Image courtesy ESA/NASA and A. Evans, UVA/NRAO/SBU

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The galaxy Arp 220, where the seven supernovae were found.

Image courtesy ESA/NASA and A. Evans, UVA/NRAO/SBU

Seven Supernovae Found in Single Galaxy—A First

Finding marks Arp 220 as potential lab for studying galaxy evolution.

In a galaxy 250 million light-years from Earth, astronomers have spotted a record-breaking seven supernovae all found at the same time.

"As far as we know, only three supernovae in a single galaxy were found at once so far, which is already an impressive number," said study leader Fabien Batejat, a Ph.D. student at Chalmers University of Technology in Onsala, Sweden.

"But we can confirm seven supernovae [in a single galaxy], thanks to a 17-year monitoring of the radio sources in Arp 220."

The unprecedented find may offer a unique cosmic laboratory for studying galaxy evolution.

The prodigious galaxy, known as Arp 220, is thought to have formed from the merger of two smaller galaxies and is well known to host a very intense burst of star formation, easily seen in visible wavelengths.

But the new data confirms that Arp 220 is also a very efficient factory for explosive star deaths, giving scientists a glimpse of how the earliest galaxies in the universe may have behaved.

Telescope Como Revealed Supernovae

Each of the supernovae found in Arp 220 spans less than a light-year, and at such a great distance, each radio signal covers an angle in the sky less than 0.5 milliarcseconds across, Batejat said.

"To give you an idea of how small this is, this size corresponds to what you would see if you would look into a straw of about 1,500 kilometers [932 miles] long," Batejat said.

"In order to see such small objects, we would need a telescope of 10,000 kilometers [6,214 miles] across, which is a bit less than the diameter of the Earth itself. But since we can't build such gigantic telescopes, we use interferometry to simulate them."

In astronomy, interferometry uses the combined power of an array of telescopes—rather than a single, huge telescope—to create high-resolution images that can probe deep into the universe. (Related picture: "New ALMA Telescope Peers Into Galaxy Smashup.")

Batejat's team used 57 of the largest radio telescopes on Earth, which are spread across two continents and five countries. The project included data from the European VLBI Network, the Very Long Baseline Array, the Green Bank Telescope, and the Arecibo Observatory.

The heart of Arp 220 is highly obscured by dust that can't be penetrated by visible wavelengths. But radio waves can travel through such a dense environment to reach telescopes on Earth.

Supernova Discovery Is "Something Amazing"

Ultimately the data revealed around 40 radio sources near the center of Arp 220. By watching how these sources changed over time in two different radio wavelengths, astronomers could tell that seven of the objects were stars that had exploded around the same time.

Astronomers estimate that our Milky Way galaxy sees only a single supernova every hundred years, on average, Batejat said. (Related: "Youngest Supernova in Milky Way Found.")

But the highly active Arp 220, with its dynamic cycles of star birth and death, behaves more like how young galaxies probably did more than ten billion years ago.

"We hope this might lead to interesting discoveries on how stars formed [and died] in the early universe," Batejat said.

What's more, such relatively fresh supernovae "are rare, and they have short lives of a few decades maximum" before they settle into supernova remnants, he said. "So discovering seven such supernovae at once is something amazing."

The supernova study appears on the research website and has been accepted for publication in the Astrophysical Journal.