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A substantial population of low-mass stars in luminous elliptical galaxies


The stellar initial mass function (IMF) describes the mass distribution of stars at the time of their formation and is of fundamental importance for many areas of astrophysics. The IMF is reasonably well constrained in the disk of the Milky Way1 but we have very little direct information on the form of the IMF in other galaxies and at earlier cosmic epochs. Here we report observations of the Na i doublet2,3 and the Wing–Ford molecular FeH band4,5 in the spectra of elliptical galaxies. These lines are strong in stars with masses less than 0.3M (where M is the mass of the Sun) and are weak or absent in all other types of stars5,6,7. We unambiguously detect both signatures, consistent with previous studies8 that were based on data of lower signal-to-noise ratio. The direct detection of the light of low-mass stars implies that they are very abundant in elliptical galaxies, making up over 80% of the total number of stars and contributing more than 60% of the total stellar mass. We infer that the IMF in massive star-forming galaxies in the early Universe produced many more low-mass stars than the IMF in the Milky Way disk, and was probably slightly steeper than the Salpeter form9 in the mass range 0.1M to 1M.

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Figure 1: Detection of the Na  i doublet and the Wing–Ford band.
Figure 2: Constraining the IMF.

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We thank R. Bezanson, J. Brinchmann, R. Larson and R. Zinn for discussions. We thank R. Schiavon for comments that improved the manuscript. This study is based on observations obtained at the W. M. Keck Observatory. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the indigenous Hawaiian community and consider ourselves fortunate to have the opportunity to conduct observations from this mountain.

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P.G.v.D. obtained and analysed the data and contributed to the analysis and interpretation. C.C. constructed the stellar population synthesis models and contributed to the analysis and interpretation.

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Correspondence to Pieter G. van Dokkum.

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The authors declare no competing financial interests.

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The file contains Supplementary Text on stellar population synthesis modelling, additional references and Supplementary Figures 1-2 with legends. (PDF 203 kb)

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van Dokkum, P., Conroy, C. A substantial population of low-mass stars in luminous elliptical galaxies. Nature 468, 940–942 (2010).

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