BLAST FROM THE PAST: Type IIn supernovae are believed to represent the collapse of massive stars surrounded by dense shrouds. Eta Carinae in the Milky Way is shown for illustration; it is a massive star (or pair of stars) surrounded by a nebula of material it cast off in the past. Image: Jon Morse/NASA
A team of astronomers has found what may prove to be the oldest supernovae identified to date, and they estimate that tens of thousands from the same vintage will be detectable in the coming years.
Jeff Cooke, a postdoctoral fellow at the Center for Cosmology at the University of California, Irvine, and his colleagues found two supernovae, or stellar explosions, that are believed to result from the collapse of massive stars some 11 billion years ago, predating the previous record holder by roughly a billion years. (The universe is estimated to be 13.7 billion years old.)
Cooke's team reports their findings in this week's Nature. (Scientific American is part of the Nature Publishing Group.) The researchers compiled years of archival data from the Canada–France–Hawaii Telescope on Mauna Kea in Hawaii, looking for galaxies that brightened as a result of exploding stars within. Their search was targeted toward type IIn supernovae, the presumed progenitors of which are massive stars whose iron cores suffer catastrophic collapses.
Supernovae are broadly classified by the presence (type II) or absence (type I) of hydrogen in their emission spectra. Type IIn supernovae are so called because their hydrogen line is narrow (hence the "n")—they are believed to mark the interaction of a supernova's ejecta with dense material around the star.
Type IIn supernovae make promising targets for long-distance detection because they are exceptionally bright emitters of ultraviolet light, short-wavelength radiation that is stretched to the visible part of the spectrum while traveling for billions of years across an expanding universe. What is more, they are long-lived events whose visible detection can be checked by spectroscopy years after the initial cataclysm reveals itself.
Douglas Leonard, an assistant professor of astronomy at San Diego State University, says that unlike the better-known type Ia supernovae that are used as cosmic distance markers and that helped make the case for an accelerating universe driven by dark energy, "the type IIn class of supernovae was kind of relegated to third-party status for a long time." But if they could be used to probe the conditions in which the earliest massive stars formed, they might find themselves more popular. "Now it's turning out," Leonard says, "that they may be really interesting objects."
Leonard calls the research carried out by Cooke and his co-authors "very difficult and creative work." At the same time, he acknowledges that the definitive spectral confirmation of the objects as supernovae rather than, say, brightly glowing galactic nuclei powered by black holes, is a tough task.
"I'd say [the supernovae] are intriguing, and I would say on balance they are convincing, but I wouldn't put it at the 99 percent level," Leonard says. "Further study with even better observations" from the biggest telescopes in the world "will help confirm," he adds. With clearer data and further characterization of nearby type IIn events for comparison, Leonard says he anticipates "the certainty of the discovery to become nearly iron-clad" in the near future.
Adam Riess, a Johns Hopkins University astrophysicist credited as a co-discoverer of dark energy for his work with type Ia supernovae, calls the Cooke team's paper "heroic work and quite exciting." He says he is not convinced beyond all doubt, "but time will tell as more of these are studied."
If the new method withstands scrutiny, it may bear bountiful fruit in the early universe. The study's authors estimate that approximately 40,000 type IIn supernovae from the same cosmological epoch could await discovery in the next decade, as well as some from even earlier times.