One of the most curious discoveries was the Fermi telescope. Bubbles: Giant symmetrical clumps extending above and below the galactic plane, 25,000 light-years on either side of the center of the Milky Way, glowing in gamma radiation, the highest energy wavelength range in the electromagnetic spectrum.
Then, in 2020, the eROSITA X-ray telescope discovered another surprise: even larger bubbles extending over 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 sort of ejection or ejections from the galactic center and the supermassive black hole in it. However, the mechanism causing the gamma and X-ray emission was a little more difficult to determine.
Now, using simulations, physicist Yutaka Fujita of Tokyo Metropolitan University in Japan has come up with a single explanation. this explains both sets of bubbles in one fell swoop.
He discovered that X-rays are the product of a powerful, fast-moving wind that crashes into the rarefied gas that fills interstellar space, producing a shock wave that reflects through the plasma, causing this energetic glow in it.
The supermassive black hole that powers the heart of the Milky Way, Sagittarius A*, is pretty quiet when compared to black holes.
Its nutritional activity is minimal; it is classified as “calm”. However, this was not always the case. And an active black hole can have all sorts of influences on the space around it.
When material falls into a black hole, it heats up and emits light. Some of the material is sent along magnetic field lines outside the black hole, which act like a synchrotron to accelerate the particles to near-light speed.
They are launched in the form of powerful jets of ionized plasma from the poles of the black hole, ejected into space for millions of light years.
Then there are the cosmic winds: streams of charged particles that are whipped up by matter spinning around a black hole, which then blasts out into space.
While Sagittarius A* may be calm now, it wasn’t always like this. If you look closely enough, traces of past activity, such as Fermi bubbles, can be found in the space around the galactic plane. By studying these relics, we can understand when and how this activity took place.
Fujita’s foray into Fermi bubbles is based on data from the now retired Suzaku X-ray satellite, which is jointly operated by NASA and the Japanese. Space Agency (JAXA).
He made Suzaku’s observations of X-ray structures associated with 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 impact age profiles of X-ray gas can be used to distinguish between 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 the fast wind from the galactic center, since it generates a strong blowback and reproduces the temperature peak 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 that was measured over 10 million years and ended relatively recently.
As the wind propagates outward, the charged particles collide with the interstellar medium, creating a shock wave that bounces back into the bubble. These backward shock waves heat the material inside the bubbles, causing it 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 Fermi bubbles.
This suggests that a slow, steady wind from the galactic center was the most likely progenitor of the mysterious structures.
And the strength of the wind can only be attributed to Sagittarius A *, and not to star formation – another phenomenon that generates cosmic winds.
“Thus,” he writes in his paper, “the wind may be the same since active ejections of galactic nuclei are often observed in other galaxies and are thought to regulate the growth of galaxies and their central black holes.”
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