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Gravitational detection of a low-mass dark satellite galaxy at cosmological distance


The mass function of dwarf satellite galaxies that are observed around Local Group galaxies differs substantially from simulations1,2,3,4,5 based on cold dark matter: the simulations predict many more dwarf galaxies than are seen. The Local Group, however, may be anomalous in this regard6,7. A massive dark satellite in an early-type lens galaxy at a redshift of 0.222 was recently found8 using a method based on gravitational lensing9,10, suggesting that the mass fraction contained in substructure could be higher than is predicted from simulations. The lack of very low-mass detections, however, prohibited any constraint on their mass function. Here we report the presence of a (1.9 ± 0.1) × 108 dark satellite galaxy in the Einstein ring system JVAS B1938+666 (ref. 11) at a redshift of 0.881, where denotes the solar mass. This satellite galaxy has a mass similar to that of the Sagittarius12 galaxy, which is a satellite of the Milky Way. We determine the logarithmic slope of the mass function for substructure beyond the local Universe to be , with an average mass fraction of per cent, by combining data on both of these recently discovered galaxies. Our results are consistent with the predictions from cold dark matter simulations13,14,15 at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter.

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Figure 1: Detection of a dark-matter-dominated satellite galaxy in the gravitational lens system B1938+666 at z = 0.881.


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Our results are based on observations made with the W. M. Keck Observatory and the Hubble Space Telescope. S.V. is supported by a Pappalardo Fellowship at the Massachusetts Institute of Technology, L.V.E.K. is supported (in part) through an NWO-VIDI program subsidy, and D.J.L. and C.D.F. acknowledge support from the National Science Foundation. The authors are grateful to P. Marshall for comments and feedback.

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Authors and Affiliations



S.V. and L.V.E.K. developed the gravitational imaging technique used for the detection of substructure. S.V. carried out the gravitational lens modelling of the data with help from L.V.E.K. and J.P.M. S.V., L.V.E.K. and J.P.M. wrote the manuscript with comments from all of the authors. C.D.F. was the principal investigator of the observing programme and was responsible, along with D.J.L., for acquiring the data. D.J.L. and M.W.A. reduced the data and performed the galaxy subtraction with help from C.D.F. M.W.A. calculated the systematic error in the substructure mass. C.D.F. calculated the galaxy luminosity.

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Correspondence to S. Vegetti.

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The authors declare no competing financial interests.

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This file contains Supplementary Text and Data, Supplementary References, Supplementary Tables 1-3 and Supplementary Figures 1-6 with legends. (PDF 436 kb)

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Vegetti, S., Lagattuta, D., McKean, J. et al. Gravitational detection of a low-mass dark satellite galaxy at cosmological distance. Nature 481, 341–343 (2012).

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