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 curious discoveries was the Fermi Bubbles: giant, symmetrical blobs stretching above and below the galactic plane, 25,000 light-years on each side from the center of the Milky Way, glowing in gamma-ray light – the highest energy wavelength regions of the electromagnetic spectrum.
Then, in 2020, an X-ray telescope named eROSITA found another surprise: even larger bubbles stretching 45,000 light-years on either side of the galactic plane, this time emitting less energetic X-rays.
Scientists have since concluded that both sets of bubbles are likely the result of some kind of outburst or eruption from the galactic center and the supermassive black hole there. However, the mechanism that produces the gamma and X-rays has been a little more difficult to determine.
Now, using simulations, physicist Yutaka Fujita of Tokyo Metropolitan University in Japan has come up with a simple explanation that explains both sets of bubbles in one fell swoop.
The X-ray emission, he has found, is the product of a powerful, fast-moving wind slamming into the tenuous gas-filled interstellar space, producing a shock wave that travels back through the plasma, causing the energetic glow.
The supermassive black hole that powers the heart of the Milky Way – Sagittarius A* – is pretty quiet as far as black holes go. Feeding activity is minimal; it is classified as “tranquil”. However, it has not always been this way. And an active black hole can have all kinds of effects on the space around it.
As the material falls towards the black hole, it heats up and blazes with light. Some of the material is channeled away along magnetic field lines outside the black hole, which acts as a synchrotron to accelerate particles to near-light speed. These are shot up as powerful jets of ionized plasma from the black hole’s poles, and strike out into space for up to millions of light years.
In addition, there are cosmic winds: streams of charged particles whipped up by the material orbiting the black hole that then explode into space.
While Sagittarius A* may be quiet now, that hasn’t necessarily always been the case. Look hard enough, and relics of past activity, such as the Fermi bubbles, can be found lurking in the space surrounding the galactic plane. By studying these relics, we can understand when and how that activity took place.
Fujita’s foray into the Fermi bubbles is based on data from the now-retired Suzaku X-ray satellite, operated jointly by NASA and the Japan Aerospace Exploration Agency (JAXA). He took Suzaku observations of the X-ray structures associated with the bubbles and performed numerical simulations to try to reproduce them based on black hole feeding processes.
“We show that a combination of density, temperature and shock age profiles of the X-ray gas can be used to distinguish the energy injection mechanisms,” he writes in his paper.
“By comparing the results of numerical simulations with observations, we indicate that the bubbles were created by a fast wind from the galactic center because it generates a strong reverse shock and reproduces the observed temperature peak there.”
The most likely scenario, he found, is a black hole wind blowing at a speed of 1,000 kilometers per second (621 miles) from a previous feeding event that was measured over 10 million years and ended very 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 reverse shock waves heat the material inside the bubbles, causing them to glow.
The numerical simulations developed by Fujita accurately reproduced the temperature profile of the X-ray structure.
He also investigated the possibility of a single explosive eruption from the galactic center and was unable to reproduce the Fermi bubbles. This suggests that a slow, steady wind from the galactic center was the most likely progenitor of 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.
“Therefore,” he writes in his paper, “the wind may be the same as active galactic nuclei that are often observed in other galaxies and are believed to regulate the growth of galaxies and their central black holes.”
The newspaper is published in Monthly Notices of the Royal Astronomical Society.