'Firecracker Galaxy' Offers Double Blast

Nov. 22, 2006 — Stars are blowing themselves to smithereens more often than usual in galaxy NGC 1316. Astronomers have stumbled onto two supernovae letting loose there just months apart, in addition to two previous mega-blasts in the last 26 years.

That makes the rate of exploding stars in NGC 1316 many times higher than any other known galaxy.

The recent twin supernovae were detected by the NASA Swift satellite observatory on June 19 and Nov. 5 of this year. In the images, the supernovae are visible on either side of the bright galaxy center.

"It is actually very puzzling," said Neil Gehrels, a Swift investigator at NASA's Goddard Space Flight Center. "That's the most (supernovae) there have ever been in such a short time." A more typical rate is around one a century in a given galaxy.

So what makes NGC 1316 different? Not a lot, at least so far as researchers can tell. It's a large elliptical galaxy about 80 million light years away, believed to have recently collided and merged with a spiral galaxy, which may have something to do with the accelerated rate of supernovae — or not.

Galactic mergers are believed to stir up cosmic dust in a way that can create supermassive stars — the sort that eventually explode as supernovae.

But the supernovae in NGC 1316 have the spectral fingerprints of Type 1A supernovae, which begin as small, dying white dwarf stars rather than supermassive ones.

There's no reason to believe that a galactic merger would create a larger number of white dwarfs than normal. So the mystery endures.

Another possible explanation is that there are an inordinate number of white dwarfs in NGC 1316, which are the corpses of an extraordinary "baby boom" of stars in NGC 1316. If that's the case, a surge of white dwarf-triggered supernovae would be expected when all the "boomer generation" stars reach a similar age.

On the other hand, it could just be chance. Of the thousands of observable galaxies, odds are in favor of an occasional random clustering of supernovae in one galaxy, said John Nousek, a professor of astronomy and astrophysics at Pennsylvania State University.

"This sort of thing is more of a cosmic coincidence than a cosmic Rosetta stone," said Nousek. He and graduate student Peter Brown have been hard at work monitoring and trying to make sense of the double explosions.

"The exact mechanism is not well understood," said Gehrels. "This is a fairly hot topic."