"With all the data in place, we can now be certain that all seven of these sources are supernovae: stars that exploded in the last 60 years," says Fabien Batejat, main author of the article about the discovery.

So many supernovae have never before been detected in the same galaxy. The number is nevertheless consistent with how fast stars are forming in Arp 220.

"In Arp 220, we see far more supernovae than in our galaxy. We estimate that a star explodes in Arp 220 once every quarter. In the Milky Way, there is only one supernova per century," says Rodrigo Parra, astronomer at the European Southern Observatory in Chile and member of the team.

John Conway is professor of observational radio astronomy at Chalmers and deputy director of Onsala Space Observatory.

"Arp 220 is well-known as a place where star formation is very efficient. Now we have been able to show that star factories like this are also supernova factories," he says.

The radio measurements have also given researchers insight into how radio waves are generated in supernovae and their remnants.

"Our measurements show that a supernova's own magnetic field is what gives rise to its radio emission, not the magnetic fields in the galaxy around it," says Fabien Batejat.

The results will be published in the October 20 issue of the journal Astrophysical Journal.

The team is composed of Fabien Batejat, John Conway and Rossa Hurley from Onsala Space Observatory at Chalmers, Rodrigo Parra (European Southern Observatory, ESO, Santiago, Chile), Philip Diamond (CSIRO, Sydney, Australia), Colin J. Lonsdale (MIT Haystack Observatory, USA) and Carol J. Lonsdale (North American Alma Science Center, NRAO, Charlottesville, USA).

The observations were carried out using telescopes which belong to the European VLBI Network (EVN) together with the Very Long Baseline Array (VLBA). The VLBA is a set of ten radio telescopes located from Hawaii to St. Croix in the U.S. Virgin Islands and operated by the National Radio Astronomy Observatory.