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arXiv 1806.01280astro-ph.GA
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宇宙时间上尘埃星暴星系中由大质量恒星支配的恒星星族

Stellar populations dominated by massive stars in dusty starburst galaxies across cosmic time

Zhi-Yu Zhang, D. Romano, R. J. Ivison, P. P. Papadopoulos, F. Matteucci

摘要:

所有宇宙恒星形成的测量都必须假设恒星质量的初始分布-恒星初始质量函数-以便从典型的稀有大质量恒星(> 8 Msun)测量的恒星形成率外推到整个恒星质量谱的总恒星形成率。不同星系群中恒星初始质量函数的形状支持了我们对整个宇宙时间内星系形成和演化的理解。传统上,对局部星系中恒星初始质量函数的经典测定是在紫外线、光学和近红外波长下进行的,而在尘埃遮蔽的星系中,特别是在遥远的恒星爆发中,无法探测到这些波长,这些恒星爆发的恒星形成率比我们银河系高几百到几千倍,选择在亚毫米(静止帧远红外)波长下进行。冷分子气体中的13 C/18 O丰度比--可以通过13 CO和C18 O同位素的旋转跃迁来探测--是恒星初始质量函数的一个非常敏感的指标,其测定不受尘埃的有害影响。在这里,我们报告的13 CO和C18 O排放的样品的四个尘埃笼罩的恒星爆发的红移约2至3,并找到明确的证据,头重脚轻的恒星初始质量函数在所有这些。我们所有目标的13 CO/C18 O比率都很低,再加上以银河系为基准的经过充分测试的详细化学演化模型,这意味着在星暴事件中有比普通恒星形成螺旋星系更多的大质量恒星。这可以使这些非凡的恒星爆发更接近恒星形成星系的“主序星”,尽管这样的主序星星系可能不会受到初始恒星质量函数变化的影响,这取决于它们的恒星形成密度。

英文摘要:

All measurements of cosmic star formation must assume an initial distribution of stellar masses -- the stellar initial mass function -- in order to extrapolate from the star-formation rate measured for typically rare, massive stars (> 8 Msun) to the total star-formation rate across the full stellar mass spectrum. The shape of the stellar initial mass function in various galaxy populations underpins our understanding of the formation and evolution of galaxies across cosmic time. Classical determinations of the stellar initial mass function in local galaxies are traditionally made at ultraviolet, optical and near-infrared wavelengths, which cannot be probed in dust-obscured galaxies, especially in distant starbursts, whose apparent star-formation rates are hundreds to thousands of times higher than in our Milky Way, selected at submillimetre (rest-frame far-infrared) wavelengths. The 13C/18O abundance ratio in the cold molecular gas -- which can be probed via the rotational transitions of the 13CO and C18O isotopologues -- is a very sensitive index of the stellar initial mass function, with its determination immune to the pernicious effects of dust. Here we report observations of 13CO and C18O emission for a sample of four dust-enshrouded starbursts at redshifts of approximately two to three, and find unambiguous evidence for a top-heavy stellar initial mass function in all of them. A low 13CO/C18O ratio for all our targets -- alongside a well-tested, detailed chemical evolution model benchmarked on the Milky Way -- implies that there are considerably more massive stars in starburst events than in ordinary star-forming spiral galaxies. This can bring these extraordinary starbursts closer to the `main sequence' of star-forming galaxies, though such main-sequence galaxies may not be immune to changes in initial stellar mass function, depending upon their star-formation densities.

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