-mass stars. In this case, neutron stars (NSs) and black holes would provide unseen mass in the UCDs. In order to test this scenario with an independent method, we use data on which a fraction of UCDs has a bright X-ray source. These X-ray sources are interpreted as low-mass X-ray binaries (LMXBs), i.e., binaries where a NS accretes matter from an evolving low-mass star. We find that LMXBs are indeed up to 10 times more frequent in UCDs than expected if the IMF was invariant. The top-heavy IMF required to account for this overabundance is the same as that needed to explain the unusually high mass-to-light ratios of UCDs and a top-heavy IMF appears to be the only simultaneous explanation for both findings. Furthermore, we show that the high rate of type II supernovae in the starburst galaxy Arp 220 suggests a top-heavy IMF in that system. This finding is consistent with the notion that starburst galaxies are sites where UCDs are likely to be formed and that the IMF of UCDs is top-heavy. It is estimated that the IMF becomes top-heavy whenever the star formation rate per volume surpasses 0.1 M☉ yr−1 pc−3 in pc-scale regions.