A galaxy tormented by the Milky Way makes the hunt for dark matter difficult

The extreme gamma rays emanating from the center of our galaxy finally have an explanation – and it appears to have nothing to do with dark matter.

When we think of the Milky Way, we often picture it as a relatively flat disc studded with stars. But beyond the range of visible light, astronomers have noticed the existence of a number of mysterious parallel structures. For example, we can quote the Fermi bubbles.

These are two structures accidentally discovered by researchers on the trail of dark matter. These are huge objects growing at the center of the galaxy on either side of the disk; Researchers estimate that they are about 25,000 light-years across, or several hundred billion times the size of our Earth.

Despite this size, they remained invisible for a long time. Researchers were only able to detect them for the first time in 2010, when they were betrayed by the immense amount of gamma rays they emit. Essentially, this radiation is a stream of particles that somewhat resemble the photons of light, with one big difference; Each of them carries an amount of energy immensely greater than that of a standard photon.

That means for instruments that track these gamma rays, like the Fermi Gamma-ray Space Telescope, these bubbles glow brightly — like two gigantic cosmic lightbulbs hanging on either side of the Milky Way.

Specialists have therefore tried to find the origin of these two curiosities. Because of its location, one particularly promising candidate quickly stood out from the crowd: Sagittarius A*, the supermassive black hole at the center of our galaxy.

The cosmic monsters of this category play a crucial role in the architecture of most galaxies; They are the source of a Dantesque gravitational force that structures these large star clusters. And active, unlike dormant black holes, usually eject huge geysers of plasma called jets.

While the shape is different, it’s difficult not to see a connection between these jets and the famous Fermi bubbles. Researchers therefore set out in search of evidence that would allow them to link them to Sagittarius A*… but unfortunately they came up empty-handed. The black hole remains the prime suspect, but there’s still no definitive proof that it’s actually the one creating these balls of plasma.

The death rattle of a galaxy torn apart

What is interesting, on the other hand, is that these observations have made it possible to highlight the presence of another mysterious structure at the center of one of the two bubbles: a dot called “Fermi cocoon” what a produces extremely intense gamma radiation.

Some teams of researchers have therefore decided to study it, hoping to discover its origin. A group of astrophysicists now think they have the answer… and unexpectedly, the answer has nothing to do with Sagittarius A* or even the Fermi bubbles surrounding it.

The suspect identified by the researchers lives on the edge of the Milky Way; it is Sagittarius, a once mighty galaxy orbiting our own. It gets shredded by its big sister with each passage, leaving pretty streaks of stars in its wake; today it is reduced to the stage of dwarf galaxy because of this progressive fragmentation.

In the Milky Way, the main sources of gamma rays are collisions between cosmic rays and gases trapped between stars by gravity. The problem is that this process does absolutely nothing to explain the gamma rays produced by the Sagittarius dwarf galaxy, as it has lost most of its gas due to our galaxy’s gravitational embrace.

In this case, how can the gamma radiation emitted by it be explained? The researchers therefore first suggested that it could be a long-awaited signature of dark matter, that invisible substance suspected of structuring the entire universe. But they didn’t find any convincing elements on this side, so they focused on the second and final lead: it’s ultrafast pulsars that would be the source of the radiation.

a pulse. © NASA’s Goddard Space Flight Center

Pulsars are 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 equal to that of the Sun in a diameter of only a few tens of kilometers.

This density gives them very special properties; they rotate around themselves at very high speeds and thus behave like giant open-air particle accelerators. This dynamic tends to create large jets of particles that periodically reach us with each rotation in the form of pulsations – hence the term pulsar. These jets, in turn, emit powerful gamma rays, similar to those from black holes.

According to the researchers, Sagittarius’ dance around the Milky Way in particular would contribute to causing heaps of neutron stars to rotate. The latter would then become pulsars, which themselves emit large amounts of gamma rays – hence the presence of this famous “cocoon”.

The search for dark matter could get complicated

For the 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 into 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 dark matter signature. explains Oscar Macias, co-lead author of this work.

Researchers are no longer allowed to exclaim ” Black Matter! as soon as they see such a signal. Now that they’ve proven that this gamma-ray radiation may well have come from pulsars, even more care is needed; Astronomers must therefore find new methods of analysis and other cosmic objects to be studied. Suffice it to say that we still have to wait until we arrive at direct evidence that irrefutably demonstrates the existence of dark matter.

The text of the study can be found here.

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