Astronomers just made a discovery in a nearby dwarf galaxy

A team of astronomers studied two nearby globular clusters, 47 Tucanae and Omega Centauri, looking for signals produced by dark matter annihilation. The search was fruitless, but it was not a failure. The lack of evidence placed strict upper bounds on the mass of the hypothetical dark matter particle.

Shine a light on dark matter

Dark matter makes up about 80 percent of all mass in the universe, although it is completely invisible. It just doesn’t interact with the electromagnetic force, and so it doesn’t glow, reflect or absorb or anything. So far we only have evidence of its existence through its gravitational effect on the rest of the universe. Because of this, astronomers aren’t entirely sure what dark matter is, although many physicists believe it’s a new type of particle previously unknown to the Standard Model of particle physics.

One possibility is that dark matter consists of an ultralight particle like an axion. And although these particles would not interact with normal matter, very rarely could they interact with themselves, colliding and annihilating. If the energy of the collision is high enough, it can result in the production of a gamma ray, which then splits into an electron and a positron.

The globular cluster Messier 54. NASA

These electrons and positrons can stick together to form bound states called positronium. However, the positronium atoms are not stable and they eventually decay, leaving behind a flash of radio emission.

So although dark matter does not directly interact with electromagnetism, there is still a chance that we will see the radio emission from the collision and decay of dark matter particles.

Look at the globular clusters

You need a lot of dark matter for this to work. If the dark matter particles collided easily enough, we would have seen it by now. The collisions must therefore be rare. The density of dark matter in our galactic neighborhood is far too low to produce detectable emissions, but the dense cores of galaxies may provide better access.

The natural place we’re looking is our galactic core, but that place is awash with all manner of radio emissions, so it’s difficult to tell if a particular signal is from dark matter annihilation or something more mundane. For this reason, a team of astronomers has been searching for two nearby globular clusters, as reported in a recent article in the preprint journal arXiv.

Two star clusters, 47 Tucanae and Omega Centauri, are only a few thousand light-years away, making them relatively easy to see. And astronomers believe they are the remnants of dwarf galaxies that have had most of their stars stripped away through interactions with the Milky Way.

This makes the clusters ideal laboratories as they are essentially spheres of dense dark matter with very little contamination. The team of astronomers searched for the unique radio signal from decaying positronium using Parkes Observatory in Australia.

They haven’t found anything that isn’t necessarily a bad thing. Based on their observations, they were able to set the best upper bounds to date on the mass and cross section (a measure of how frequently the particles interact) of these light-dark matter models. Sure, it would have been great to see a confirmed signal and finally solve this dark matter mystery, but new knowledge in any direction is always welcome and always helpful.

This article was originally published on universe today by Paul M Sutter. Read the original article here.

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