Substructure Maps Show that Dark Matter Clumps in Galaxies

New Haven, CT — Hubble Space Telescope data, analyzed by a Yale
astronomer using gravitational lensing techniques, has generated a spatial
map demonstrating the clumped substructure of dark matter inside clusters of
galaxies.

Clusters of galaxies (about a million, million times the mass of our sun),
are typically made up of hundreds of galaxies bound together by gravity.
About 90 percent of their mass is dark matter. The rest is ordinary atoms in
the form of hot gas and stars.

Although little is known about it, cold dark matter is thought to have
structure at all magnitudes. Theoretical models of the clumping properties
were derived from detailed, high resolution simulations of the growth of
structure in the Universe. Although previous evidence supported the
“concordance model” of a Universe mostly composed of cold, dark matter, the
predicted substructure had never been detected.

In this study, Yale assistant professor of astronomy and physics Priyamvada
Natarajan and her colleagues demonstrate that, at least in the mass range of
typical galaxies in clusters, there is an excellent agreement between the
observations and theoretical predictions of the concordance model.

Using gravitational lensing made it possible for the observers to visualize
light from distant galaxies as it bent around mass in its way. This allowed
the researchers to measure light deflections that indicated structural
clumps in the dark matter.

“We used an innovative technique to pick up the effect of precisely the
clumps which might otherwise be obscured by the presence of more massive
structures,” said Natarajan. “When we compared our results with theoretical
expectations of the concordance model, we found extremely good agreement,
suggesting that the model passes the substructure test for the mass range we
are sensitive to with this technique.”

“We think the properties of these clumps hold a key to the nature of dark
matter – which is presently unknown,” said Natarajan. “The question remains
whether these predictions and observations agree for smaller mass clumps
that are as yet undetected.”

Co-author on the study, funded by Yale University, is Volker Springel, MPA,
Garching, Germany. Other collaborators include. Jean-Paul Knee, LAM – OAMP,
Marseille, France, Ian Smail, University of Durham, U.K., and Richard Ellis
of Caltech.

Citation: Astrophysical Journal Letters 617: L13-L16 (December 10, 2004)

There is an accompanying image (captioned: ” Hubble gravitational lensing
image of galaxies (yellow to red) and haloes from clumped dark matter
(blue)” located on the Yale Office of Public Affairs release website:
http://www.yale.edu/opa/newsr/05-01-06-02.all.html

• Priyamvada Natarajan: http://www.astro.yale.edu/priya/
• Yale Astronomy: http://www.astro.yale.edu/
• Yale Physics: http://www.physics.yale.edu/

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