The largest matter concentration on large scale of the local Universe
1er novembre 2005
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.
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)
Dernière modification le 4 mars 2013