Galaxy Fountains: Gas and Dust Dynamics
Observing gas streams emitting in the X-ray range, fountains escaping from the plane of our galaxy, the European probe XMM-Newton has collected data revealing their origin in more detail. This is similar to the water cycle on Earth: gas flowing out of the galaxy’s disk into the halo, which is invisible to the eye, cools here and falls back again, onto the disk. But, unlike the water cycle, this movement is largely determined by supernova explosions.
In general, the interstellar medium filling the inner space of galaxies is not at all so empty and lifeless. This is a complex and dynamic system, including gas, and dust of different densities and temperatures, and magnetic fields, and all types of electromagnetic radiation, cosmic rays and, of course, invisible dark matter. This "empty space", in fact - the complex interaction between gas and dust clouds and flows of different phases, densities and temperatures, their movement under the influence of the attraction of dark matter - play an important role in the formation of stars in the galaxy. The largest stars, both during their life and at the time of death in a supernova burst, are strongly influenced by the interstellar medium.
One of the components of the interstellar medium is an extremely low-density gas heated to millions of degrees, which, due to its high temperature, radiates noticeably in the x-ray range. Its presence was first assumed in the 1970s, shortly after the advent of x-ray astronomy. Since then, his presence has not only been proven, but his important role in the dynamics of the interstellar medium has been shown. And for scientists, its fluxes easily visible in X-rays have become an excellent tool for studying the motion of energy flows from stars to the interstellar medium.
Supernova explosions heating this medium can emit streams of hot gas so powerfully that they shoot above the plane of the disk of the galaxy, gradually accumulating outward and forming the halo of the Milky Way. The existence of a galactic halo was clearly shown in the early 1990s, during the operation of the ROSAT orbital x-ray telescope; similar halo are found in other spiral galaxies. As the streams of hot gas rise above the surface of the galaxy’s disk and reach a “height” of several kiloparsecs, they emit radiation, lose energy and cool. This cooled gas gradually condenses into denser clouds, which are attracted back to the “territory” of the galactic disk. Outwardly, everything resembles a fountain, raising a fast stream of water, which then falls down. This process provides global gas circulation, connecting the main part of the galaxy with its halo, and the exchange of matter between them.
Unfortunately, it has not yet been possible to observe these “fountains” in their most interesting part — the one where gas escapes from the surface of the galactic disk and rushes toward the halo. The problem is that the x-ray emitted by it is quickly absorbed by the matter of the galaxy - in fact, we have to observe these “fountains” from the edge of the galactic disk. However, the radiation that it emits is already visible to us, already being within the galactic halo. And the scientists, led by David Henley, who modeled these “galactic fountains, ” came to the conclusion that this scheme perfectly predicts the X-ray parameters of gas fixed in the halo.
Scientists also considered other scenarios of the origin of hot gas in the galactic halo, including its condensation from extragalactic matter; gas heating by supernova explosions, already occurring within the halo; finally, in the third version, supernovae generated the same “fountains” of hot gas that we talked about. It is the third version that is consistent with the observed data in the best way.
Meanwhile, according to some reports, the Milky Way may reveal not one, but two halos at once. Why - read in the note "Oddities of the halo."
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