SNe July 2009

Most distant supernovae in the known universe found in the CFHT Legacy Survey

With is wide-field CCD camera MegaCam, CFHT took the last images for the CFHT Legacy Survey (CFHTLS) in January of this year. This Canadian-French project spread over 500 nights and nearly 6 years. Even though the full analysis of its images for the main goals of the survey is still work in progress, the CFHTLS has already provided amazing discoveries form the our solar system to the remote universe.

The SNLS (SuperNovae Legacy Survey), one of the components of the CFHTLS, observed four areas of the sky each the size of the MegaCam field (one square degree) to detect the explosion of SuperNoave (SNe). Each of the "Deep" fields has been monitored every few nights around new moon for half a year over each year of the duration of the survey. An amazing number of SNe were discovered by comparing each new block of observation (generally worthone hour of exposure time) with a reference image taken earlier.

An international team of astronomers, led by Jeff Cooke (University of California, Irvine), used the images of the SNLS in a different way: all images taken on a given field on the same semester were stacked together. The stacks, going now very deep as they corresponded to tens of hours of exposure time, were compared to each other from semester to semester. While the method does not allow to follow the explosion itself, with the raising of the light up to to the maximum of its explosion and the slow decay over the following weeks, it allows to discover very distant SNe, too faint to be seen in one hour of exposure time, as shown on the figure below.

This image shows the host galaxy containing one of the newly discovered supernovae. Comparing the images shows how the galaxy visibly brightens in 2004 and then returns to normal. This suggested that in 2003 the supernova was not detected; it appeared in 2004 and had mostly faded in 2005. The last frame subtracts the images from the years that the supernova was not detected as well as the galaxy’s light to reveal only the supernova. Credit: Jeff Cooke/CFHT

Cooke then used a Keck telescope to look more closely at the spectrum of light each object emitted and confirmed they were indeed supernovae. Two of the four newly SNe discovered happened about 11 billion years ago, 2 billion years earlier than the previous record holder, which happened nine billion years ago.

8' x 4', or 1% of the whole image of one of the four deep fields of the CFHTLS used in Cooke' study. More images from one of the CFHT Deep fields can be found here.