Steward Observatory graduate student and team discover most distant supernovas ever found

Peering deeply into the cosmos, NASA's James Webb Space Telescope is giving scientists their first detailed glimpse of supernovas from a time when the universe was young.

A team using JWST data has identified 10 times more supernovas in the early universe than were previously known. A few of the newfound exploding stars are the most distant examples of their type, including those used to measure the universe's expansion rate.

"Webb is a supernova discovery machine," said Christa DeCoursey, a University of Arizona graduate student in the Department of Astronomy and Steward Observatory who led the research. "The sheer number of detections, plus the great distances to these supernovae, are the two most exciting outcomes from our survey."

DeCoursey presented the findings this week at a press conference at the 244th meeting of the American Astronomical Society in Madison, Wisconsin.

'A supernova discovery machine'

To make these discoveries, the team analyzed imaging data obtained as part of the JWST Advanced Deep Extragalactic Survey, or JADES, program. Webb is ideal for finding extremely distant supernovas because their light is stretched into longer wavelengths by the expansion of the universe, a phenomenon known as cosmological redshift. The more distant the supernova, the more dramatic the redshift.

Prior to JWST's launch, only a handful of supernovas had been found above a redshift of two, which corresponds to when the universe was only 3.3 billion years old, just 25% of its current age. 

Previously, researchers used NASA's Hubble Space Telescope to view supernovas from when the universe was the human equivalent to a young adult, according to the team. With JADES, scientists are seeing supernovas when the universe was in its teenage years. In the future, they hope to look back to even younger phases of the universe.

To discover the supernovas, the team compared multiple images taken up to one year apart and looked for sources that disappeared or appeared in those images. These objects that vary in observed brightness over time are called transients, and supernovas are a type of transient. In all, the JADES Transient Survey Sample team uncovered about 80 supernovas in a patch of sky only about the thickness of a grain of rice held at arm's length. 

"This is really our first sample of what the high-redshift universe looks like for transient science," said teammate Justin Pierel, a NASA Einstein Fellow at the Space Telescope Science Institute, or STScI, in Baltimore, Maryland. "We are trying to identify whether distant supernovae are fundamentally different from or very much like what we see in the nearby universe."

Pierel and other STScI researchers provided expert analysis to determine which transients were actually supernovas and which were not, because often they looked very similar.

The team identified a number of high-redshift supernovas, including the farthest one ever confirmed, at a redshift of 3.6. Its progenitor star exploded when the universe was only 1.8 billion years old. It is a so-called core-collapse supernova.

Looking toward the future

The early universe was a very different place with extreme environments. Scientists expect to see ancient supernovas that come from stars that contain far fewer heavy chemical elements than stars like the sun. Comparing these supernovas with those in the local universe will help astrophysicists understand star formation and supernova explosion mechanisms at these early times.

"We're essentially opening a new window on the transient universe," said STScI fellow Matthew Siebert, who is leading the spectroscopic analysis of the JADES supernovas. "Historically, whenever we've done that, we've found extremely exciting things — things that we didn't expect."

"Because Webb is so sensitive, it's finding supernovae and other transients almost everywhere it's pointed," said JADES team member Eiichi Egami, a UArizona research professor in the Department of Astronomy and Steward Observatory. "This is the first significant step toward more extensive surveys of supernovae with the James Webb Space Telescope."