The detection of one of the most distant superluminous supernova to have its details observed has provided us with a brief flash of insight into a period known as “cosmic high noon”, the time when star formation in the universe reached its peak.
Supernovae are spectacular enough, but sometimes the universe puts on something extra special – a superluminous supernova, which can be 10 to 100 times brighter than ordinary supernovas. There are so few opportunities to study these events that we're still debating the processes that drive them, so when the Blanco four-meter telescope in Chile detected DES15E2mlf, it was a cause for excitement.
DES15E2mlf comes from a particularly interesting period in the universe's evolution. Some 3.5 billion years after the Big Bang, star formation reached a maximum, before slowly declining as more and more galaxies used up most of their initial gas. The stars that become supernovae, particularly super-luminous ones, have short lifespans, so anything exploding during cosmic high noon formed around the same time.
Since the cosmic high noon was 10 billion years ago, anything we detect from this period is at a distance of 10 billion light-years. It is only because DES15E2mlf was so phenomenally bright, three times brighter than the entire Milky Way, that we are able to see it at all.
Already DES15E2mlf has shaken up our understanding of such events, according to a paper in the Monthly Notices of the Royal Astronomical Society. Most previous superluminous supernovae have been in low-mass or dwarf galaxies, but this one is in a galaxy with a mass 3.5 billion times that of the Sun – still much smaller than the Milky Way, but larger than the galaxies that usually host such explosions.
The association between superluminous supernovae and small galaxies has been explained by the low concentrations of elements heavier than helium in these galaxies. The presence of these elements was thought to prevent the formation of such supernovae, but the galaxy in which DES15E2mlf formed would be expected to have been rich in them.
“Even high-mass galaxies had low enough metal content to create these extraordinary stellar explosions,” said study co-author Dr Ryan Foley of the University of California, Santa Cruz, said in a statement. The presence of such a bright supernova in a massive galaxy indicates this was the case at this point in the universe's evolution.
Many questions remain, though. "It's important simply to know that very massive stars were exploding at that time," Foley said. "What we really want to know is the relative rate of superluminous supernovae to normal supernovae, but we can't yet make that comparison because normal supernovae are too faint to see at that distance.”
A sample of the brightest explosions from the same era may give us some answers.
