A recent research conducted by Dr. HOU Xian from Yunnan Observatories (YNAO) of Chinese Academy of Sciences and her coworker Pierrick Martin from France could possibly indicate mysterious dark matter in neighboring Andromeda galaxy (M31). This study was published in The Astrophysical Journal.

As the highest-energy form of light, gamma rays can be produced by the most energetic phenomena in the universe. Galaxies like the Milky Way (MW) are shinning in gamma rays which predominantly originate from interactions between cosmic rays (CRs), particles traveling near the speed of light, and the interstellar medium (ISM).

Such emission is observed to be correlated with regions rich in gas or star formation activities (usually along the disk of the galaxy) and extended from the center of the galaxy.

However, surprisingly, latest analysis performed by the Fermi Large Area Telescope (Fermi LAT) collaboration discovered that M31 emits gamma rays in confined inner regions rather than filling the disk of the galaxy or extending far from it. What does this uncommon phenomenon mean?

This gamma-ray signal is similar to the one Fermi has seen at the center of the MW. But in this case, the CRs-ISM mechanism cannot explain M31’s gamma-ray emission, suggesting that the emission is not ISM in origin. Scientists thought that other mechanisms or scenarios may account for the observed unusual spatial distribution.

One possibility is dark matter, an unidentified type of matter which constitutes more than 80% of the total mass of the universe. Dark matter is expected to accumulate in the inner center of galaxies like the MW. Theories predict that gamma rays can be produced through decay or annihilation of dark matter particles.

Another possible explanation of the central emission from M31 is related with a population of unresolved pulsars, highly spinning neutron stars which are dominant gamma-ray emitters in the MW.

While Fermi-LAT can observe the MW center more easily and in more details but partially obscured by the emission of its disk, M31 can be observed from an outside vantage point providing a cleaner view of the central emission from a grand-design galaxy.

In this context, the finding of similar gamma-ray signal both in the MW and M31 would enable scientist to use both galaxies as models for each other when making difficult observations. Consequently, it can help scientists to better understand the properties of both galaxies including the still mysterious dark matter.

"We still don’t fully understand the roles cosmic rays play in galaxies, or how they travel through them,” said HOU Xian, “M31 lets us see how cosmic rays behave under conditions different from those in our own galaxy.”

M31 will be a key to understand what this means for both Andromeda and the Milky Way.

Figure: The gamma-ray excess (shown in yellow-white) at the heart of M31 (Credits: NASA/DOE/Fermi LAT Collaboration and Bill Schoening, Vanessa Harvey/REU programAO/AURA/NSF)