Halo in blue: In this illustration, the radius of the gas cloud around the Milky Way (center) is 300, 000 light-years. In fact, he could be much larger, the researchers say. To the left of the Milky Way are the two dwarf galaxies known as the Large and Small Magellanic Clouds. Illustration: NASA / CXC / M.Weiss; NASA / CXC / Ohio State / A Gupta et al
Even our home galaxy, the Milky Way, is always good for surprises: now US astronomers have discovered that it is shrouded in a thin but gigantic cloud of hot gas. Although astronomers already knew that there is matter around the Milky Way, the new data shows the dimensions of the cloud for the first time. The gas accumulation is not only hotter than previously thought, but also much larger and more massive? and therefore could well accommodate a previously missing part of the matter. At the heart of the current study are observations of the Chandra space X-ray telescope: it had measured the radiation of eight bright X-ray sources many hundreds of millions of light years away from us. It showed that the X-rays on the way into the Milky Way are partially absorbed by oxygen ions in the neighborhood of our galaxy. Their temperature is apparently extremely high, calculated the team around Anjali Gupta of Ohio State University: It is probably between one and 2.5 million degrees Celsius.

Gas cloud in the weighing pan

This result fits the data of earlier studies, the researchers conclude. There was also evidence of the presence of gas at temperatures in excess of one million degrees Celsius. "So we know the gas is around the galaxy, and we know how hot it is, " commented Gupta. "The big question is: how big is the cloud and how much mass does it possess?"

Again, Chandra, along with the X-ray Observatory XMM-Newton and the Japanese X-ray telescope Suzaku, provided a first answer? And the scientists are astounded: although the gas cloud is very thin, it contains a total mass of more than 10 billion suns, possibly even 60 billion suns. Thus, the mass of the gas cloud is of the same order of magnitude as the mass of all stars of the Milky Way taken together. display

How much matter is actually gathered there, astronomers can not yet say? for many reasons. For example, it is unclear what the ratio of oxygen to hydrogen is in the thin cloud, the latter appearing to be the dominant gas in the cloud. Also unclear is the actual size of the halo. The only certainty is that it is very extensive: "It could extend around the Milky Way for a few 100, 000 light-years or even further into the local group of galaxies in the neighborhood, " says co-author Smita Mathur, also of the Ohio State University.

Baryons desperately wanted

The new data may help explain the mystery of the missing baryonic mass? one of the great unsolved mysteries of the cosmos. Baryons are the particles that make up "normal", visible matter. The core particles protons and neutrons are included.
The Problem: If we sum up the baryonic mass of all stars, the interstellar gas and the dust in the galaxies of the cosmos, we come to a value that is not even half as high as the theoretical extrapolations. The team around Gupta now suspects that this missing baryonic mass hides in hot gas clouds like the one Chandra has now discovered in the Milky Way. Presumably, these clouds have been overlooked so far, since they have such a low density, so the conclusion of the researchers.

Anjali Gupta (Ohio State University, Colombo) et al .: The Astrophysical Journal Letters, Vol. 756, doi: 10.1088 / 2041-8205 / 756/1 / L8 © science.de? Ilka Lehnen-Beyel

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