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RE: The Great Attractor

The Great Attractor.kmz
Google Sky File

-- Edited by Blobrana at 16:47, 2008-01-12



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Large Scale Structures

Title: Large Scale Self-Similar Skeletal Structure of the Universe
Authors: Valentin A. Rantsev-Kartinov

An analysis of the redshift maps of galaxies and quasars has revealed large-scale self-similar skeletal structures of the Universe of the same topology which had been found earlier in a wide range of phenomena, spatial scales and environments. The "cartwheel" type of structure with diameter ~ 1.5 10^27 cm is discovered in this analysis by means of the method of multi-level dynamical contrasting. Similar skeletal structures in size up to 1.5 10^28 cm are found also in the redshift maps of quasars.

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Title: The Signature of Large Scale Structures on the Very High Energy Gamma-Ray Sky
Authors: A. Cuoco, S. Hannestad, T. Haugbølle, G. Miele, P. D. Serpico, H. Tu

If the diffuse extragalactic gamma ray emission traces the large scale structures of the universe, peculiar anisotropy patterns are expected in the gamma ray sky. In particular, because of the cutoff distance introduced by the absorption of 0.1-10 TeV photons on the infrared/optical background, prominent correlations with the local structures within a range of few hundreds Mpc should be present. We provide detailed predictions of the signal based on the PSCz map of the local universe. We also use mock N-body catalogues complemented with the halo model of structures to study some statistical features of the expected signatures. The results are largely independent from cosmological details, and depend mostly on the index of correlation (or bias) of the sources with respect to the large scale distribution of galaxies. The predicted signal in the case of a quadratic correlation (as expected e.g. for a dark matter annihilation contribution to the diffuse gamma flux) differs substantially from a linear correlation case, providing a complementary tool to unveil the nature of the sources of the diffuse gamma ray emission. The chances of the present and future space and ground based observatories to measure these features are discussed.

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Cluster-Supercluster Alignments

Title: Cluster-Supercluster Alignments
Authors: Jounghun Lee (Seoul Nat'l U.), August E. Evrard (U. Michigan)
(revised v2)

We study correlations in spatial orientation between galaxy clusters and their host superclusters using a Hubble Volume N-body realization of a concordance cosmology and an analytic model for tidally-induced alignments. We derive an analytic form for distributions of the alignment angle as functions of halo mass (M), ellipticity (epsilon), distance (r) and velocity (v) and show that the model, after tuning of three parameters, provides a good fit to the numerical results. The parameters indicate a high degree of alignment along anisotropic, collapsed filaments. The degree of alignment increases with M and epsilon while it decreases with r and is independent of v. We note the possibility of using the cluster-supercluster alignment effect as a cosmological probe to constrain the slope of the initial power spectrum.

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Largest full-sky, three-dimensional survey of galaxies

A team of American, Australian and British astronomers has released maps from the largest full-sky, three-dimensional survey of galaxies ever conducted.

Their detailed maps show the ‘local’ cosmos out to a distance of 600 million light years, identifying all the major superclusters of galaxies and voids. They also provide important clues regarding the distribution of the mysterious ‘dark matter’ and ‘dark energy’ which are thought to account for up to 96% of the apparent mass of the Universe.
Within this vast volume, the most massive galaxy supercluster is 400 million light years away. It was named after its identifier, the American astronomer Harlow Shapley. The Shapley supercluster is so big that it takes light at least 20 million years to travel from its one end to the other. However, Shapley is not the only massive supercluster in our vicinity.
The Great Attractor supercluster, which is three times closer than Shapley, plays a bigger role in the motion of our Galaxy. According to the team, our Milky Way galaxy, its sister galaxy Andromeda and other neighbouring galaxies are moving towards the Great Attractor at an amazing speed of about a million miles per hour. The researchers also established that the Great Attractor is indeed an isolated supercluster and is not part of Shapley.
The new maps are based on the observation that, as the Universe expands, the colours of galaxies change as their emitted light waves are stretched or “redshifted”. By measuring the extent of this redshift, astronomers are able to calculate approximate distances to galaxies.
The new survey, known as the 2MASS Redshift Survey (2MRS), has combined two dimensional positions and colours from the Two Micron All Sky Survey (2MASS), with redshifts of 25,000 galaxies over most of the sky. These redshifts were either measured specifically for the 2MRS or they were obtained from an even deeper survey of the southern sky, the 6dF Galaxy Redshift Survey (6dFGS).
The great advantage of 2MASS is that it detects light in the near-infrared, at wavelengths slightly longer than the visible light. The near-infrared waves are one of the few types of radiation that can penetrate gases and dust and that can be detected on the Earth’s surface. Although the 2MRS does not probe as deeply into space as other recent narrow-angle surveys, it covers the entire sky.

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Title: Reconstructed Density and Velocity Fields from the 2MASS Redshift Survey
Authors: Pirin Erdo¢gdu, Ofer Lahav, John P. Huchra., Matthew Colless, Roc M. Cutri, Emilio Falco. Teddy George, Thomas Jarrett, D. Heath Jones, Lucas M. Macri, Jeff Mader, Nathalie Martimbeau, Michael A. Pahre, Quentin A. Parker, Anais Rassat, Will Saunders

We present the reconstructed real-space density and the predicted velocity fields from the Two Mass Redshift Survey (2MRS). The 2MRS is the densest all-sky redshift survey to date and includes about 23,200 galaxies with extinction corrected magnitudes brighter than Ks = 11.25. Our method is based on the expansion of these fields in Fourier-Bessel functions.
Within this framework, the linear redshift distortions only affect the density field in the radial direction and can easily be deconvolved using a distortion matrix. Moreover, in this coordinate system, the velocity field is related to the density field by a simple linear transformation. The shot noise errors in the reconstructions are suppressed by means of a Wiener filter which yields a minimum variance estimate of the density and velocity fields.
Using the reconstructed real-space density fields, we identify all major superclusters and voids. At 50 h-1 Mpc, our reconstructed velocity field indicates a back-side infall to the Great Attractor region of vinfall = (491 ± 200)(β/0.5) kms-1 in the Local Group frame and vinfall = (64 ± 205)(β/0.5) kms^-1 in the cosmic microwave background (CMB) frame and β is the redshift distortion parameter. The direction of the reconstructed dipole agrees well with the dipole derived by Erdo¢gdu et al. (2006). The misalignment between the reconstructed 2MRS and the CMB dipoles drops to 13. at around 5000 kms^-1 but then increases at larger distances.

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The Great Attractor

A new survey by University of Hawaii astronomers has found that, in a tug-of-war of cosmic proportions, our Milky Way galaxy is being pulled toward the largest concentration of matter in the observable Universe.
This finding is being presented today by University of Hawaii graduate student Dale D. Kocevski and collaborators at the American Astronomical Society meeting held in Washington, D.C.

The University of Hawaii scientists used a new X-ray survey to determine what region is winning the tug-of-war: a massive association of galaxies over 500 million light-years away. The study shows that our galaxy's journey through space is not entirely due to the pull of nearby galaxies, but is affected by much farther regions of the Universe than previously thought.
Kocevski worked with Dr. Harald Ebeling, Dr. R. Brent Tully, both also with the University of Hawaii's Institute for Astronomy, and Dr. Chris R. Mullis, a UH alumnus who is now a research fellow at the University of Michigan.
Astronomers have long known that the Milky Way is moving toward the constellation Centaurus at a speed of 1.4 million mph, but the reason for the movement remained a topic of debate. Over 20 years ago, it was suggested that the motion was due to the gravitational pull of a nearby large concentration of matter dubbed the Great Attractor. The Great Attractor is what is known as a supercluster, that is, a group of clusters of galaxies, and was estimated to contain matter equal to more than over 10 million billion times the mass of the sun.

Expand (1.5mb, 3000 x 2426)
Position(2000): RA 13 27 38.9 Dec -31 32 23
This image of the core of the Shapley Supercluster shows a small portion of the thousands of galaxies that comprise Abell 3558, the galaxy cluster at the centre of the largest mass concentration in the observable Universe. Image taken with NASA's Hubble Space Telescope;
courtesy J. Blakeslee, Washington State University.

Until now, efforts to find the Great Attractor were hampered by its location in the "zone of avoidance," an area behind the plane of the Milky Way where gas and dust within our galaxy block much of the visible light from objects outside it. The new survey, Clusters in the Zone of Avoidance (CIZA), is the first to search for the X-ray signatures of galaxy clusters behind the Milky Way and investigate the nature of the Great Attractor. Due to the difficulty of observing through the Milky Way, this region was the final portion of the sky in which the cluster population had yet to be mapped.

"X-rays can penetrate even regions that are extremely obscured by gas and dust, and galaxy clusters are sources of X-rays. This is what prompted us to attempt to map the distribution of galaxy clusters behind the plane of the Milky Way using X-ray observations" - Dr. Harald Ebeling, who initiated the survey in 1998.

Kocevski and collaborators report finding far fewer massive cluster systems near the Great Attractor than would be expected given the region's proposed mass.

"One of our goals was to uncover the true mass of the Great Attractor. What we found is that it is not that great after all" - Dale D. Kocevski.

Instead, the CIZA team identified a significant concentration of galaxies behind the Great Attractor, near the Shapley Supercluster, which lies 500 million light-years away or four times the distance to the Great Attractor region. The Shapley Supercluster, first identified in 1930 by Harlow Shapley, is the most massive association of galaxies out of the 220 identified superclusters in the observable Universe. It contains the equivalent of nearly 10,000 Milky Ways, or four times the amount of mass currently observed in the Great Attractor region.

Two-dimensional projection of the cluster population within 800 million light-years of the Milky Way. Each blue halo represents a cluster of galaxies. Superclusters are located where multiple halos group together. The Milky Way's motion through space is due to a combination of the gravitational pull of the Great Attractor (small arrows) and the pull of the Shapley Supercluster, which produces a large-scale flow in which much of the Universe near our galaxy is streaming toward the more massive supercluster (large arrows)
Credit: IfA.

With the galaxy cluster population mapped over the entire sky for the first time, Kocevski analysed how all the clusters surrounding the Milky Way would affect it and found that only 44% of our galaxy's motion through space is due to the gravitational pull of galaxies in the nearby Great Attractor region. The remaining portion is the result of a large-scale flow in which much of the local Universe, including perhaps the Great Attractor itself, is being pulled toward the Shapley Supercluster.
The results confirm previous work, which suggested the Milky Way's motion was influenced by structures more distant than the Great Attractor, but this study is the first to reach this conclusion after having fully mapped the Great Attractor and regions behind it.
The finding resolves one of the long-standing problems associated with the Great Attractor. The presence of a massive overdensity relatively close to the Milky Way suggested that extreme mass concentrations such as the Great Attractor were fairly common in the Universe. This implied that the Universe contained much more matter than was measured by other means such as supernova Ia observations. The finding of a less massive Great Attractor and the large distance to the Shapley supercluster implies that extremely massive overdensities are rare in the Universe, which brings the suggested density of the Universe in line with the density established by independent means.

Preprints of papers submitted to The Astrophysical Journal on the Milky Way's motion and the CIZA survey can be found at:
* http://arxiv.org/abs/astro-ph/0510106
* http://arxiv.org/abs/astro-ph/0512321

This work was supported by the NASA Graduate Student Research Program.

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