1. Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, California 92697-4574, USA
2. Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
3. Department of Astronomy and Astrophysics, University of Toronto, Toronto, Ontario M5S 3H4, Canada
4. Benoziyo Center for Astrophysics, Weizmann Institute of Science, 76100 Rehovot, Israel

Correspondence to: Jeff Cooke¹ Correspondence and requests for materials should be addressed to J.C. (Email: cooke@uci.edu).

Abstract

Supernovae have been confirmed to redshift z  1.7 (refs 1, 2) for type Ia (thermonuclear detonation of a white dwarf) and to z  0.7 (refs 1, 3–5) for type II (collapse of the core of the star). The subclass type IIn (ref. 6) supernovae are luminous^{7, 8, 9} core-collapse explosions of massive stars^{8, 9, 10, 11} and, unlike other types, are very bright in the ultraviolet^{12, 13, 14, 15}, which should enable them to be found optically at redshifts z  2 and higher^{14, 16}. In addition, the interaction of the ejecta with circumstellar material creates strong, long-lived emission lines that allow spectroscopic confirmation of many events of this type at z  2 for 3–5 years after explosion (ref. 14). Here we report three spectroscopically confirmed type IIn supernovae, at redshifts z = 0.808, 2.013 and 2.357, detected in archival data using a method¹⁴ designed to exploit these properties at z  2. Type IIn supernovae directly probe the formation of massive stars at high redshift. The number found to date is consistent with the expectations of a locally measured¹⁷ stellar initial mass function, but not with an evolving initial mass function proposed^{18, 19, 20} to explain independent observations at low and high redshift.

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