Looking Towards the Great Attractor The Great Attractor is a gravity anomaly in intergalactic space within the vicinity of the Hydra-Centaurus Supercluster at the center of the Laniakea Supercluster that reveals the existence of a localized concentration of mass tens of thousands of times more massive than the Milky Way. This mass is observable by its effect on the motion of galaxies and their associated clusters over a region hundreds of millions of light-years across. There’s a region of space about 150 million light years away that is gravitationally attracting the galaxies in its region, including ours. It is known as the great attractor, and we’re not entirely sure what’s there. The problem is it happens to lie in the direction of the zone of avoidance, so our own galaxy is blocking our view. When we measure the motion of our galaxy relative to other nearby galaxies, we find that the Milky Way is moving in the general direction of the great attractor at a speed of about 2.2 million kilometers per hour. Observations of other galaxies also indicated motion in that direction, which gave rise to idea that a large concentration of galaxies must exist within that region. But since the great attractor is within the zone of avoidance, it was difficult to determine what is there. The zone of avoidance is the region of sky where the plane of our galaxy is located. Because of the gas and dust in the plane it is difficult to see beyond our galaxy in that direction. But gas and dust obscure some wavelengths more than others. Radio, infrared and x-rays, for example, can penetrate the region more readily, so with the rise of astronomy at these wavelengths (particularly x-rays) we began map the region of the great attractor. By the 1980s we were able to pin down the location of the great attractor, and found that it was centered at a large cluster of galaxies known as the Norma cluster. Later observations found that this region doesn’t have enough mass to be the sole source of the attraction. Instead a larger supercluster known as the Shapley supercluster. It contains about 10,000 Milky Way sized galaxies, and is the largest known supercluster in the visible universe. So now we have an understanding of the clusters pulling our galaxy. These galaxies are all redshifted, in accordance with the Hubble Flow, indicating that they are receding relative to us and to each other, but the variations in their redshift are sufficient to reveal the existence of the anomaly. The variations in their redshifts are known as peculiar velocities, and cover a range from about +700 km/s to −700 km/s, depending on the angular deviation from the direction to the Great Attractor. Debate over apparent mass. In 1992, much of the apparent signal of the Great Attractor was attributed to the effect of Malmquist Bias. adsabs.harvard.edu/abs/1992ApJ...391..494L In 2005, astronomers conducting an X-ray survey of part of the sky known as the Clusters in the Zone of Avoidance (CIZA) project reported that the Great Attractor was actually only one tenth the mass that scientists had originally estimated. The survey also confirmed earlier theories that the Milky Way galaxy is in fact being pulled towards a much more massive cluster of galaxies near the Shapley Supercluster, which lies beyond the Great Attractor. The dark flow The dark flow is a purported velocity tendency of galaxies to move in a particular direction, which was formerly thought to be caused by the Great Attractor, but then theorized to be outside the observable universe. These findings were published in 2008 but were later contradicted by data from the Planck satellite. Laniakea Supercluster. The proposed Laniakea supercluster is defined as the Great Attractors basin, encompassing the former superclusters of Virgo and Hydra-Centaurus. Thus the Great Attractor would be the core of the new supercluster. Image 1: Credit: ESA/ESO Great Attractor. This image shows the central region of the Great Attractor at the original resolution of the WFI. Note the strong warping of the galaxy above the center, which may be caused by gravitational interaction with one or both of the bright galaxies that are seen above and below it. Image 2: ESA/Hubble Space Telescope & NASA - nasa.gov/mission_pages/hubble/science/great-attractor.html The Hubble Telescope turned its lens to the region of the sky where the Great Attractor is located. Credit: EL telescopio ESA/Hubble dirigió sus ojos a la región del cielo donde se encuentra el Gran Atractor.: Image 3: Panoramic view of the entire near-infrared sky. The location of the Great Attractor is shown following the long blue arrow at bottom-right. Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond the Milky Way. The image is derived from the 2MASS Extended Source Catalog (XSC)—more than 1.5 million galaxies, and the Point Source Catalog (PSC)--nearly 0.5 billion Milky Way stars. The galaxies are color coded by redshift (numbers in parentheses) obtained from the UGC, CfA, Tully NBGC, LCRS, 2dF, 6dFGS, and SDSS surveys (and from various observations compiled by the NASA Extragalactic Database), or photo-metrically deduced from the K band (2.2 μm). Blue/purple are the nearest sources (z < 0.01); green are at moderate distances (0.01 < z < 0.04) and red are the most distant sources that 2MASS resolves (0.04 < z < 0.1). The map is projected with an equal area Aitoff in the Galactic system (Milky Way at center). IPAC/Caltech, by Thomas Jarrett - Large Scale Structure in the Local Universe: The 2MASS Galaxy Catalog, Jarrett, T.H. 2004, PASA, 21, 396 Source: Great Attractor BY BRIAN KOBERLEIN · https://briankoberlein/2014/01/28/great-attractor/ nature/nature/journal/v513/n7516/full/nature13674.html nasa.gov/mission_pages/hubble/science/great-attractor.html
Posted on: Sun, 02 Nov 2014 13:00:32 +0000
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