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TOPIC: Dark matter
Blobrana


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RE: Dark matter
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Title: Discovery of a Ringlike Dark Matter Structure in the Core of the Galaxy Cluster Cl 0024+17
Authors: M.J. Jee, H.C. Ford, G.D. Illingworth, R.L. White, T.J. Broadhurst, D.A. Coe, G.R. Meurer, A. Van Der Wel, N. Benitez, J.P. Blakeslee, R.J. Bouwens, L.D. Bradley, R. Demarco, N.L. Homeier, A.R. Martel, S. Mei

We present a comprehensive mass reconstruction of the rich galaxy cluster Cl 0024+17 at z~0.4 from ACS data, unifying both strong- and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies (~120 per square arcmin) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly-lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of Cl 0024+17 obtained in such a way is remarkable. It reveals a ringlike dark matter substructure at r~75" surrounding a soft, dense core at r~50". We interpret this peculiar sub-structure as the result of a high-speed line-of-sight collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated by the cluster velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the pre-collision cores. Cl 0024+17 can be likened to the bullet cluster 1E0657-56, but viewed $along$ the collision axis at a much later epoch. In addition, we show that the long-standing mass discrepancy for Cl 0024+17 between X-ray and lensing can be resolved by treating the cluster X-ray emission as coming from a superposition of two X-ray systems. The cluster's unusual X-ray surface brightness profile that requires a two isothermal sphere description supports this hypothesis.

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Blobrana


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This rich galaxy cluster, catalogued as Cl 0024+17, is allowing astronomers to probe the distribution of dark matter in space. The blue streaks near the centre of the image are the smeared images of very distant galaxies that are not part of the cluster. The distant galaxies appear distorted because their light is being bent and magnified by the powerful gravity of Cl 0024+17, an effect called gravitational lensing.



Dark matter cannot be seen because it does not shine or reflect light. Astronomers can only detect its influence by how its gravity affects light. By mapping the distorted light created by gravitational lensing, astronomers can trace how dark matter is distributed in the cluster. While mapping the dark matter, astronomers found a dark-matter ring near the cluster's center. The ring's discovery is among the strongest evidence that dark matter exists.
The Hubble observations were taken in November 2004 by the Advanced Camera for Surveys.

Galaxy Cluster Cl 0024+17, ZwCl 0024+1652
Position (J2000): R.A. 00h 26m 35s Dec. +17° 09' 43"

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-- Edited by Blobrana at 20:30, 2007-05-15

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Blobrana


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Dark matter makes up most of the Universes material. Ordinary matter, which makes up stars and planets, comprises only a few percent of the Universes matter.
Tracing it is not an easy task, because it does not shine or reflect light. Its influence can be detected by its gravitational effects on light. To locate dark matter, astronomers study how faint light from more distant galaxies is distorted or bent into arcs and streaks by the gravity of the dark matter in the foreground. This technique is called gravitational lensing. By mapping the distorted light, the mass of the cluster and the distribution of dark matter can be deduced.

"The collision between the two galaxy clusters created a ripple of dark matter which left distinct footprints in the shapes of the background galaxies. It is comparable to looking at pebbles on the bottom of a pond with ripples on the surface. The shape of the pebble appears to change as the ripples pass over them. In a similar manner, galaxies behind the ring show changes in their shape due to the presence of the dense ring" - astronomer M. James Jee of Johns Hopkins University, USA.

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Blobrana


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A cloud of dark matter has been detected expanding like a smoke ring from a giant collision between galaxy clusters, a team of astronomers says. If confirmed, the ring could offer new clues to the nature of the mysterious matter.
Dark matter is an enigmatic material that does not emit, absorb or reflect light. It reveals itself only by the way its gravity influences normal matter around it and seems to outweigh the universe's normal matter by a factor of six.
Now, astronomers have discovered what looks like a ring of dark matter expanding from a cosmic clash involving two massive galaxy clusters. If confirmed, the ring will help astronomers investigate how dark matter behaves when disturbed, perhaps providing hints to its nature.
Myungkook James Jee of Johns Hopkins University in Maryland, US, led the team of researchers that made the discovery. They detected the ring in Hubble Space Telescope (HST) observations of a galaxy cluster called Cl 0024+17, which is 5 billion light years from Earth in the direction of the constellation Pisces.
They carefully observed how matter in the cluster bends the paths of light rays coming from distant background galaxies. This 'gravitational lensing' effect allowed them to map out the distribution of matter, including dark matter, in the cluster.

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Blobrana


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Hubble Finds Ghostly Ring of Dark Matter
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The most common substance in the universe is called dark matter. It doesnt shine or reflect light. We cant even see it.

DarkmatterRing2
Expand (1.42mb, 2400 x 3000)
Credit: NASA, ESA, M.J. Jee and H. Ford (Johns Hopkins University)

This Hubble Space Telescope composite image shows a ghostly 'ring' of dark matter in the galaxy cluster Cl 0024+17.

It is an invisible substance composed of atoms that are far different from those that make up the universes normal matter, such as stars and galaxies.
In fact, if you drove into a wall made of dark matter, you wouldnt crack a headlight or inflate an airbag. You wouldnt even know it happened. But what happens to dark matter during a collision?
Astronomers using NASAs Hubble Space Telescope got a first-hand view of how dark matter behaves during a titanic collision between two galaxy clusters. The wreck created a ripple of dark matter, which is somewhat similar to a ripple formed in a pond when a rock hits the water.
The ring's discovery is among the strongest evidence yet that dark matter exists. Astronomers have long suspected the existence of the invisible substance as the source of additional gravity that holds together galaxy clusters. Such clusters would fly apart if they relied only on the gravity from their visible stars. Although astronomers don't know what dark matter is made of, they hypothesize that it is a type of elementary particle that pervades the universe.
The ring-like structure is evident in a composite image of the cluster made from Hubble observations. The ring can be seen in the blue map of the clusters dark matter distribution, which is superimposed on an image of the cluster.
The Hubble astronomers say it is the first time they have detected dark matter as having a unique structure that is different from the gas and galaxies in the cluster. The researchers spotted the ring unexpectedly while they were mapping the distribution of dark matter within the galaxy cluster Cl 0024+17 (ZwCl 0024+1652), located 5 billion light-years from Earth. The ring measures 2.6 million light-years across.

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Blobrana


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Galaxy cluster CL0024+1654
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Astronomers have created the first detailed map of dark matter in a cluster of galaxies. The map indicates that such clusters grow by swallowing individual galaxies, as well as providing a timely reality check for cosmological theories of dark matter.
The galaxy cluster studied, CL0024+1654, is one of the largest structures in the Universe. It is 4.5 billion light years away and, although too faint to be seen with the naked eye, spans an area of sky as large as the full Moon.

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Blobrana


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CL0024+17
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Title:    Discovery of a Dark Matter Ring in the Core of the Galaxy Cluster CL0024+17 at z=0.4
Authors:    Jee, Myungkook J.; Ford, H. C.; Illingworth, G. D.; White, R. L.; Broadhurst, T. J.; Coe, D. A.; Meurer, G. R.; van der Wel, A.; ACS Science Team

We present a comprehensive mass reconstruction of the z = 0.4 rich galaxy cluster CL0024+17 from Advanced Camera for Surveys data, unifying both strong and weak-lensing constraints. The weak-lensing signal from a dense distribution of background galaxies ( 120 per arcminČ) across the cluster enables the derivation of a high-resolution parameter-free mass map. The strongly-lensed objects tightly constrain the mass structure of the cluster inner region on an absolute scale, breaking the mass-sheet degeneracy. The mass reconstruction of CL0024+17 obtained in such a way is remarkable. It reveals a ring-like dark matter substructure at r 75" surrounding a soft, dense core at r<50". We interpret this peculiar sub-structure as the result of a high-speed line-of-sight collision of two massive clusters 1-2 Gyr ago. Such an event is also indicated by the cluster bimodal velocity distribution. Our numerical simulation with purely collisionless particles demonstrates that such density ripples can arise by radially expanding, decelerating particles that originally comprised the pre-collision cores. ACS was developed under NASA contract NAS5-32865, and this research was supported by NASA grant NAG5-7697.

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Blobrana


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RE: Dark matter
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Title: Self Interacting Dark Matter in the Solar System
Authors: Avijit K. Ganguly, Pankaj Jain, Subhayan Mandal, Sarah Stokes
(version v2)

Weakly coupled, almost massless, spin 0 particles have been predicted by many extensions of the standard model of particle physics. Recently, the PVLAS group observed a rotation of polarisation of electromagnetic waves in vacuum in the presence of transverse magnetic field. This phenomenon is best explained by the existence of a weakly coupled light pseudoscalar particle. However, the coupling required by this experiment is much larger than the conventional astrophysical limits. Here we consider a hypothetical self-interacting pseudoscalar particle which couples weakly with visible matter.
Assuming that these pseudoscalars pervade the galaxy, we show that the solar limits on the pseudoscalar-photon coupling can be evaded.

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Blobrana


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CL0024+17
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NASA will hold a media teleconference at 1 p.m. EDT on May 15 to discuss the strongest evidence to date that dark matter exists. This evidence was found in a ghostly ring of dark matter in the cluster CL0024+17, discovered using NASA's Hubble Space Telescope. The ring is the first cluster to show a dark matter distribution that differs from the distribution of both the galaxies and the hot gas. The discovery will be featured in the May 15 issue of the Astrophysical Journal.

Briefing participants are:
-- Dr. Myungkook James Jee, associate research scientist, Johns Hopkins University, Baltimore
-- Dr. Richard White, astronomer, Space Telescope Science Institute, Baltimore

Images and supporting graphics will be posted on the Web at: http://www.nasa.gov/mission_pages/hubble/news/dark_matter_ring.html

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Blobrana


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Dark matter and the first stars
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Title: Dark matter and the first stars: a new phase of stellar evolution
Authors: Douglas Spolyar, Katherine Freese, and Paolo Gondolo

A mechanism is identified whereby dark matter (DM) in protostellar halos dramatically alters the current theoretical framework for the formation of the first stars. Heat from neutralino DM annihilation is shown to overwhelm any cooling mechanism, consequently impeding the star formation process and possibly leading to a new stellar phase.
A dark star may result: a giant ( ~ > 1 AU) hydrogen-helium star powered by DM annihilation instead of nuclear fusion, and detectable via annihilation products (gamma-rays, neutrinos, antimatter) possibly in combination with hydrogen lines.

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