A long time ago, from an explosion far, far away

An international team of astronomers, including Penn researchers, has confirmed the discovery of the most distant supernova ever detected, a huge cosmic explosion that took place 10.5 billion years ago.

The exploding star, named DES16C2nm, was detected by the Dark Energy Survey (DES), an international collaboration to map several hundred million galaxies in order to learn more about dark energy, the mysterious force believed to be causing the accelerated expansion of the universe.

As detailed in a new study published in The Astrophysical Journal, light from the event has taken 10.5 billion years to reach Earth, making it the oldest supernova ever discovered and studied. The universe itself is thought to be 13.8 billion years old.

The research was led by Mathew Smith of the University of Southampton in collaboration with Chris D’Andrea, a postdoctoral fellow at Penn, and Masao Sako, an associate professor of astronomy in the School of Arts and Sciences. Researchers at the University of Portsmouth also contributed to the research.

A supernova is the explosion of a massive star at the end of its life cycle. DES16C2nm is classified as a superluminous supernova, or SLSN, the brightest and rarest class of supernovae, which were first discovered 10 years ago. It is thought to be caused by material falling onto the densest type of object in the universe, a rapidly rotating neutron star newly formed in the explosion of a massive star. This violent explosion, Sako says, is brighter than even the brightest galaxies.

DES16C2nm was first detected in August 2016, and its distance and extreme brightness was confirmed in October of that year using three of the world’s most powerful telescopes: the Very Large Telescope and the Magellan in Chile, and the Keck Observatory in Hawaii.

According to Smith, the ultraviolet light from these supernovae “informs us of the amount of metal produced in the explosion and the temperature of the explosion itself, both of which are key to understanding what causes and drives these cosmic explosions.”

“What we think could be happening here,” D’Andrea says, “is that the stellar explosion produces a magnetar at its core: a rapidly spinning neutron star with a magnetic field 100 trillion times stronger than that on Earth. If we look at how the light from the superluminous supernova evolves in time, it matches very well with models of the amount of energy that magnetars emit as they spin. This energy is hitting the winds of the ejected material from the stellar explosion and dramatically brightening what we’re seeing.”

These particular supernovae could lead to advances in both stellar astrophysics and cosmology, Sako says. By investigating why and how stars die, scientists could discover new information about how compact objects such as black holes and neutron stars are created.

For cosmology, the typical “standard candles” for measuring the expansion evolution of the universe are type 1a supernovae. Scientists are able to determine the exact brightness of these exploding white dwarf stars, which led to the Nobel Prize-winning discovery that the universe is expanding at an accelerating rate. Since SLSN can be anywhere from 50 to 100 times brighter than type 1a supernovae, they can be seen at even greater distances. This could allow researchers to probe even more deeply into the expansion history of the universe to learn more about the nature of dark energy.

“The trick to being able to do this,” D’Andrea says, “is to find as many of these objects as possible to try to find patterns between the light they emit and how bright each object is. Given a large enough sample of SLSN, scientists may then be able to ‘standardize’ them as they did with type 1a supernovae.”

D’Andrea and other researchers will continue to hunt for these rare supernovae, both in the final year of DES as well as in future, more powerful surveys, such as the Large Synoptic Survey Telescope. They will also comb through previous data from DES and other surveys to see what may have been missed.

“Such supernovae were not thought of when we started DES over a decade ago,” says study co-author Bob Nichol, a professor of astrophysics at the University of Portsmouth. “Such discoveries show the importance of empirical science; sometimes you just have to go out and look up to find something amazing.”

superluminous supernova