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 stranger discoveries were the Fermi bubbles: giant, symmetrical blobs that stretch above and below the galactic plane, 25,000 light-years on each side of the center of the Milky Way, and glow in gamma-ray light — the highest-energy wavelength ranges in the electromagnetic Spectrum.
Then, in 2020, an X-ray telescope called eROSITA found another surprise: even larger bubbles stretching 45,000 light-years on each side of the galactic plane, this time emitting lower-energy X-rays.
Scientists have since concluded that both sets of bubbles are likely the result of some sort of eruption or eruptions from the galactic center and the supermassive black hole within. However, the mechanism that produces the gamma and X-rays has been a little more difficult to pin down.
Now, physicist Yutaka Fujita from Tokyo Metropolitan University in Japan has used simulations to find a single explanation that explains both groups of bubbles at once.
He has found that X-ray emission is the product of a strong, fast-moving wind slamming into the faint gas that fills interstellar space, creating a shockwave that radiates back through the plasma, giving it this energetic glow.
The supermassive black hole powering the heart of the Milky Way – Sagittarius A* – is pretty quiet about black holes. Its feeding activity is minimal; it is classified as “quiet”. However, it wasn’t always like this. And an active black hole can have all sorts of effects on the space around it.
As material falls toward the black hole, it heats up and blazes with light. Some of the material is carried away along magnetic field lines outside the black hole, which act as a synchrotron to accelerate particles to near the speed of light. These are fired as powerful jets of ionized plasma from the black hole’s poles, shooting up millions of light years into space.
In addition, there are cosmic winds: streams of charged particles whipped up by the matter orbiting the black hole and then flung into space.
While Sagittarius A* may be calm now, that wasn’t necessarily always the case. Look closely enough and relics of past activity, like the Fermi bubbles, can be found in space around the galactic plane. By studying these relics, we can understand when and how this activity took place.
Fujita’s push into the Fermi bubbles is based on data from the now-defunct Suzaku X-ray satellite, operated jointly by NASA and the Japan Space Agency (JAXA). He took Suzaku observations of the X-ray structures associated with the bubbles and ran numerical simulations to try to reproduce them based on black hole feeding processes.
“We show that a combination of X-ray gas density, temperature and impact age profiles can be used to distinguish the energy injection mechanisms,” he writes in his paper.
“By comparing the results of numerical simulations with observations, we suggest that the bubbles were created by a fast wind from the galactic center, as it creates strong recoil and reproduces the temperature spike observed there.”
The most likely scenario, he found, is a black hole wind blowing at 1,000 kilometers per second (621 miles) from a past feeding event measured over a 10-million-year period that ended relatively recently . As the wind spreads outward, the charged particles collide with the interstellar medium, creating a shock wave that bounces back into the bubble. These reverse shock waves heat the material in 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 failed to reproduce the Fermi bubbles. This suggests that a slow, steady wind blowing from the galactic center was the most likely precursor to the mysterious structures. And the power of the wind can only be attributed to Sagittarius A*, not star formation – another phenomenon that creates cosmic winds.
“Hence,” he writes in his paper, “the winds could be the same as outflows from active galactic cores, which are often observed in other galaxies and are thought to regulate the growth of galaxies and their central black holes.”
The paper was published in Monthly Bulletins of the Royal Astronomical Society.