Particles play pinball in galactic superbubbles

The X-ray emission of the two superbubbles is shown here in purple.

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By studying the superbubbles of Galaxy NGC 3079, an American researcher was able to determine that the latter emitted X-rays produced within them. Yelena Stein, a postdoctoral fellow at the Strasbourg Astronomical Observatory, analysed the intensity of the magnetic field, which makes the smallest of these gas bubbles a real particle accelerator. The discovery was published in The Astrophysical Journal.

Galaxy NGC 3079 contains two superbubbles, younger cousins of the "Fermi Bubbles", identified in our Milky Way for the first time in 2010. A large one to the north, 4,900 light-years* away, and a small one to the south, 3,600 light-years away. "This spiral galaxy seen from the side can be analysed in detail because it is not too far from us," said Yelena Stein, a researcher at the Strasbourg Observatory. Some 67 million light-years all the same...

Superbubbles of NGC 3079 are gas bubbles that emit light in the form of X-rays and radio emissions. The researchers believe that these bubbles are fed by the central supermassive black hole they surround, which is very active, unlike the one located in the Milky Way. While studying the X-rays emitted by these bubbles, and more particularly the smaller of the two, thanks to the NASA Chandra satellite, Jiangtao Li, a researcher at the University of Michigan, found indications that energy was also produced in the very heart of the bubble.

Cosmic rays entering the atmosphere

"To produce X-rays you need particles with a lot of energy. On the periphery of the bubble, there is a high magnetic field that acts as a pinball on the particles, and accelerates them to higher energies than those reached in the best particle accelerators on Earth," explains Yelena Stein, whom Jiangtao Li called on to analyse the radio data, and thus determine the intensity of the magnetic field.

These particles, by escaping with considerable energy, could be the source of certain "cosmic rays" that regularly hit the Earth. "When they enter the atmosphere, they produce sprays of particles detected on the ground by our devices," says the researcher. Some cosmic rays come from the Sun, but other much more powerful ones arrive from other galaxies. "On Earth, we detect one per year at high energy for every km2. We thought they came from supermassive black holes", continues Yelena Stein, who says they are not dangerous for humans. The discovery will give rise to further comments. The superbubbles have not yet delivered up all of their mysteries.

*A light-year represents about 9,000 billion kilometres.