The galaxy M82 is to the left. The VLA image (top left) shows the supernova taken in May 2008. The high-resolution image (lower right) shows an expanding shell at the scale of a few light days.
Credit: Milde Science Communication/NASA/ESA/STScI/AURA/A. Brunthaler/MPIfR
SYDNEY: European radio astronomers have discovered the hidden explosion of a massive star, in the nearby galaxy M82.
Despite being the closest supernova discovered in the last five years, the explosion was only detected at radio wavelengths, as dense gas and dust surrounding it have rendered it invisible at other wavelengths.
Without this, the explosion would have been visible even with amateur telescopes, say the authors of a study in this week's Astronomy & Astrophysics Letters.
Exploding galaxy
M82, or Messier 82, is an irregular galaxy in a nearby group found 12 million light-years from Earth. Despite being smaller than the Milky Way, M82 harbours a vigorous central region of starburst in its innermost few hundred light-years – here more stars are born than in our entire galaxy.
M82 is often called an 'exploding galaxy', because, in optical and infrared images, it looks as if being torn apart as a result of numerous supernovas from massive stars.
Many remnants from previous supernovas are seen on radio images of M82 and a new one has been long overdue. For a quarter of a century astronomers have tried to catch this cosmic catastrophe in the act and were starting to wonder why the galaxy has been so quiet in recent years.
The new explosion was discovered in April when Andreas Brunthaler of the Max Planck Institute For Radio Astronomy in Bonn, Germany, examined data from the Very Large Array (VLA) observatory in New Mexico, USA.
A mystery solved
"I then looked back into older data we had from March and May last year, and there it was as well, outshining the entire galaxy!" he said. Observations taken before 2008 showed neither pronounced radio nor X-ray emissions at the position of this supernova.
Observations of M82 taken last year with optical telescopes showed no signs of this explosion, however. And the supernova is hidden on newer ultraviolet and X-ray images.
This is because the supernova exploded close to the centre of the galaxy in a very dense interstellar environment, say the astronomers. This could also explain the mystery about the long silence of M82. Many of these events may actually be something like "underground explosions", where the light is obscured by huge clouds of gas and dust and only radio waves can penetrate, said Brunthaler.
"This cosmic catastrophe shows that using our radio telescopes we have a front-row seat to observe the otherwise hidden universe," added co-author Heino Falcke of Radboud University, Nijmegen in the Netherlands.
Radio emissions are only detected from 'core collapse' supernovas, where the core of a massive star collapses and produces a black hole or a neutron star.
4% the speed of light
It is produced when the shock wave of the explosion propagates into dense material surrounding the star; usually material that was shed from the massive progenitor star before it exploded.
By combining data from the 10 telescopes of the Very Long Baseline Array (VLBA), the VLA, the Green Bank Telescope in the U.S., and the Effelsberg 100-metre telescope in Germany, the team was able to produce images that show a ring-like structure expanding at more than 40 million km/h or 4% of the speed of light, which is typical for a supernova.
"By extrapolating this expansion back in time, we can estimate the explosion date [to be] late January or early February 2008," said Brunthaler.
With the Max Planck Institute For Radio Astronomy.
