When the Fermi Gamma Ray Space Telescope entered low Earth orbit in 2008, it opened our eyes to a whole new universe of high-energy radiation.
One of the more interesting discoveries was the Fermi Bubbles: giant, symmetrical bubbles stretching above and below the plane of the galaxy, 25,000 light-years on either side of the center of the Milky Way, glowing in the gamma-ray range, the highest-energy wavelength on Earth. electromagnetic spectrum.
Then in 2020, the X-ray telescope eROSITA discovered another surprise: even larger bubbles stretching over 45,000 light years on either side of the galaxy’s plane, this time emitting less energetic X-rays.
Scientists have since concluded that both sets of bubbles are likely the result of some sort of burst or bursts from the galactic center and its supermassive black hole. However, the mechanism by which the gamma and X-rays were produced was a bit more difficult to pin down.
Now, using simulations, physicist Yutaka Fujita of Tokyo Metropolitan University in Japan has come up with one explanation that explains both sets of bubbles in one fell swoop.
He discovered that the X-ray emission is the product of a strong, fast-moving wind that hits the rarefied gas filling interstellar space, creating a shock wave that bounces back through the plasma, causing it to glow energetically.
The supermassive black hole that powers the heart of the Milky Way, Sagittarius A*, is pretty quiet when it comes to black holes. Its feeding activity is minimal; it is classified as “peaceful”. However, this was not always the case. And an active black hole can have different effects on the space around it.
As matter falls towards the black hole, it heats up and glows with light. Some of the material is drawn off along the magnetic field lines outside the black hole, which act as a synchrotron accelerating the particles to speeds close to the speed of light. They are fired as powerful jets of ionized plasma from the poles of the black hole, projecting into space up to millions of light years away.
In addition, there are cosmic winds: streams of charged particles that are carried along by material orbiting a black hole and then ejected into space.
While the Sagittarius A* may be quiet now, it wasn’t necessarily always that way. Look closely enough and in the space around the galactic plane you can find relics of past activity, such as Fermi bubbles. By studying these relics, we can understand when and how this activity took place.
Fujita’s Fermi bubble expedition is based on data from the now-defunct Suzaku X-ray satellite, operated jointly by NASA and the Japan Space Agency (JAXA). He made Suzaku observations of X-ray structures associated with bubbles and performed numerical simulations to try to recreate them based on black hole power processes.
“We show that a combination of X-ray gas density, temperature and impact age profiles can be used to discriminate energy injection mechanisms,” he writes in his paper.
“Comparing the results of the numerical simulations with the observations, we indicate that the bubbles were created by a fast wind from the center of the galaxy, as it generates a strong back shock and reproduces the temperature peak observed there.”
He found that the most likely scenario is a black hole wind blowing at 1,000 kilometers per second (621 miles) from a past feeding event that lasted 10 million years and ended quite recently. As the wind propagates outward, the charged particles collide with the interstellar medium, producing a shock wave that bounces back into the bubble. These inverted shock waves heat the material inside the bubbles, causing it to glow.
Numerical simulations developed by Fujita accurately reproduced the temperature profile of the X-ray structure.
He also studied the possibility of a single explosive eruption from the center of the galaxy and was unable to reproduce the Fermi bubbles. This suggests that the slow, steady wind from the center of the galaxy was most likely the precursor to the mysterious structures. And the force of the wind can only be attributed to Sagittarius A*, not star formation, another phenomenon that produces cosmic winds.
“Thus,” he writes in his paper, “the wind may be the same as the outflows of active galactic nuclei often observed in other galaxies and thought to regulate the growth of galaxies and their central black holes.”
The article was published in Monthly Notices of the Royal Astronomical Society.