A New Look at Gamma-Rays From Our Galaxy’s Next-Door Neighbor

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Title: On the Gamma-Ray Emission of the Andromeda Galaxy M31

Authors: Yi Xing, Zhongxiang Wang, Dong Zheng, and Jie Li

First Author’s Institute: Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai

Status: Submitted to ApJL, open access on arXiv

Gamma rays represent the most powerful photons in our universe. Naturally, they come from some of the most extreme places in the universe, such as pulsars, active galactic nuclei (AGN), supernovae, and possibly dark matter. Although many sources of gamma-rays have been found both in the Milky Way and outside, the type of gamma-ray emission from our neighboring galaxy, Andromeda (or M31) remains secret.

Fermi-LAT is a gamma-ray observatory that has been scanning the atmosphere for high-energy gamma rays since 2008, with lots of data taken from Andromeda throughout its flight. Many groups have analyzed this data, with many data providing insight into what produces these gamma rays.

To expand or not to expand?

Until today’s paper, it appeared that the gamma rays from Andromeda were coming from a blob-like structure (called extended emission) in the center of the galaxy (similar to Figure 1, left). This was very exciting, since the extended shape produced by gamma rays usually suggests the distribution of cosmic rays or the presence of a large dark light.

Cosmic rays are the fastest moving particles in the universe but are easily deflected by gravity, making it very difficult to determine their origin on Earth. Fortunately, since there are processes that produce gamma rays from charged particles (hadronic processes), identifying areas of extended gamma rays can be traced to the places where cosmic rays interact with their environment. . On the other hand, much of the dark matter in the center of Andromeda may decay or disappear, producing gamma rays in the process.

Figure 1: Key map of Andromeda at energies from 0.1-500 GeV (left) and 2-500 GeV (right). The optical emission region is represented by the white contour. Colourbar corresponds to test statistics, which are similar to significance. A test score of 25 is consistent with the finding. Green symbols correspond to nearby sources found in the SIMBAD database. The left image shows a hint of additional structure in the southeast region of Andromeda, but the two data sources come from an area that appears to be extended only at the lowest power cut off. (Source: Figure 1 from the paper).

Where do gamma rays come from?

A new analysis of 14 years of Fermi-LAT data by the authors reveals that gamma ray production is not increasing. In fact, it appears to have two point sources: one in the center of the galaxy, and the other about 6 kpc to the southeast (see Fig. 1). This only became apparent when the authors cut out the lowest-energy gamma rays, which still make the data appear to be greatly expanded when included. Even more surprising, the authors found that both regions are much brighter than expected, compared to the gamma-ray emission of our Galactic center (see Figure 2).

Figure 2: A spectral energy distribution (SED) showing the flux (the amount of gamma rays received) plotted against the power of the regions from the center of Andromeda (black) and the southeast (red), as well as the center of Milky Way’s Galactic (blue). It is clear that both sources are not only similar in brightness, but also emit much more radiation than our Galactic center. (Source: Figure 2 from the paper).

This new image of Andromeda’s gamma rays revolutionizes our understanding of the galaxy. It’s unlikely that Andromeda’s central gamma-ray hotspot comes from a dark halo or cosmic ray distribution, so the authors looked at the Galactic center of the Milky Way to find out what kinds of objects might be there. responsible for gamma rays. One of the main theories of our Galactic gamma-rays center is the population of ancient, unresolved objects, such as millisecond pulsars (MSPs). However, in the case of Andromeda, at least 15,000 MSPs would be needed to account for the bright gamma-ray emission. Although it is still unclear whether the Andromeda center can host this large number of MSPs or not, we have only seen about 200 MSPs in the Milky Way center, so this explanation seems unlikely.

The authors also investigated the southeast source that appeared in their new analysis. Because the galaxies are so far apart, the chance of finding two or more galaxies by chance in a ring around the central and southeastern sources is only ~0.4%. This means that the output may have come from within Andromeda. As can be seen in Figure 2, the off-center source is almost identical to the Andromeda center source (which is unique in itself!), leading to the same problem of identifying sources capable of emitting bright light. like that. After looking at X-ray and optical studies, the authors concluded that there are no good counterparts to this region at other wavelengths. Although there is a low probability that this is the mysterious source of Andromeda, there are no known companions in the sky where this hotspot is located.

The results obtained by today’s authors are truly unexpected and open a new can of worms when it comes to finding the origin of gamma rays in our neighborhood galaxy. Although there are still many unknowns, future observations and analyzes of these newly restricted regions will help us understand how bright gamma rays are produced near the centers of clusters. of the stars, and may even help us better understand our own Galactic center.

Astrobite edited by Ivey Davis and Katya Gozman

Featured image credit: Adam Evans, Wikimedia Commons

About Samantha Wong

I am a graduate student at McGill University, where I study high energy astronomy. This includes studying all kinds of extreme environments in the universe such as active galactic nuclei, pulsars, and supernova remnants with the VERITAS gamma-ray telescope.

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