The discovery of two superluminous supernovae at large distances from Earth pushes the frontier of supernova studies to just 1.5 billion years after the Big Bang, and suggests that they may be common in the young Universe. See Letter p.228
Astronomers thrive in unexplored territory — whether that is the depths of dust clouds in our Galaxy, the immediate surroundings of a black hole or the farthest reaches of the Universe. For several decades, they have used the largest telescopes on Earth and in space to find the most distant sources of light in the Universe. The more distant the object, the longer its light has taken to travel to Earth, and the further back in time we can look. The most distant galaxies tell us how the Universe looked just 500 million years after the Big Bang1,2. Of course, the more luminous an object, the easier it is to detect at large distances. The discovery of new types of luminous stellar explosion, which only a few years ago were unknown, has opened a new window on the distant Universe. On page 228 of this issue, Cooke et al.3 present the discovery of two distant stellar explosions, both of which seem to be superluminous supernovaeFootnote 1.
Supernovae are stellar explosions that, until recently, were thought to come in two distinct varieties: type Ia supernovae and core-collapse supernovae. Type Ia supernovae originate from white-dwarf stars in two-star systems that reach a critical mass by gaining mass from the companion star. Core-collapse supernovae derive from massive stars that fuse increasingly heavier elements in their cores, starting from hydrogen and helium and going all the way up to iron. This iron core eventually becomes too massive to support itself against gravity, collapsing and releasing about 1046 joules of energy. The release of this energy, mostly in the form of neutrinos, drives a shock wave that causes the visible supernova when it hits the surface of the star. The resulting supernova radiates energy for several months, at a rate 1 billion times that of the Sun.
The search for type Ia supernovae in the distant Universe is well documented, and their use as standard candles to discover the existence of 'dark energy' resulted in last year's Nobel Prize in Physics. This discovery was made possible by finding these type Ia supernovae at large distances, corresponding to a redshift of about 0.7 — a time when the Universe was 7 billion years old, half its present age. The most recent surveys for type Ia supernovae, conducted with the Hubble Space Telescope, have detected4 them out to redshift 1.55, whereas high-redshift core-collapse supernovae have been found5 associated with γ-ray bursts at around redshift 1.
Cooke et al. have used data from the Canada-France-Hawaii Telescope (CFHT) Legacy Survey in a new way to find, at high redshift, much more luminous supernovae than type Ia and core-collapse supernovae. In the past few years, surveys of the nearby Universe covering a wide field of view discovered a hitherto unknown group of supernovae that has been labelled6,7,8 as “superluminous” and “ultraluminous” (Fig. 1). These are typically 10 times more luminous than type Ia supernovae and 100 times more luminous than normal core-collapse supernovae. A flurry of activity by astronomers getting to grips with these unusual events has resulted in their categorization into at least three distinct groups on the basis of their observed characteristics9. These results led Cooke et al. to apply a clever technique10 to search for the most distant events. The authors added up all the available images from the CFHT Legacy Survey, which were taken at monthly intervals, to perform a search of a large volume of the high-redshift Universe. They found two transient events that look similar to some of the lower-redshift, superluminous supernovae.
One theory for the origin of superluminous supernovae, known as the pair-instability hypothesis, is that the progenitors were very massive stars, about 100–300 solar masses. In the 1960s, theorists predicted that if such massive stars could retain much of their initial mass, then their cores could get big enough and hot enough to create electron–positron pairs11(the positron is the electron's antiparticle). This process can reduce the internal pressure of the stars such that their cores contract and reach temperatures greater than 109 °C — some 250 times hotter than the Sun's core. This core contraction could result in 60 solar masses of carbon and oxygen instantly undergoing a massive thermonuclear explosion and fusing to make heavier elements all the way up to the iron group. This explosion produces unstable, radioactive nickel-56, which can decay to stable iron-56, releasing γ-rays that heat the supernova to superluminous intensities.
Models predict that 4–10 solar masses of nickel-56 could be produced. This process is similar to what goes on in a type Ia supernova, but in that case only 0.7 solar masses of nickel-56 are generated from a tiny white-dwarf star. Cooke et al. propose that the two superluminous supernovae that they have found are pair-instability supernovae.
Given their redshifts of 2.05 and 3.9, the new supernovae exploded 3 billion and 1.5 billion years after the Big Bang, respectively. The redshift-3.9 event is the most distant supernova discovered to date, although γ-ray bursts that might be associated with supernovae have been found at higher redshifts2,5.These new supernovae are undoubtedly intriguing transient events. Spectra of their host galaxies, obtained using the Keck telescope in Hawaii, allowed the unambiguous measurement of the redshifts. However, there are no spectra of the supernovae themselves, which would be crucial for confirming their physical nature. In addition, there is only one well-documented case of a pair-instability supernova candidate, called SN2007bi, in the local Universe with which to compare the new systems12,13. Papers and posters presented at supernova-related conferences in the past year indicate that wide-field surveys shallower than that undertaken by Cooke et al. have found a few more examples — but they are rare9. The physical nature of SN2007bi has also been subject to debate, with some authors doubting its pair-instability nature and suggesting a normal core-collapse mechanism in a very massive star13 or a supernova enhanced by a magnetic neutron star14.
For many years, astronomers have dreamed of finding the first supernovae in the Universe at redshifts beyond 10, and theory has predicted that very massive stars and pair-instability supernovae could be prevalent at such redshifts. In one leap, Cooke et al. have expanded our horizons to show that it is now possible to find supernovae out to redshift 4. Uncertainty remains over the nature of these superluminous explosions both at high and low redshifts. Yet the work of Cooke et al. shows that superluminous supernovae, of some type, existed in the young Universe, and that their frequency might be much higher than we see locally. The next challenge is to find them as they reach peak luminosity, obtain their spectra to understand exactly what they are, and use them as probes of the high-redshift Universe15.
*This article and the paper under discussion3 were published online on 31 October 2012.
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