Runaway Stars Supernova Then Die As Exiles

Crisp images taken by the Hubble Space Telescope have disclosed that three Type Ia supernovae first discovered several years ago exploded in the dark emptiness of intergalactic space.

These supernovae exploded after being expelled from their home galaxies millions or billions of years earlier. Usually, supernovae are observed within galaxies having hundreds of billions of stars, per century per galaxy one of them might explode.

But these solitary supernovae were found in-between galaxies, in three large clusters of several thousand galaxies each. Most likely, the stars’ closest neighbors were 300 light years away.

That is a distance equivalent to almost 100 times more than our to sun’s nearest stellar neighbor, Proxima Centauri, 4.24 light years distant.

Annihilated Planets

Study leader Melissa Graham, University of California at Berkeley postdoctoral fellow, said that any planets around these intracluster stars were doubtlessly annihilated by the explosions, but they would have had a night sky devoid of bright stars

“It would have been a fairly dark background indeed,” she said, “populated only by the occasional faint and fuzzy blobs of the nearest and brightest cluster galaxies.”

Typically, stars and supernovae live in galaxies. But galaxies located in massive galactic clusters undergo gravitational forces that pull away about 15 percent of the stars, according to a recent survey.

These clusters have so much mass, however, that the displaced stars remain gravitationally trapped inside thinly populated intracluster regions.

Best Evidence Yet

 Credit: Melissa Graham, CFHT and HSTOnce scattered, these lonesome stars are too faint to be detected individually, unless they go supernovae. Graham and her team are looking for bright supernovae in intracluster space as tracers to determine the population of unseen stars. This data gives clues about the formation and evolution of large scale structures in the universe.

“We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae,” Graham said, “and confirmed that hostless supernovae can be used to trace the population of intracluster stars, which is important for extending this technique to more distant clusters.”

If the supernova is indeed part of a globular cluster, this would be the first time a supernova has been verfied to explode inside these small, dense clusters of less than a million stars.

“Since there are far fewer stars in globular clusters, only a small fraction of the supernovae are expected to occur in globular clusters,” Graham said. “This might be the first confirmed case, and may indicate that the fraction of stars that explode as supernovae is higher in either low-mass galaxies or globular clusters.”

Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging
Melissa Graham et al., 2015
The Astrophysical Journal Volume 729 Number 2

Illustration: Artist’s concept of a Type Ia supernova exploding in the region between galaxies in a large cluster of galaxies. Credit: Dr. Alex H Parker, NASA and the SDSS