In the news this month... new method detects ancient stellar explosions
This image was taken with CFHT as part of the telescope’s Legacy Survey and shows one of the deep fields used to find the most distant supernovae to date. CREDIT: Jeff Cooke / CFHT
Using telescopes high on the islands of Hawaii, astronomers have detected light from supernovae which occurred roughly 11 billion years ago, smashing the previous distance record for such objects. Led by Jeff Cooke, a cosmologist at the University of California, the team used a new technique to look for the tiny change in brightness of a distant galaxy due to a stellar explosion. They were looking for the signatures of a class of explosion known as type IIn supernovae, caused by stars between 50 and 100 times as massive as the Sun. These stars are different because they shed a large amount of material before they die. When the final catastrophic explosion occurs, the remaining material and the resulting shock wave plough into the surrounding gas previously expelled from the star, resulting in a remnant so bright that it is still visible many years after the event. Because these type IIn supernovae are the brightest class of stellar explosion, they are the most likely to be detected at large distances.
The normal method of searching for supernovae is to compare two images of the same galaxy taken on different nights and look for new objects in the image. While this technique works well for nearby galaxies where supernovae will appear relatively bright, it becomes increasingly difficult in more distant galaxies. Rather than looking at individual images taken on single nights, Cooke's team took five years of images covering four separate patches of sky and stacked them together creating one composite image per field for each year of observation. By comparing the brightnesses of each galaxy in the stacked images, the astronomers identified four potential supernovae. They then used the Keck telescope to observe the spectra of each of the candidates, using the light collected to determine the object's composition and distance. This follow-up work showed that three of the candidates were supernovae, two of which occurred more than 11 billion years ago, beating the previous record by 2 billion years.
The results were published in the 9th July issue of the journal Nature where the authors suggest that using this method with planned synoptic surveys on 8-m class telescopes could identify an estimated 40,000 type IIn supernovae at this distance, as well as even older explosions caused by some of the first stars created following the Big Bang, probing stellar processes all the way back to the very early universe.