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Date and Time of the Query: 2019-08-26 T03:06:07 PDT
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For refcode 2018Natur.558..260Z:
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2018Natur.558..260Z Stellar populations dominated by massive stars in dusty starburst galaxies across cosmic time Zhang, Zhi-Yu; Romano, D.; Ivison, R. J.; Papadopoulos, Padelis P.; Matteucci, F. Abstract. 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 (of more than eight solar masses) to the total star-formation rate across the full stellar mass spectrum^1^. 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^2^. 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^2,3^, especially distant starbursts, whose apparent star-formation rates are hundreds to thousands of times higher than in the Milky Way, selected at submillimetre (rest-frame far-infrared) wavelengths^4,5^. The ^13^C/^18^O isotope abundance ratio in the cold molecular gas---which can be probed via the rotational transitions of the ^13^CO and C^18^O 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 ^13^CO and C^18^O 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 ^13^CO/C^18^O ratio for all our targets---alongside a well tested, detailed chemical evolution model benchmarked on the Milky Way^6^---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^7^, although such main-sequence galaxies may not be immune to changes in initial stellar mass function, depending on their star-formation densities.
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