A survey by the Chandra X-ray Observatory has revealed in detail, for the first time, the effects of a shock wave blasted through a galaxy by powerful jets of plasma emanating from a supermassive black hole at the galactic core. The observations of Centaurus A, the nearest galaxy that contains these jets, have enabled astronomers to revise their picture of how jets affect the galaxies in which they live. The results were presented April 22 at the European Week of Astronomy and Space Science in the United Kingdom by Judith Croston of the University of Hertfordshire, the institution hosting the event.
A team led by Croston and Ralph Kraft, of the Harvard-Smithsonian Center for Astrophysics in the United States, used deep X-ray observations from Chandra to get a new view of the jets in Centaurus A. The jets inflate large bubbles filled with energetic particles, which drives a shock wave through the stars and gas of the surrounding galaxy. By analyzing in detail the X-ray emission produced where the supersonically expanding bubble collides with the surrounding galaxy, the team was able to show for the first time that particles are being accelerated to very high energies at the shock front, which causes them to produce intense X-ray and gamma-ray radiation. Another team of researchers using the High Energy Stereoscopic System (HESS) telescope in Namibia recently detected high-energy gamma-ray radiation from Centaurus A for the first time.
“Although we expect that galaxies with these shock waves are common in the universe, Centaurus A is the only one close enough to study in such detail,” Croston said. “By understanding the impact that the jet has on the galaxy, its gas and stars, we can hope to understand how important the shock waves are for the life cycles of other, more distant galaxies.”
The powerful jets are found in only a small fraction of galaxies but are most common in the largest galaxies, which are thought to have the biggest black holes. Astronomers believe the jets form near a central supermassive black hole and travel close to the speed of light for distances of up to hundreds of thousands of light-years. Recent progress in understanding how galaxies evolve suggests that these jet-driven bubbles, called radio lobes, may play an important part in the life cycle of the largest galaxies in the universe.
Energetic particles from radio galaxies may also reach us directly as cosmic rays hitting Earth’s atmosphere. Centaurus A is thought to produce many of the highest-energy cosmic rays that arrive at Earth. The team believes that its results are important for understanding how such high-energy particles are produced in galaxies as well as for understanding how massive galaxies evolve.
The results of this research will be published in a forthcoming issue of the Monthly Notices of the Royal Astronomical Society.