Oldest, farthest supernova explosions discovered
Super-luminous supernovae are the 'bringers of death or the bringers of life to stars.'
Thu, Nov 01, 2012 at 10:54 AM
High-resolution simulation of a galaxy hosting a super-luminous supernova and its chaotic environment in the early universe. (Image: Adrian Malec and Marie Martig/Swinburne University)
The most distant star explosions in the universe have now been discovered, suggesting scientists may one day see the deaths of the first stars to arise after the Big Bang, researchers say.
Future research into such remote, powerful explosions could shed light on the evolution of the universe since the Big Bang, investigators added.
The most powerful star explosions are supernovas, which are bright enough to briefly outshine all the stars in their home galaxies. In the past 12 years, astronomers have detected a new class of supernova, so-called super-luminous supernovas, which areup to 100 times brighter than all the others.
"Super-luminous supernovae are very energetic events and extremely rare," lead study author Jeff Cooke, an astronomer at the Swinburne University of Technology in Hawthorn, Australia, told SPACE.com. "They are very destructive as well. In the early universe, many galaxies were quite small but vigorously forming stars. A single supernova of this type could disrupt a significant fraction of such a galaxy and, in some cases, cause the star formation process to come to a halt."
However, in larger galaxies where super-luminous supernovas make less of an overall impact, the material blown off them "provides the seeds to form new stars, and the shock waves from the explosions can help to compress gas in those galaxies to accelerate the star formation process," Cooke added. "So they can be the bringers of death or the bringers of life to stars. Detecting and measuring the rate of super-luminous supernovae in the early universe helps clarify their role in the formation and evolution of galaxies." [Supernova Photos: Great Images of Star Explosions]
The origins of super-luminous supernovas remain mysterious. Researchers think some of them result from the detonation of extraordinarily large stars 100 to 250 times the mass of the sun. These are the rarest kind of super-luminous supernova, known as radioactively powered SLSNe-R, or as pair-instability supernovas.
Astrophysicists suspect that within the stars that gave rise to these supernovas, conditions are just right for gamma-ray light to convert into pairs of electrons and their antimatter counterparts, known as positrons. These gamma-rays normally help exert pressure that helps support the star against the crushing effects of gravity. As gamma-rays get converted to matter, the star loses this support, collapsing in on itself. This collapse triggers a runaway thermonuclear explosion that completely obliterates the star.
"The progenitor stars to these supernovae have really interesting physics going on inside them prior to explosion that has been long theorized but only recently observed," Cooke said.
The giant stars that give rise to pair-instability supernovas are far larger than any existing today, and are thought to have been more common in the early universe. This is because elements heavier than helium did not exist in any significant amounts back then that could help suck up heat so that gas could cool and collapse to form stars. These relatively heavy elements — including carbon, oxygen, iron and most of the matter seen on Earth — only began to be forged in large amounts during the lives and violent deaths of the first stars.
"The first generation of stars born after the Big Bang formed from pristine gas," Cooke said. "Their subsequent supernova deaths polluted the universe with heavier elements and the following generation of stars formed from this enriched gas. Thus, the first generation of stars was truly unique."
Instead, to overcome the lack of cooling ingredients and create the first stars, huge amounts of mass were needed to generate powerful gravity fields. This gravitational pull helped collapse gas together to set off star formation.
Gazing at the edge of time
To find these ancient giant stars and their super-luminous supernovas, researchers gazed into distant reaches of the universe. Since light takes time to travel, the more distant the star, the farther its light has traveled and the older the star is when observed. This means the most distant stars astronomers can see are also the oldest.
Scientists focused on ancient super-luminous supernovas that exploded more than 10 billion years ago, back when the universe was less than one-quarter of its current 13.7-billion-year age. By combining all the available images from the Canada-France-Hawaii Telescope's Legacy Survey to create the deepest images possible, the scientists discovered two supernovas, one that occurred about 10.4 billion years ago, the other about 12.1 billion years ago. Until now, the most distant supernova seen came from an explosion about 10.8 billion years ago.
One, and possibly both, of these newfound super-luminous supernovas are pair-instability supernovas. Until now, astronomers had only detected one pair-instability supernova with any certainty.
The pattern of light seen from these newfound supernovas suggests they did not actually come from the very first generation of stars. Still, "one of the most exciting things is that this work demonstrates that we have the technique and technology right now to detect the deaths of the first generation of stars that formed after the Big Bang," Cooke said. "Previous to this work, it was assumed that we would need to wait until the next decade for the upcoming instruments and large-aperture telescopes to make these detections."
"The first stars to form after the Big Bang laid the framework for the long process of enriching the universe that eventually produced the diverse set of galaxies, stars and planets we see around us today," Cooke said. "Our discoveries mean that we now have the means to investigate this process from the beginning."
The researchers are now concentrating on events most likely linked with the first generation of stars. "The pristine gas that these stars require is predicted to be found in the far outskirts of galaxies or in areas where there is no visible host galaxy," Cooke said. "We call the supernovae in these remote regions 'orphan supernovae' because there is no obvious host associated with them. This project is already finding some interesting events."
In addition, the scientists aim to use super-luminous supernovas "as bright beacons that temporarily light up their host galaxy from the inside," Cooke added. "Events detected when they are at their brightest in upcoming surveys will reveal the properties of the gas in the galaxies that host them that would otherwise be invisible."
The scientists detailed their findings online Oct. 31 in the journal Nature.
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