NGC 3079, an edge-on spiral galaxy located about 67 million light-years from Earth, contains two ‘superbubbles.’ New observations from NASA’s Chandra X-ray Observatory show that a cosmic particle accelerator in this galaxy is producing ultra-energetic particles in the rims of the superbubbles. These particles can be much more energetic than those created by the Large Hadron Collider at CERN.
They stretch out on opposite sides of the center of NGC 3079: one is 4,900 light-years across and the other is only slightly smaller, with a diameter of about 3,600 light-years.
These structures provide evidence that they and structures like them may be the source of high-energy particles called cosmic rays that regularly bombard the Earth.
“Shock waves associated with exploding stars can accelerate particles up to energies about 100 times larger than those generated in the Large Hadron Collider,” said lead author Dr. Jiang-Tao Li from the University of Michigan and colleagues.
“The outer regions of the superbubbles in NGC 3079 generate shock waves as they expand and collide with surrounding gas.”
The astronomers think charged particles scatter or bounce off tangled magnetic fields in these shock waves, much like balls rebounding off bumpers in a pinball machine.
“When the particles cross the shock front they are accelerated, as if they received a kick from a pinball machine’s flipper,” they explained.
“These energetic particles can escape and some may eventually strike the Earth’s atmosphere in the form of cosmic rays.”
The amount of radio waves or X-rays at different wavelengths, or ‘spectra,’ of one of NGC 3079’s bubbles suggest that the source of the emission is electrons spiraling around magnetic field lines, and radiating by a process called synchrotron radiation.
This is the first direct evidence of synchrotron radiation in high energy X-rays from a galaxy-sized superbubble, and it tells scientists about the maximum energies that the electrons have attained.
“The radio and X-ray spectra, along with the location of the X-ray emission along the rims of the bubbles, imply that the particles responsible for the X-ray emission must have been accelerated in the shock waves there, because they would have lost too much energy while being transported from the center of the galaxy,” Dr. Li and co-authors said.
Jiang-Tao Li et al. 2019. Detection of non-thermal hard X-ray emission from the ‘Fermi bubble’ in an external galaxy. ApJ, in press; arXiv: 1901.10536