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Saturday, March 19, 2011

Looking at the Bending of Light Around the Multiple Galaxy Collisions in "Pandora's Cluster"

A new arXiv paper has been released analyzing Pandora's Cluster of galaxies.
"Creation of cosmic structure in the complex galaxy cluster merger Abell 2744"

As you can see from the paper's title, the technical name for Pandora's Cluster is Abell 2744.  This area of the sky is in a state of extreme cosmic violence.  We're witnessing a rare situation where there are multiple galaxies smashing into each other with multiple collisions.

A team of scientists have just taken more precise measurements of Pandora's Cluster.  Updated analysis of the data is the core purpose of the paper.  This new data is essentially a playground for astrophysicists and mathematicians. 

Image: Embedded in cited arXiv pdf, Fig 1. "The central field of Abell 2744 as revealed by VLT VRI imaging..."

The bulk of this analysis was focused on Gravitational Lensing.  By examining very carefully the bending of light around massive objects, you can further calculate the mass and location of the matter causing the lensing.  When you hear that scientists have found indirect evidence for Dark Matter, the biggest phenomena they're referring to is quantifiable gravitational lensing.  Light bends around matter, so the (dark) matter therefore has to be there when light is seemingly bent by nothing.  It's sneakily hiding from the light that we use to gather our cosmic information.

This has been seen in Pandora's Cluster, as shown in the paper.  Using gravity lensing, they map out Pandora's Cluster in more detail, inferring where the dark matter is.  Both the green and red circles here shows the inferred presence of dark matter in Pandora's Cluster.

The breakdown of what this image actually is, is fascinating:

Image: Embedded in cited arXiv pdf, Fig 7. 
"The different merging bodies in the field of Pandora’s cluster. The false-colour background is provided by HST/ACS, VLT and Subaru images..." "...Overlaid in cyan are the surface-mass density contours most concentrated in the ’core’ area and in magenta the more evenly-distributed X-ray luminosity contours. The positions of the dark matterclumps are indicated by the small circular green annuli, showing also an aggressive and a conservative error estimate on these positions. The small red circles show the position of the local overdensities in the gas distribution , associated with each individual dark matter clump. The white rulers show the separation between dark matter peaks and the bright clusters galaxies and local gas peaks."

 ArXiv is a publicly accessible website where scientists post academic papers.  New papers are submitted here for the world to see, often even before publication.  You can sometimes find a gem of a story on here a week or more before it pops up elsewhere online.  They are a 1st generation resource, written by the scientists themselves.  Let me point out that arXiv can be browsed over and read by non-academics.  True, some papers are incredibly dense with technical language, but others are almost entirely layman-friendly.  The more you learn about a certain field, the more you pick up and understand when reading these papers.  Plus, some of them, like this one, have really awesome pictures.

A final gem of a picture from the paper, illustrating the results of their strong lensing data:

Image: Embedded in cited arXiv pdf, Fig 2. "Shown as a continuous white line is the critical curve of the cluster as it is derived from the strong-lensing model. It assumes a source redshift zs = 2.0. Also shown are the approximate positions of the identified multiple-image systems..."

J. Merten, D. Coe, R. Dupke, R. Massey, A. Zitrin, E. S. Cypriano, N. Okabe, B. Frye, F. Braglia, Y. Jimenez-Teja, N. Benitez, T. Broadhurst, J. Rhodes, M. Meneghetti, L. A. Moustakas, L. Sodre Jr., J. Krick, & J. N. Bregman (2011). Creation of cosmic structure in the complex galaxy cluster merger Abell
2744 MNRAS arXiv: 1103.2772v1

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  1. The abstract says "11 galaxies". Is there an indication on the photo above of the division between the original galaxies masses? Eleven colliding seems extraordinary. Why so many at this location, do we know?

  2. It's a good question. That part of the abstract can easily be misconstrued. What's they're looking at in the southern "core" clump of mass is 11 galaxies in the background, which supply us with the light sources that are actually bending. Here's a quote from page page 3 on Lensing Analysis:

    "We have identified strong gravitational lensing of 11 background galaxies producing 34 multiple images around the Southern core, with an Einstein radius of rE ∼ 30 (see below). These enable us to tightly constrain the position and shape of the core mass distribution."

    So far as collisions, I know there were at least 2. If there were more, I missed it in the jumble. If anyone knows for certain there, definitely feel free to chime in!

  3. If anyone is adept at math and would like further reading, this arXiv paper gives a lot of insight on the technical (mathematic) approach to the lensing effect. "Perturbative Approach to the Gravitational Lensing by a Non-spherically Distorted Compact Object"


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