Space and Physics
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There are excesses of gamma-ray emissions from the center of the Milky Way that are not fully understood. It has been hypothesized that dark matter could be responsible for it, though there has not been an exhaustive explanation. Dark matter itself is hypothetical and has not been confirmed directly yet. A new dark matter model aims to solve at least two, and maybe even three mysteries at the heart of our galaxy.
The rest of this article is behind a paywall. Please sign in or subscribe to access the full content.Regular matter that makes us, planets, and stars interact according to the four fundamental forces: gravity, electromagnetism, strong and weak nuclear forces. Dark matter only interacts through gravity, influencing regular matter but remaining invisible. Dark matter outweighs regular matter five to one, so it forms large halos around galaxies. At the core of galaxies, there’s an overdensity, and that gives an opportunity for some interesting stuff to happen.
The new model, put forward by researchers at King's College London, suggests the existence of "excited dark matter". At the center of the Milky Way, dark matter particles would be present at a higher density and moving at a higher speed. In the model, these particles are in the ground state – the lowest energy state for them – and they interact thanks to a fifth unknown force. Once they slam together, they can produce an excited state, which decays over time, and in that decay, we can find the sources of the signals.
There are three different signals, seemingly disconnected, but the... excited dark matter model can explain at least two and potentially significantly contribute to the third one as well.
Dr Shyam Balaji
Forces have mediator particles. Electromagnetism is mediated by the photons, for example. So most of the decay of this excited dark matter particle would be this unknown mediator. The excited state might also decay into an electron and its antimatter equivalent, the positron. The positrons would produce the signals in different ways.
“We have multiple different signals from the galactic center at what's typically called the low-energy regime,” co-lead author Dr Shyam Balaji, from King’s College London, told IFLScience.
“There are three different signals, seemingly disconnected, but the point that we are making was that a particular dark matter model, which is this excited dark matter model, can explain at least two and potentially significantly contribute to the third one as well.”
The three mysteries are the 511-keV emission line, the 2 MeV gamma-ray continuum, and the Central Molecular Zone’s unusual gas ionization. The excited dark matter model suggests an excess of positrons in the center of the Milky Way. As the positrons zip away from the dark matter, they lose energy. If they lose enough energy and encounter another electron, they can form a special bound state called a positronium. This decay emits a photon with an energy of 511 keV.
If instead they encounter that electron when they still have too much energy, they can instead produce a more powerful gamma-ray emission, forming the 2 MeV gamma-ray continuum. The production of the matter-antimatter pair could also be a contributing factor to the ionization of gas in the region, with the powerful light from those events stripping some of the atoms’ electrons away.
The model is neat. While dark matter remains hypothetical, this suggestion provides some intriguing possibilities to explain observations that have remained puzzling.
“The importance of this kind of work in general, which is drawing the connection between theory and experiment, is looking for hints like a detective, accumulating all the evidence. and seeing where that points us,” Dr Balaji told IFLScience.
Upcoming observatories will provide new and more detailed insights into the processes at the center of our galaxy and hopefully help us solve this mystery, whether it is dark matter-related events or something else.
The study is published in The Astrophysical Journal Letters.
