The largest matter concentration on large scale of the local Universe |
 |
The largest matter concentration on large scale of the local Universe
Since the years 80's, we know that our Galaxy, the Milky Way, and about thirty nearby galaxies which form the "Local Group" of galaxies, move at high speed in the direction of Centaurus. Undoubtedly there must be in this region a "Great Attractor", i.e. a very massive supercluster of galaxies, whose gravity attracts the Local Group in its direction. An international collaboration, led by an astronomer of the Paris Observatory, has just highlighted the greatest matter concentration on large scale in the local Universe, at less than 500 million light-years from us. This concentration accounts for half of the "Great Attraction" searched for, but it still remains to find the second half.
The supercluster of galaxies Shapley 8 (Figure 1), located in the north of the constellation of Centaurus (right ascension: 13h 25mn declination -30°) was observed in 1930 by the American astronomer Harlow Shapley; he then noticed a cloud of galaxies in Centaurus which seems one of the richest to be currently detected, [... ] of oval form, having dimensions of approximately 2.8° by 0.8°. In the years 1980, this structure interested the astronomers particularly because they discovered that the Local Group, formed by about thirty galaxies surrounding us, deviated from the general expansion motion of the Universe at the speed of 366 ± 125 km/s in the direction of Centaurus. This meant that an enormous quantity of matter attracted our Galaxy and the surrounding galaxies; it took the generic name of Great Attractor.
In order to know the nature of the Great Attractor, systematic surveys of recession velocities of galaxies were undertaken in the direction of Centaurus, using various telescopes equipped with multi-objects spectrographs with fibers, allowing to analyze simultaneously the light emitted by several tens of galaxies. In this direction there is already the Hydra-Centaurus supercluster whose galaxies have in average a recession velocity of 4000 km/s. However, in spite of its wealth of galaxies, this huge complex has only a negligible gravitational effect on the Local Group. This is why an international team of astronomers French, Chilean, Australian, Brazilian and Argentinian under the responsibility of Dominique Proust (Meudon) and Hernan Quintana (Santiago of Chile) undertook a dynamical analysis of the supercluster of Shapley by studying the galaxies until a magnitude mB= 18.0, beyond the Hydra-Centaurus complex, in an area of the sky extending on 30° x 12°.
Figure 1: The center of the supercluster Shapley 8 -- Palomar Observatory Sky atlas - National Geographic Society. The galaxies were selected from photographic plates of the European Southern Observatory (ESO), digitized on the MAMA (automatic measuring machine) at Paris Observatory. The observations were carried out with the 2.5m telescope Du Pont of the Observatory of Las Campanas (Chile) equipped with the spectrograph 2D-frutti, the ESO telescope of 3.60m and spectrographs OPTOPUS and MEFOS in La Silla (Chile), and the 1.8m UKST telescope of Anglo-Australian Observatory (Australia) equipped with the spectrographs FLAIR and 6dF. By supplementing the observations with velocities contained in the NASA extragalactic data base (NED), 10529 speeds were thus gathered, corresponding to 8632 galaxies. In the central part of the supercluster, 90% of the galaxies could be measured until the magnitude 18.0, and on the whole 61% of the objects of the supercluster were observed.
Figure 2 shows the diagram "in cone" of these galaxies. Velocity is plotted versus right ascension, ranging between 12h 30 and 14h 30. One notices on this figure the Hydra-Centaurus supercluster, whose galaxies have a mean recession velocity of 4000 km/s, in the foreground (with respect to the line of sight). It is connected by a bridge of galaxies to the supercluster of Shapley itself whose 5701 galaxies have a mean recession velocity of 15400 km/s. This enormous structure is made up of three "sheets" of galaxies, connected by several clusters as figure 3 shows , where velocity is plotted versus declination (view perpendicular to the previous one), between -20° and -40°.
Figure 2: Cone diagram (right ascension) of the galaxies observed in the area of the supercluster of Shapley until a recession velocity of 30 000 km/s. Many ramifications connect the supercluster of Shapley to other superclusters, underlining the distribution in filaments of the matter in the local Universe. Moreover, the analysis of the supercluster shows that it is composed of 44 galaxy clusters; those located at the center are X-ray sources, indicating the presence of gas at very high temperature, more than ten million degrees. Each cluster is characterized by an apparent elongated structure along the line of sight, due to the velocity dispersion inside the cluster. This structure bears the generic name of "finger of God". The galaxies contained in the supercluster of Shapley present a matter surdensity by a factor 5.4 ± 0.2 times the average density, that is to say a value of the density definitely larger than the nearby superclusters, such that of the Horologium-Reticulum whose excess of density is only 2.4. The supercluster of Shapley extends on more than 120 million light-years, its volume being equivalent to that of a sphere of 80 million light-years in radius: it is the largest matter concentration on large scale in the local Universe, at less than 500 million light-years from us.
Using the obtained data, the astronomers determined the luminosity and the mass of the supercluster by using various models such as the determination of the mass by X-ray properties, the analysis of the velocity fields of each galaxy cluster or spherical model of collapse. They obtained a total luminosity of about 1.4 x 1014 times that of the Sun for a mass of the supercluster of about 5 x 1016 times that of the Sun. Although very high, this mass represents only half of that required to attract the Local Group in the direction of the supercluster. In addition to the error bars, it remains to discover a identical amount of matter, in the same direction, to be able to account for the particular motion of our Galaxy.
Figure 3: Cone diagram (declination) of the galaxies observed in the area of the supercluster of Shapley until a recession velocity of 30 000 km/s. Reference
Dominique Proust, Hernan Quintana, Eleazar R. Carrasco, Andreas Reisenegger, Eric Slezak, Hernan Muriel, Rolando Dünner, Laerte Sodré Jr., Michael J. Drinkwater, Quentin A. Parker, Cinthia J. Ragone,
Structure and dynamics of the Shapley Supercluster,
astro-ph/0509903,
Astronomy and Astrophysics, in press.
Contact
Dominique Proust (Observatoire de Paris, GEPI)