Scientists believe that there could be a mysterious barrier in the center of the Milky Way. Credit: NASA

Mystery “Galactic Wall” is Keeping Cosmic Rays Out of Our Galaxy’s Center

Scientists found that the energy density of cosmic rays within the central molecular zone differs markedly from the density outside it.

Astronomers analyzed their observations of the central molecular zone in the gamma range and, from the observed luminosity, estimated the energy density of cosmic rays in this cloud. It turned out that inside the cloud this density is, on average, noticeably lower compared to the surrounding outer region. Scientists note that this may indicate a physical barrier that does not allow cosmic rays to enter the galactic center of the Milky Way.


Scientists found evidence of a mysterious barrier in the Milky Way

Uncertainties

To date, it is not entirely clear where and how cosmic rays in our galaxy are accelerated to high energies (and especially to extremely high ones – about 10 15 electron volts). There are many theoretical models that do not yet have reliable observational confirmation.

The acceleration of cosmic rays

To study the mechanisms of acceleration of cosmic rays, it is useful to look for regions in which the distribution of cosmic rays stands out with respect to the average galactic smooth quasi-stationary background. In such regions, there should be cosmic ray accelerators, which supplied the neighborhood with a portion of “fresh” rays, which did not have time to merge with the established background.

Due to this, from such observations, it is possible to obtain more detailed information both on the parameters of the accelerator itself and on the propagation of cosmic rays in the galaxy than from the usual average background.

Focus of the research

Astronomers from the Chinese Academy of Sciences focused their attention on the central molecular zone – a dense molecular cloud in the galactic center, where the accelerator (or accelerators) of high-energy cosmic rays is likely to be located. The researchers analyzed archival observations of the Fermi gamma-ray telescope from August 4, 2008, to February 1, 2020, in the energy range of the order of 10 9 -10 12 electron volts and in the direction that corresponds to the center of the galaxy.

How did scientists analyze the data?

To trace the properties of cosmic rays inside and outside the central molecular zone, the authors split the angular map into sections with a thickness of the order of 0.5 degrees and estimated the observed luminosities of each section in the gamma range and the mass of gas within the sections. Taking advantage of the fact that gamma radiation is the result of the interaction of cosmic rays with the matter, the authors calculated the average energy density of cosmic rays (in the corresponding energy range) at each of the sites.

The density of cosmic rays as a function of the angular distance to the center of the galaxy for the central molecular cloud (red and orange dots) and the region outside the cloud (blue and green dots). The graph on the left additionally takes into account the contribution of excess gamma radiation from the center of the Milky Way, which has no unambiguous interpretation. Credit: Xiaoyuan Huang, Qiang Yuan & Yi-Zhong Fan / Nature Communications, 2021
The density of cosmic rays as a function of the angular distance to the galactic center for the central molecular cloud (red and orange dots) and the region outside the cloud (blue and green dots). The graph on the left additionally takes into account the contribution of excess gamma radiation from the center of the Milky Way, which has no unambiguous interpretation. Credit: Xiaoyuan Huang, Qiang Yuan & Yi-Zhong Fan / Nature Communications, 2021

Results

It turned out that the energy density of cosmic rays within the central molecular zone differs markedly from the density outside it. In magnitude, inside the cloud, the density value turned out to be close to that measured in the vicinity of the Sun, and up to two times less than in the outer region at equal distances to the center of the galaxy.

In terms of behavior in space, the energy density at the edges of the cloud was approximately half that at its center, while in the surrounding matter outside the cloud, the density practically did not depend on the distance to the galactic center.

Is there a barrier in the center of the Milky Way?

According to astronomers, such a picture may indicate a physical mechanism like a barrier that prevents cosmic rays from penetrating the central molecular zone of the Milky Way – for example, a favorable configuration of the magnetic field at the edges of a cloud. If in the future the authors’ assumption is confirmed, then this kind of cloud isolation can help to study in more detail the design of the supposed cosmic ray accelerators inside it.


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Sources:

Bergan, B. (2021, November 11). Scientists detected a mysterious barrier near the center of the galaxy. Interesting Engineering.
Crane, L. (2021, November 9). A strange barrier is keeping cosmic rays out of the milky way’s centre. New Scientist.
Huang, X., Yuan, Q., & Fan, Y.-Z. (2021, November 9). A Gev-TEV particle component and the barrier of cosmic-ray sea in the Central Molecular Zone. Nature News.
Kelvey, J. (2021, November 10). Scientists link an odd reaction to a barrier in the center of the milky way. Inverse.
Starr, M. (n.d.). There’s a mysterious barrier keeping cosmic rays out of the Galactic Center. ScienceAlert.

Written by Vladislav Tchakarov

Hello, my name is Vladislav and I am glad to have you here on Curiosmos. As a history student, I have a strong passion for history and science, and the opportunity to research and write in this field on a daily basis is a dream come true.

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