The circumgalactic medium may be impacting the Milky Way

Recently, researcher Mou Guobin and professor Wang Wei of the School of Physics of Wuhan University and Fang Taotao, professor of the Department of Astronomy of Xiamen University, for the first time theoretically revealed that the cirternal galactic medium has a violent lateral motion relative to the Milky Way, and called it the cirternal medium wind. The findings were published in Nature Communications.

Schematic diagram of the formation mechanism of silver halo multi-band bubbles and their asymmetry (Photo courtesy of Mou Guobin).

The circumgalactic medium of the Milky Way refers to gas outside the interstellar medium of the Milky Way , within the virial radius ( approximately 250-300 kiloparsecs from the silver center ) . These gases consist mainly of hot gases with temperatures of millions of kelvins, and the kinematic characteristics of these gases remain unclear due to the performance of current observing instruments. This question is key to understanding the interaction of the circumgalactic medium with the Milky Way and its evolution.

In the last decade or so, from radio to gamma rays, more and more large, bubble-like remains have been found in silver halos, with physical dimensions of tens of thousands of light-years. These bubbles include the WMAP fog discovered and named by the Wilkinson Anisotropy Detector (WMAP) in 2008, the Fermi bubble discovered and named by the Fermi Large Field of View Telescope in 2010, the polarized radio lobe discovered at 2.3 GHz by the S-Band Polarization Sky Survey (S-PASS) in 2013, and the southern halo X-ray bubble paired with the Northern Halo One Circle discovered by the eROSITA X-ray Telescope in 2020.

Mu Guobin told China Science News that because these bubbles are roughly symmetrically distributed on both sides of the silver center, it is generally believed that they are the remains of the silver halo formed by the accretion activity of the supermassive black hole of the silver center or the violent star formation process driven by the accretion activity of the silver center or the violent star formation process millions to tens of millions of years ago. Interestingly, the northern halo bubbles all show obvious east-west asymmetry, while the southern halo Fermi bubbles and polarized radio flaps also show east-west asymmetry, while the X-ray bubbles of the southern halo are much weaker than the northern halo.

Mu Guobin believes that if there is obvious lateral motion in the circumgalactic medium, it will leave an imprint on these silver halo bubbles, so the asymmetry of the bubbles can provide an excellent entry angle for the kinematics of the circumgalactic medium, especially the part of the gas near the silver disk (as shown in the figure).

Previously, the results of Mou Guobin’s team showed that if the asymmetry of the northern halo X-ray bubble (the southern halo X-ray bubble had not been found at that time) was caused by an undetected lateral wind, then the wind speed estimated by the analytical model would be as high as 100-200 km/s. However, at that time, whether the nature of the northern halo X-ray bubble was a nearby structure hundreds to thousands of light-years from the solar system, or a large silver halo structure tens of thousands of light-years was still controversial, and many researchers in the field held the former view, so this conclusion did not receive attention.

It is worth mentioning that at the end of 2020, the discovery of X-ray bubbles in the southern halo confirmed the latter view and brought an opportunity to solve the problem of the origin of asymmetric features.

In the new study, the researchers further proposed that the bubble is formed by the interaction between the outflow of the silver center and the perigalactic medium, then the asymmetry of the bubble must be caused by the outflow of the silver center or a certain property of the perigalactic medium, which leads to three possibilities, namely, there is a deflection of the silver center outflow, the density of the perigalactic medium is asymmetrically distributed under hydrostatic equilibrium (the density on the left side is higher), and there is lateral movement in the perigalactic medium.

For the first time, the research team quantitatively investigated three possibilities for the origin of asymmetry with the help of numerical simulations of fluid dynamics. The results show that neither of the first two can explain the observations, while only the third can fit the observations and self-consistent with multiple other independent observations, including metal abundance distribution, high-speed clouds, etc.

The simulation results show that the circumgalactic medium sweeps the silver halo at a speed of 200km/s towards the lower right in the galactic coordinate system, which profoundly affects the physical properties of the silver halo gas. The researchers speculate that this wind may be triggered by the motion of the Milky Way in its own group.

“Unlike the previous view that the cirternal medium is regarded as a hydrostatic equilibrium or rotational state, this study reveals for the first time the lateral motion characteristics of the cirternal medium from a theoretical point of view, indicating that the cirternal medium is acting significantly with the Milky Way.” Mou Guobin said that this result will help to understand the intensity and mode of past activity of the silver heart, the evolution of the Milky Way, the formation of high-speed clouds and warped neutral hydrogen disks, and other frontier questions. (Source: China Science News Xin Yu)

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