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Victoria gives Opportunity a view into Mars' history

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M82's hidden supernova

BY KEITH COOPER

ASTRONOMY NOW

Posted: 27 May, 2009

A supernova has recently exploded in the nearby galaxy M82, but you won’t be able to see it with any ordinary telescope. Shrouded in obscuring gas and dust, only the radio emission of the stellar explosion is able to penetrate through to the outside and be detected by the radio telescopes.

M82, the 'Cigar Galaxy', is an extremely dusty galaxy that is hiding its supernovae. Image: NASA/ESA/Hubble Heritage Team (STScI/AURA).


The detection was made purely by accident by Dr Andreas Brunthaler, of the Max Planck Institute in Germany, as he was checking radio images of M82 taken by the Very Large Array in New Mexico, USA, just this April.


“I then looked back into older data that we had from March and May last year and there it was, outshining the entire galaxy!” says Brunthaler. Not a thing was seen in visible light or even X-rays. Ordinarily the supernova would have been obvious to any amateur astronomer with a 150mm telescope or larger.


Such a discovery required a closer look, particularly considering how rare supernovae seem to be in M82. Despite its prodigious rate of star formation and the numerous supernova remnants that litter the galaxy, there has been a dearth of supernovae in M82 with only one ever recorded (SN 2004am). Brunthaler teamed up with colleagues from the Max Planck Institute, the University of California, Berkeley, and the University of Nijmegen in the Netherlands to combine data from four separate radio observatories. Linking together the 27 telescopes of the VLA, the ten telescopes belonging to the Very Long Baseline Array (VLBA) across the United States, the Green Bank Telescope in West Virginia, USA, and the Effelsberg 100-metre telescope in Germany, the team were able to image an asymmetric ring of debris racing away from the site of the supernova at four percent the speed of light (about 40 million kilometres per hour). Tracking back the expansion of this ring suggests that the supernova would have been seen to explode in either late January or early February 2008.


The asymmetric nature of the ring could mean one of two possibilities: either the explosion itself was lop-sided, or that the gas and dust surrounding the deceased star was unevenly spread, and is slowing down the expansion of the ring in one direction. This material surrounding the star was probably ejected from the star during earlier outbursts leading up to the supernova. As the shockwaves from the explosion slam into this material, the collision generates the radio emission that we detect. Only a so-called core-collapse supernova, whereby a star runs out of nuclear fuel and collapses under its own gravity, has these earlier outbursts.

The ring of supernova debris over a year after it exploded, imaged at radio wavelengths with very long baseline interferometry. Image: A Brunthaler/MPIfR.

The supernova was located close to M82’s centre, in an area featuring high star-formation rates. In fact M82, which is a cigar-shaped galaxy 12 million light years from us, is known as a starburst galaxy. It is clouded with opaque gas and dust, with streamers emanating from the main body of the galaxy. It is suspected that its distorted shape and extreme starburst activity are the result of gravitational tides unleashed during a previous close encounter with another galaxy, the spiral M81, which is much larger and appears to have remained unscathed.

Starbursts can create a lot of hot, massive stars that live relatively short lives of just a few million years, leading to a higher rate of supernovae than in other galaxies.
The discovery highlights the possibility that many more supernovae may have exploded in M82 in the past but remained unseen. Astronomers will be keeping a closer eye on M82 with their radio telescopes from now on.
The research is published in a paper in Astronomy and Astrophysics Letters.

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