How galactic bubbles produce energetic particles
ANN ARBOR—We’re all familiar with everyday soapy bubbles on earth, but in space the term “bubble” takes on a much more colossal meaning. In space, a galactic bubble is composed of a lighter gas that is trapped inside a heavier one, and these bubbles can be huge.
A team of astronomers led by Jiangtao Li, assistant research scientist in the Department of Astronomy at the University of Michigan, has described the results of their observations and research on galactic bubbles in a study published in the Astrophysical Journal.
Galaxy NGC 3079, which is located approximately 67 million light years from earth, contains two galactic bubbles—one that is 4,900 light years across, and another that is 3,600 light years. These bubbles give off light in the form of X-ray, optical and radio emission, which make them detectable by NASA telescopes, such as those at the NASA Chandra X-ray Observatory.
New observations from Chandra by Li and his team of researchers show that a cosmic particle accelerator is producing ultra-energetic particles on the edges of NGS 3079’s cosmic bubbles, therefore providing evidence that these bubbles—and structures like them—may be the source of “cosmic rays.”
As the outer regions of galactic bubbles expand and collide with surrounding gas, researchers believe that charged particles scatter and bounce off magnetic fields in the shock waves. When the particles cross the shock front, they are super accelerated to energies about 100 times stronger than those generated by the world’s most powerful human-made particle accelerator. Some of these energy particles may escape and even strike Earth’s atmosphere in the form of cosmic rays.
The amount of radio waves generated by one of the galactic bubbles suggests that the source of the X-ray emission is electrons swirling around the magnetic fields of the shock waves on the rim of the bubble. The electrons are radiated by a process called synchrotron radiation, and these observations by Li and his team present the first direct evidence of synchrotron radiation in high energy X-rays from a massive galactic bubble.
The radio emission has been detected on both sides of the bubble, but the hard X-ray emission is detected only on the southwest side. It is strange that the northeast side, which is brighter in every other wavelength (radio, h-alpha, soft X-ray), is not detected in hard X-ray, Li said. It is not yet understood why synchrotron emission has not been detected from the other bubble.