Superluminous supernovae at redshifts of 2.05 and 3.90.

A rare class of 'superluminous' supernovae that are about ten or more times more luminous at their peaks than other types of luminous supernova has recently been found at low to intermediate redshifts. A small subset of these events have luminosities that evolve slowly and result in radiated energies of up to about 10(51) ergs. Therefore, they are probably examples of 'pair-instability' or 'pulsational pair-instability' supernovae with estimated progenitor masses of 100 to 250 times that of the Sun. These events are exceedingly rare at low redshift, but are expected to be more common at high redshift because the mass distribution of the earliest stars was probably skewed to high values. Here we report the detection of two superluminous supernovae, at redshifts of 2.05 and 3.90, that have slowly evolving light curves. We estimate the rate of events at redshifts of 2 and 4 to be approximately ten times higher than the rate at low redshift. The extreme luminosities of superluminous supernovae extend the redshift limit for supernova detection using present technology, previously 2.36 (ref. 8), and provide a way of investigating the deaths of the first generation of stars to form after the Big Bang.

Type IIn supernovae at redshift z approximately 2 from archival data.

Supernovae have been confirmed to redshift z approximately 1.7 (refs 1, 2) for type Ia (thermonuclear detonation of a white dwarf) and to z approximately 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 core-collapse explosions of massive stars and, unlike other types, are very bright in the ultraviolet, which should enable them to be found optically at redshifts z approximately 2 and higher. 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 approximately 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 method designed to exploit these properties at z approximately 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 measured stellar initial mass function, but not with an evolving initial mass function proposed to explain independent observations at low and high redshift.