A top-heavy stellar initial mass function in starbursts as an explanation for the high mass-to-light ratios of ultra-compact dwarf galaxies
A top-heavy stellar initial mass function in starbursts as an explanation for the high mass-to-light ratios of ultra-compact dwarf galaxies
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星暴中头重脚轻的恒星初始质量函数可以解释超紧凑矮星系的高质光比
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发表时间：
2009
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通讯作者：
Holger Baumgardt
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作者：
J. Dabringhausen;P. Kroupa;Holger Baumgardt
最近的研究表明，质量在10 6 ~ 10 8 M之间的致密恒星系统的V波段动态质光比与简单星族模型的预测不一致。在这些所谓的超紧凑矮星系（UCD）中，头重脚轻的恒星初始质量函数（IMF）为这一发现提供了一个有吸引力的解释，恒星残骸和保留的恒星包层提供了不可见的质量。因此，我们构建了一个模型，该模型量化了UCD的IMF在中等和高质量范围内必须偏离规范IMF多少，以解释UCD的增强M/LV比。推导出的高质量IMF在UCD取决于UCD的年龄和恒星演化的微弱的产品，他们保留的数量。假设UCD中的IMF是一个三部分幂律，等于低质量范围内的正则IMF，并将20%作为UCD保留的高质量恒星残余部分的合理选择，该模型表明，如果UCD的年龄为13 Gyr，则大质量IMF的指数为1.6（即几乎和宇宙一样古老）或者如果UCD是7 Gyr，则为1.0，而Salpeter-Massey IMF为2.3。如果IMF像这里所建议的那样头重脚轻，那么UCD的稳定性可能会受到大质量恒星辐射和演化引起的严重质量损失的威胁。UCD的中心密度一定在10 6 ~ 10 7 Mpc −3之间，当它们以10 ~ 100 Myr −1的星星形成速率形成时。
It has been recently shown that the dynamical V-band mass-to-light ratios of compact stellar systems with masses from 10 6 to 10 8 Mare not consistent with the predictions from simple stellar population models. Top-heavy stellar initial mass functions (IMFs) in these so-called ultra-compact dwarf galaxies (UCDs) offer an attractive explanation for this finding, the stellar remnants and retained stellar envelopes providing the unseen mass. We therefore construct a model which quantifies by how much the IMFs of UCDs would have to deviate in the intermediate- and high-mass range from the canonical IMF in order to account for the enhanced M/LV ratio of the UCDs. The deduced high-mass IMF in the UCDs depends on the age of the UCDs and the number of faint products of stellar evolution retained by them. Assuming that the IMF in the UCDs is a three-part power law equal to the canonical IMF in the low-mass range and taking 20 per cent as a plausible choice for the fraction of the remnants of high-mass stars retained by UCDs, the model suggests the exponent of the high-mass IMF to be ≈1.6 if the UCDs are 13 Gyr old (i.e. almost as old as the Universe) or ≈1.0 if the UCDs are 7 Gyr old, in contrast to 2.3 for the Salpeter-Massey IMF. If the IMF was as top heavy as suggested here, the stability of the UCDs might have been threatened by heavy mass loss induced by the radiation and evolution of massive stars. The central densities of UCDs must have been in the range 10 6 to 10 7 Mpc −3 when they formed with star formation rates of 10 to 100 Myr −1 .