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 it has nothing to do with dark matter.

When we imagine the Milky Way, we often visualize a relatively flat disk studded with stars. But stepping outside of visible light, astronomers have noticed the existence of a number of mysterious parallel structures. For example, we can quote 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 either side of the disk; scientists 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. It was only in 2010 that scientists were able to detect them for the first time, when they were betrayed by the enormous amount of gamma radiation they emit. Essentially, this radiation is a stream of particles somewhat similar to photons of light, with one major difference; each of them carries an amount of energy vastly greater than that of a standard photon.

That means that for instruments that track this gamma radiation, like the Fermi Gamma Ray Space Telescope, these bubbles shine brightly — like two giant cosmic bulbs hanging from either side of the Milky Way.

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

The cosmic monsters in this category play a crucial role in the architecture of most galaxies; they are the origin of a Dantesque gravity which structures these great clusters of stars. And active, unlike dormant black holes, typically emit huge geysers of plasma called jets.

Although the shape is different, it is hard not to see a connection between these jets and the famous Fermi Bubbles. The researchers therefore set out to find a piece of evidence that would allow them to link them to Sagittarius A*… but unfortunately they came back empty-handed. The black hole remains the prime suspect, but there is still no definitive proof that it is actually the one generating these plasma spheres.

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 in the middle of one of the two bubbles: a point called “Fermi Cocoon” which produces a extremely intense gamma radiation.

A couple of research teams have therefore decided to investigate it in the hope of discovering its origin. A group of astrophysicists now think they’ve found the answer… and unexpectedly, the answer has nothing to do with Sagittarius A*, or even the Fermi bubbles that surround it.

The suspect identified by the researchers lives on the outskirts of the Milky Way; it’s Sagittarius, a once-massive galaxy orbiting our own. It is torn apart by its elder sister at each of its passages, leaving beautiful streaks of stars; today it is reduced to the stage of dwarf galaxy because of this progressive division.

In the Milky Way, the main sources of gamma radiation 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 radiation produced by the Sagittarius Dwarf Galaxy, since it has lost most of its gas due to our galaxy’s gravitational embrace.

How to explain the gamma radiation coming from it in this case? The researchers therefore started by proposing that it could be a long-awaited signature of dark matter, the invisible substance suspected of structuring the entire universe. But they found no convincing elements on this side, and therefore concentrated on the second and final clue: it is ultrafast pulsars that would be the starting point of the radiation.

A pulse. © NASA’s Goddard Space Flight Center

Pulsars set in motion by the dance of galaxies

These objects are actually special examples of neutron stars. They are extremely heavy stars despite their ridiculous size in astronomical terms; traditionally, they concentrate a mass equal to that of the Sun in a diameter of just a few tens of kilometers.

This density gives them very special properties; they ignite themselves at very high speed, thus behaving like huge open-air particle accelerators. These dynamics tend to produce large jets of particles that come at us at regular intervals with each rotation, in the form of pulsations – hence the term pulsar. These jets, in turn, emit powerful gamma rays, just like those from black holes.

According to the researchers, Sagittarius’ dance around the Milky Way would precisely contribute to putting heaps of neutron stars into rotation. The latter would then turn into pulsars, which themselves would emit a large amount of gamma radiation – hence the presence of this famous “cocoon”.

The search for dark matter can get complicated

For the researchers, it 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 disappears.

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

From now on, researchers will no longer be able to break out ” Black fabric! as soon as they see such a signal. Now that they have proven that this gamma radiation can very well come from pulsars, it will therefore be necessary to be even more careful; astronomers will therefore have to find new methods of analysis and other cosmic objects to study. Suffice it to say that we will still have to wait until we arrive at a direct proof that would show the existence of dark matter in an irrefutable way.

The text of the study is available here.

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