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Astrophysics

The great magnet of the Milky Way

The gravitational force of a huge structure located 500 million light years awaypulls our galaxy in its direction

NASAThe mysterious black material (in blue) around the group of galaxies: Shapley has seven times more invisible particles than visible particles NASA

At the beginning of 1986, a group of astronomers rooted in the Unites States and the United Kingdom, self denominated as the Samurai Seven, showed that the Milky Way and other neighboring galaxies are moving faster than the velocity estimated for the expansion of the Universe. A cosmological anomaly seems to be pulling them at 400 km/sec above the expected speed in the direction of the constellation of Hydra and Centaur. The explanation for such an unexpected acceleration must be the presence of a colossal quantity of non-identified mass in this region of the Universe, which would function as a gravitational magnet for our and other neighboring galaxies. This is like a Great Attractor, a term made up by scientists to define this phenomenon.

Since then many astrophysicists have attempted (and are still attempting) to locate the origin of this perturbation, without having reached a definite answer. In an article to be published this year in the European scientific magazine Astronomy and Astrophysics, an international research team, including the participation of a Brazilian, is saying that they have identified the structure that is responsible for half of the Great Attractor. This will be the Shapley supercluster, a mega grouping of distant galaxies a little less than 500 million light-years from us (a light-year is equivalent to the distance covered by light in a year, 9.5 trillion kilometers).

Discovered during the 1930’s by the American astrophysicist Harlow Shapley, this supercluster, composed of 44 smaller clusters, each one with hundreds or thousands of galaxies, is located to the north of the Centaur constellation and is visible only from lands in the southern hemisphere, always with the use of telescopes. Its format is that of an oval shaped cloud of galaxies. From these central clusters X-rays emanate, indicating that within there is gas at temperatures higher than 10 million degrees Celsius. Due to its gigantic proportions, Shapley is considered by some astrophysicists as the major structure located in the so-called Local Universe, which encapsulates all that exists at a distance of 500 million light-years from the Earth. “It’s around 40 times greater than the Local Group of galaxies”, compares Laerte Sodré, from the Astronomy, Geophysics and Atmospheric Sciences Institute of the University of São Paulo (IAG/USP), one of the work’s authors. The Local Group, of which the Milky Way, Andromeda, and another three dozens galaxies make up a part, measures approximately 3 million light-years.

The Shapley supercluster provides impressive numbers. One can begin with its length, which extends for 120 million light-years. It’s density as well is almost unimaginable: if it were to be a ball, its volume would be equivalent to that of a sphere with a radius of
80 million light-years. Its mass, in accordance with the study’s calculations, is approximately 5 x 101,016 times greater than that of the Sun. The astrophysicists do not know how many galaxies exist in all of its structure, but know the velocity of 5,701 of them. In the end, the supercluster is a very uncommon structure in the Cosmos, and whose gravitational force will pull in its direction our galaxy and its neighbors. “What we have to do now is to discover a quantity of material identical to the Shapley, in the same direction of the Cosmos, in order for us to explain the particular movement of our galaxy”, says the astrophysicist Dominique Proust, from the Paris Observatory, the main author of the new study concerning the nature of the Great Attractor, which also included Chilean, Argentinean and Australian researchers.

One of the difficulties of this line of research is to obtain trustworthy data concerning the mass contained in a supercluster. Without this type of information, it is difficult to estimate the order of the magnitude of the gravitational field that can emerge from this mega-structure. In search of a possible source of the Great Attractor, the multinational team of astrophysicists first calculated, or consulted the available registers, the recession velocity of 8,632 galaxies situated in the direction of the Hydra and Centaur constellations, the region of the Cosmos to which the Milky Way is being pulled.

The majority of these galaxies belong to the Shapley supercluster. Others lie in the neighboring areas, such as the Hydra-Centaur supercluster, or smaller clusters. The recession velocity registers the rhythm with which an object distances itself from an observer. In 1929, the American astrophysicist Edwin Hubble – the first man to show that the Universe was in expansion and was not static – established the cosmological law that, the further away a new found a galaxy is from the Milky Way, the quicker it has been moving away from us. In other words, the further away a galaxy, the greater is its recession velocity. And to have on hand this type of register, as well as the data about the luminosity of stars and galaxies, greatly helps to fill out the models used by astrophysicists to estimate the density of objects and the formation of a major part of the heavens.

If the new study’s proposal were to be correct, at least half of the Great Attractor is located at a distance from the local group of galaxies that is much greater than previous estimates had pointed towards. The major part of the work defends the idea that this gravitational anomaly is brought about by cosmic structures even further from the Earth, located between 150 million and 250 million light-years. Before and in the direction of Shapley, there is another mega-grouping of galaxies and galaxy clusters, the Hydra-Centaur supercluster, at a distance of approximately some 200 million light-years. Certain researchers believe that the material responsible for the Great Attractor effect is found in some point of this cluster, which is closest to the Milky Way. “Certainly it’s possible that the Shapley supercluster provides a significant part of the great Attractor, but the evidence that has come up until now suggests that some two thirds of the total effect comes from closer regions”, says the British astrophysicist Donald Lynden-Bell, from the University of Cambridge, one of the Seven Samurai who discovered this gravitational disturbance.

The team, led by Dominique Proust from the Paris Observatory, as had been expected, does not agree with the Britisher’s vision. “The Hydra-Centaur supercluster is situated in front of Shapley, but it couldn’t produce the gravitational field necessary in order to justify the displacement of the local group of galaxies in its direction”, the French astrophysicist ponders. “This movement must be associated with a structure of much greater mass, the Shapley.” Laerte Sodré is of the same opinion. “The issue isn’t settled, but our study indicates that the Shapley gives an important contribution to the Great Attractor”, says the researcher from the IAG/USP. In truth, the article in the Astronomy and Astrophysics sustains the thesis that this supercluster is much greater than had been imagined – consequently capable of originating gravitational fields that are even stronger – and that it possess “bridges” that link it to the Hydra-Centaur supercluster, located closer to the Milky Way.

Dynamics of the Universe
To determine the nature of the Great Attractor, is, without doubt, important for the understanding of celestial structures that alter the movements of the Milky Way, the galaxy in the interior of which, in the middle of hundreds of billions of stars, the Sun, the Earth and the other planets of the solar system are located. But the relevance of this field of study could have repercussions that are even more fundamental: it could help to better understand the variables that act upon the dynamics of the Universe, which,  according to the theory most accepted within the scientific community, has been in expansion since the Big Bang, the hypothetical primordial explosion that created the Cosmos some 13.7 billion years ago. Today there is evidence that the distribution of the material in the Universe is not uniform.

Some regions of space are apparently great vacuums, without visible material, whilst others show enormous concentrations of stars and galaxies, giving origin to a cosmic mega-structure, such as the superclusters. Even in the inside of these gigantic cosmic formations the present of material is not equal in all of their sectors. In other words, it is not easy to have a clear notion of the density of the entire Universe or even of some of its zones. “The cosmological models depend a lot on this type of data”, says the astrophysicist Proust, who, besides being astrophysicist, is in spite of his literary name, a musician (he plays the organ in the Notre-Dame Church of the Ascension in Meudon, on the outskirts of Paris, and has already recorded CDs with works of musicians such as the Britisher William Herschel, an astronomer and composer who lived between 1738 and 1822). “One of the current questions is to discover why there appears to be a lack of material in the Universe.”

The region of the Universe that would have been the source of half of the Great Attractor is not an exception to this rule. The visible part of Shapley appears to be only the point of the supercluster, the scientists believe. In the apparent empty space that exists between its thousands of galaxies there must be lots of black material, a mysterious type of particle that apparently does not emit or absorb light. “There’s more or less seven times more black material than visible material in the Shapley”, estimates astrophysicist Sodré. The existence of this type of material, accepted by the majority of astrophysicists, can only be inferred by the influence of its gravitational field upon neighboring bodies.

If, for example, the movement of a galaxy or a star is affected in a proportion not compatible with the visible mass of the cosmic objects in its surroundings, this disturbance is usually explained by the presence, in this region of space, of invisible particles to direct forms of cosmic observation. Up until the end of the last decade, it had been thought that more than 90% of he Universe was composed of black material. Since then, however, with the discovery of even more intriguing black energy, a force that will function as a counterbalance to gravity, pushing away, instead of attracting, the mass of celestial bodies, the quantity of black material went on to be calculated as around 23% of the Universe’s total (the visible material would respond to a mere 4% of the Cosmos and black energy for 73%). If it is correct, Shapley, the heart of the Great Attractor, is possibly one of the points f the local Universe with more material and black energy.

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