New observations with the Advanced Camera for Surveys on the NASA/ESA Hubble Space Telescope confirm that two Type Ia supernovae discovered six years ago exploded in the intergalactic space, having been ejected from their host galaxies millions of years earlier.
This is an artist’s concept of a Type Ia supernova exploding in the intergalactic space between galaxies within a galactic cluster. Image credit: Alex Parker / NASA / SDSS.
The observations confirm the discovery in 2009 of two hostless supernovae by a team of astronomers using the Canada-France-Hawaii Telescope (CFHT), Hawaii.
The team was unable to rule out a faint galaxy hosting these supernovae. But the sensitivity and resolution of images from the Hubble’s Advanced Camera for Surveys are ten times better and clearly show that the supernovae exploded in the intergalactic space – far from any galaxy – within the galactic clusters Abell 1650 and Abell 85.
“They belong to a population of solitary stars that exist in most if not all clusters of galaxies,” said Dr Melissa Graham from the University of California, Berkeley.
While stars and supernovae typically reside in galaxies, galaxies situated in huge galactic clusters experience gravitational forces that wrench away about 15 percent of the stars.
The clusters have so much mass, though, that the displaced stars remain gravitationally bound within the sparsely populated intracluster regions.
Once dispersed, these lonely stars are too faint to be seen individually unless they explode as supernovae.
The CFHT image, left, shows no galaxy near a Type Ia supernova in the galactic cluster Abell 85, but is not sharp enough to rule out the possibility that the supernova was embedded in a galaxy too faint to detect. The higher-resolution Hubble image confirms that the exploding star, though no longer visible, was not part of a galaxy, but was instead a solitary supernova. Image credit: Melissa Graham / CFHT / HST.
“We have provided the best evidence yet that intracluster stars truly do explode as Type Ia supernovae,” said Dr Graham, who is the first author of the paper accepted for publication in the Astrophysical Journal (arXiv.org preprint), “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.”
“Any planets around these intracluster stars were no doubt obliterated by the explosions, but they would have had a night sky depleted of bright stars. The density of intracluster stars is about one-millionth what we see from Earth,” she said.
According to the astronomers, most theoretical models for Type Ia supernovae involve a binary star system, so the exploding stars would have had a companion throughout their lifetimes.
“This is no love story, though. The companion was either a lower-mass white dwarf that eventually got too close and was tragically fragmented into a ring that was cannibalized by the primary star, or a regular star from which the primary white dwarf star stole sips of gas from its outer layers,” Dr Graham said.
“Either way, this transfer of material caused the primary to become unstably massive and explode as a Type Ia supernova.”
_____
Melissa L. Graham et al. 2015. Confirmation of Hostless Type Ia Supernovae Using Hubble Space Telescope Imaging. ApJ, accepted for publication; arXiv: 1505.03407
