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The galaxies in the universe tend to stay together : astronomers noticed it already in the thirties, analyzing photographic plates made at the time that those were the most powerful observatories the world. Recognize the "clusters" of galaxies in an image in two dimensions is not trivial: often, two or more neighboring galaxies appear only for an effect of projection, which makes them fall into the same point of time Heavenly although in reality very far from each other. In other cases, however, it is physically nearby galaxies, one o'clock that feel the gravitational pull of the other: depending on the number of objects involved, astronomers speak of groups (a few dozen) or clusters (up to some thousand) of galaxies.

The clusters of galaxies are the largest structures in the universe, to be held together by gravity. They extend up to tens of millions of light years and have a mass of up to a few million billion times the Sun Yet this mass of galaxies accounts for less than ten percent! Astronomers continue to call them clusters galaxies for historical reasons, but as it turned out later, the main components of these giants are the cosmic gas, hot mixture of electrons and protons at a temperature of at least ten million degrees, emitting X-rays, and dark matter , which does not emit light and therefore can not be observed, but whose presence can be intuited indirectly.

isolate the contributions due to galaxies and gas is relatively simple: the first can be observed with conventional optical telescopes, the second thanks to satellites that operate in the band X-ray The dark matter, however, represents a sensitive subject and, in some circles, still controversial. The total mass

of a cluster can be studied by the effect of gravitational lensing, gravitational lensing or that it exerts on galaxies, located "behind" the cluster: their shape is distorted in a characteristic way, described by the theory of Einstein's general relativity, which depends on the total amount of matter contained in the cluster. Many astrophysicists, but not all, believe that the clusters are dominated by this elusive dark matter, and attach it to the total mass reconstructed by gravitational lensing.


In 2006, a very particular object has shed light on these obscure topics: "Bullet Cluster" (English, Bullet Cluster). This is actually two clusters that have "recently" clash "the cloud" of gas in the cluster on the right (the bullet) does have the typical form of a shock wave, shaped by the collision with the other . The gas has slowed down during the clash, a sort of "friction" that is separated from the galaxies from regions where it is saved most of the two clusters of matter as it was "discovered" by the study of lensing gravity. This separation shows that most of the material has interacted with the gas only via gravity. The appearance of these "scrap" cosmic is thus interpreted as evidence of the existence of dark matter.

CLAUDIA MIGNONE

In this image of the "Bullet Cluster observations of galaxies (white / yellow) and gas (pink) are coupled with the reconstruction of the mass is brought about by gravitational lensing (in blue). Image NASA / CXC / CfA / M.Markevitch et al. (Observations X) NASA / STScI, Magellan / U.Arizona / D.Clowe et al. (Optical observations), NASA / STScI, ESO WFI, Magellan / U.Arizona / D.Clowe et al. (Reconstruction by lensing).