NASA’s Fermi telescope has detected a strong gamma-ray signal coming from the core of the Andromeda galaxy (also known as Messier 31) that could indicate the presence of dark matter — and this signal is similar to one seen by Fermi at the Milky Way’s center. A paper on the results will appear in the Feb. 23 issue of the Astrophysical Journal.
NASA’s Fermi telescope has detected a gamma-ray excess (shown in yellow-white) at the center of the Andromeda galaxy that’s similar to a signature Fermi previously detected at the center of our own Milky Way Galaxy. Scientists think the signal could be produced by a variety of processes, including a population of pulsars or even dark matter. Image credit: NASA / DOE / Fermi LAT Collaboration / Bill Schoening / Vanessa Harvey / REU Program / NOAO / AURA / NSF.
The gamma rays in the Andromeda galaxy are confined to its center instead of spread throughout, the new Fermi data show.
To explain this unusual distribution, a large team of astronomers and astrophysicists is proposing that the emission may come from several undetermined sources, including pulsars or even dark matter.
“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 co-author Dr. Pierrick Martin, an astrophysicist at the National Center for Scientific Research and the Research Institute in Astrophysics and Planetology in Toulouse, France.
“Messier 31 will be a key to understanding what this means for both Andromeda and the Milky Way.”
Another possible source for this emission could be a population of pulsars at the center of Messier 31.
These spinning neutron stars weigh as much as twice the mass of the Sun and are among the densest objects in the Universe.
Because Messier 31 is approximately 2.5 million light-years away, it’s difficult to find individual pulsars.
To test whether the gamma rays are coming from these objects, researchers can apply what they know about pulsars from observations in the Milky Way Galaxy.
“The Milky Way 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 Dr. Regina Caputo, a researcher at NASA’s Goddard Space Flight Center.
“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.”
While more observations are necessary to determine the source of the gamma-ray excess, the discovery provides an exciting starting point to learn more about both galaxies, and perhaps about the still elusive nature of dark matter.
Now that Fermi has detected a similar gamma-ray signature in both Messier 31 and the Milky Way, scientists can use this information to solve mysteries within both galaxies.
For example, Messier 31 emits few gamma rays from its large disk, where most stars form, indicating fewer cosmic rays roaming there.
Because cosmic rays are usually thought to be related to star formation, the absence of gamma rays in the outer parts of Messier 31 suggests either that the galaxy produces cosmic rays differently, or that they can escape the galaxy more rapidly.
Studying Messier 31 may help astronomers understand the life cycle of cosmic rays and how it is connected to star formation.
“We don’t fully understand the roles cosmic rays play in galaxies, or how they travel through them,” said lead co-author Dr. Xian Hou, an astrophysicist at Yunnan Observatories in Kunming, China.
“Messier 31 lets us see how cosmic rays behave under conditions different from those in our own Galaxy.”
_____
M. Ackermann et al. 2017. Observations of M31 and M33 with the Fermi Large Area Telescope: A Galactic Center Excess in Andromeda? ApJ 836, 208; doi: 10.3847/1538-4357/aa5c3d
This article is based on a press-release from NASA.
