NASA's Fermi Telescope has looked at the gamma-ray emission of M31, the Andromeda Galaxy, and discovered the largest fraction of this powerful radiation comes from the core of the galaxy, very much like in our own Milky Way. The international team of researchers has considered this signature as potential indirect evidence of dark matter.
Some theoretical models predict gamma-ray emissions when dark matter particles interact with each other. Dark matter doesn’t like interacting at all, it doesn’t form clumps or clouds, so these gamma-ray signals might only happen in dense regions, like at the core of galaxies.
“We expect dark matter to accumulate in the innermost regions of the Milky Way and other galaxies, which is why finding such a compact signal is very exciting,” said lead scientist Pierrick Martin, an astrophysicist at the National Center for Scientific Research and the Research Institute in Astrophysics and Planetology in Toulouse, France, in a statement. “M31 will be a key to understanding what this means for both Andromeda and the Milky Way.”
While the signal is clearly there, the cause is very much a matter of debate. The researchers themselves state how this could be caused from several unidentified sources. The signature in the Milky Way could have been caused by a very powerful pulsar according to another study.
The research is going to be published in the Astrophysical Journal and highlights how we can use a different galaxy to learn more about our own. Andromeda is the closest galaxy to the Milky Way of comparable mass, and a distance of 2 million light-years is a stroll in the park away in cosmic terms.
“Our galaxy is so similar to Andromeda, it really helps us to be able to study it, because we can learn more about our galaxy and its formation,” said co-author Regina Caputo, a research scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s like living in a world where there’s no mirrors but you have a twin, and you can see everything physical about the twin.”
The study highlights one important difference, though. Andromeda seems to produce high-energy particles, known as cosmic rays, unlike the Milky Way. Cosmic rays also produce gamma rays when they interact with gas clouds, but there’s no gamma ray emission outside the core of Andromeda, while gamma rays from cosmic rays are found everywhere in the Milky Way.
“We don’t fully understand the roles cosmic rays play in galaxies, or how they travel through them,” said Xian Hou, an astrophysicist at Yunnan Observatories, Chinese Academy of Sciences in Kunming, also a lead scientist of this study. “M31 lets us see how cosmic rays behave under conditions different from those in our own galaxy.”
More observations will be necessary to understand the source of Andromeda’s gamma rays and we’ll likely learn something about our own island-universe as well.