A galaxy tortured by the Milky Way complicates the hunt for dark matter

The extreme gamma radiation emanating from the center of our galaxy finally has an explanation and apparently has nothing to do with dark matter.

When we imagine the Milky Way, we often visualize a relatively flat disk studded with stars. But out of the realm of visible light, astronomers have noticed the existence of a series of mysterious parallel structures. We can, for example, mention the Fermi bubbles.

These are two structures accidentally discovered by researchers on the trail of dark matter. These are huge objects that grow in the center of the galaxy, on both sides of the disk; researchers estimate they are about 25,000 light-years in diameter, or several hundred billion times the size of our Earth.

Despite these dimensions, they remained invisible for a long time. It wasn’t until 2010 that researchers were able to detect them for the first time, when they were betrayed by the immense amount of gamma radiation they emit. In essence, this radiation is a stream of particles that somewhat resemble photons of light, with one big difference; each of them carries an amount of energy immensely greater than that of a standard photon.

This means that for instruments that track this gamma radiation, such as the Fermi Gamma Ray space telescope, these bubbles glow brightly, like two giant cosmic light bulbs hanging on either side of the Milky Way.

The specialists then tried to find the origin of these two curiosities. Due to their location, one particularly promising candidate immediately stood out from the group: Sagittarius A *, the supermassive black hole that resides at the center of our galaxy.

Cosmic monsters of this category play a decisive role in the architecture of most galaxies; they are at the origin of a Dantesque gravitational force that structures these large clusters of stars. And active ones, as opposed to dormant black holes, typically emit huge geysers of plasma called jets.

Even though the shape is different, it’s hard not to see a link between these jets and the famous Fermi Bubbles. The researchers then decided to find evidence that would allow them to link them to Sagittarius A * … but unfortunately they returned empty-handed. The black hole remains the prime suspect, but there is still no definitive evidence that it is actually the one that generates these plasma spheres.

The death rattle of a torn galaxy

What is interesting, however, is that these observations made it possible to highlight the presence of another mysterious structure in the middle of one of the two bubbles: a point called “Fermi cocoon “ which produces a extremely intense gamma radiation.

Some teams of researchers have therefore decided to investigate it in the hope of discovering its origin. A group of astrophysicists now believe they have found the answer … and unexpectedly, the answer has nothing to do with Sagittarius A *, nor with the Fermi bubbles surrounding it.

The suspect identified by the researchers lies on the periphery of the Milky Way; it is Sagittarius, a once massive galaxy orbiting our own. He is torn apart by his older sister in each of her passages, leaving behind graceful streaks of stars; today it is reduced to the stage of dwarf galaxy due to this progressive dismemberment.

In the Milky Way, the main sources of gamma radiation are collisions between cosmic radiation and the gases trapped between the stars by gravity. The problem is that this process absolutely does not explain the gamma radiation produced by the dwarf galaxy of Sagittarius, as it has lost much of its gas due to the gravitational embrace of our galaxy.

How to explain the gamma radiation emanating from it, in this case? The researchers then began by suggesting that it could be a long-awaited signature of dark matter, this invisible substance suspected of structuring the entire universe. But they did not find convincing elements on this front, and therefore they focused on the second and last trace: they are the ultrafast pulsars that would be at the origin of the radiation.

An impulse. © NASA’s Goddard Space Flight Center

Pulsars set in motion by the dance of the galaxies

These objects are indeed special examples of neutron stars. They are extremely heavy stars despite their ridiculous size on an astronomical scale; traditionally they concentrate a mass equivalent to that of the Sun in a diameter of a few tens of kilometers.

This density gives them very special properties; they spin on themselves at very high speed, and therefore behave like huge particle accelerators in the open sky. This dynamic tends to produce large jets of particles that reach us periodically with each rotation, in the form of pulsations, hence the term pulsar. These jets in turn emit powerful gamma radiation, just like that of black holes.

According to the researchers, the Sagittarius dance around the Milky Way would help to spin heaps of neutron stars. The latter would then become pulsars which would in turn emit a large amount of gamma radiation – hence the presence of this famous “cocoon”.

The search for dark matter could get complicated

For researchers, this is both a success and a disappointment. On the one hand, they have finally provided a coherent and plausible explanation for the existence of this structure. But if their interpretation turns out to be correct, it also means that a line of research on dark matter will disappear.

This is important because researchers have long believed that observing gamma rays from a satellite dwarf galaxy would be an indisputable signature of dark matter. “, Explains Oscar Macias, co-lead author of this work.

From now on, researchers will no longer be able to exclaim ” Black matter! as soon as they see such a signal. Now that they have shown that this gamma radiation may very well come from pulsars, it will therefore be necessary to be even more meticulous; astronomers will then have to find new methods of analysis and other cosmic objects to study. Suffice it to say that we will still have to wait before arriving at direct evidence that would prove the existence of dark matter in an irrefutable way.

The text of the study is available here.

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