Observations of faint and distant galaxy groups made with the European Space Agency's XMM-Newton observatory have been used to probe the evolution of dark matter. The results of the study are reported in the 20 January issue of The Astrophysical Journal.

Dark matter is a mysterious, invisible constituent of the Universe which only reveals itself through its gravitational influence. Understanding its nature is one of the key open questions in modern cosmology.

In one of the approaches used to address this question astronomers use the relationship between mass and luminosity that has been found for clusters of galaxies which links their X-ray emissions, an indication of the mass of the ordinary (baryonic) matter alone, and their total masses (baryonic plus dark matter) as determined by gravitational lensing.

To date the relationship could only be established for nearby clusters. New work by an international collaboration, including the Max Planck Institute for Extraterrestrial Physics (MPE), the Laboratory of Astrophysics of Marseilles (LAM), and Lawrence Berkeley National Laboratory (Berkeley Lab), has made major progress in extending the relationship to more distant and smaller structures than was previously possible.

To establish the link between X-ray emission and underlying dark matter, the team used one of the largest samples of X-ray-selected groups and clusters of galaxies, produced by the ESA's X-ray observatory, XMM-Newton.

Groups and clusters of galaxies can be effectively found using their extended X-ray emission on sub-arcminute scales. As a result of its large effective area, XMM-Newton is the only X-ray telescope that can detect the faint level of emission from distant groups and clusters of galaxies.

"The ability of XMM-Newton to provide large catalogues of galaxy groups in deep fields is astonishing," said Alexis Finoguenov of the MPE and the University of Maryland, a co-author of the Astrophysical Journal (ApJ) paper.

Since X-rays are the best way to find and characterise clusters, most follow-up studies have until now been limited to relatively nearby groups and clusters of galaxies.

"Given the unprecedented catalogues provided by XMM-Newton, we have been able to extend measurements of mass to much smaller structures, which existed much earlier in the history of the Universe," says Alexie Leauthaud of Berkeley Lab's Physics Division, the first author of the ApJ study.

Mass as a lens
Gravitational lensing occurs because mass curves the space around it, bending the paths along which rays of light travel: the more mass (and the closer it is to the centre of mass), the more space bends, and the more the image of a distant object is displaced and distorted. Thus measuring distortion, or 'shear', is key to measuring the mass of the lensing object.

In the case of weak gravitational lensing (as used in this study) the shear is too subtle to be seen directly, but faint additional distortions in a collection of distant galaxies can be calculated statistically, and the average shear due to the lensing of some massive object in front of them can be computed. However, in order to calculate the lens's mass from average shear, one needs to know its centre.

"The problem with high-redshift (i.e. very distant) clusters is that it is difficult to determine exactly which galaxy lies at the centre of the cluster," says Leauthaud. "That's where X-rays help. The X-ray luminosity from a galaxy cluster can be used to find its centre very accurately."

Knowing the centres of mass from the analysis of X-ray emission, Leauthaud and colleagues could then use weak lensing to estimate the total mass of the distant groups and clusters with greater accuracy than ever before.

The final step was to determine the X-ray luminosity of each galaxy cluster and plot it against the mass determined from the weak lensing, with the resulting mass-luminosity relation for the new collection of groups and clusters extending previous studies to lower masses and higher redshifts.

Within calculable uncertainty, the relation follows the same straight slope from nearby galaxy clusters to distant ones; a simple consistent scaling factor relates the total mass (baryonic plus dark) of a group or cluster to its X-ray brightness, the latter measuring the baryonic mass alone.

"By confirming the mass-luminosity relation and extending it to high redshifts, we have taken a small step in the right direction toward using weak lensing as a powerful tool to measure the evolution of structure," says Jean-Paul Kneib a co-author of the ApJ paper from LAM and France's National Center for Scientific Research (CNRS).

In the beginning
The origin of galaxies can be traced back to slight differences in the density of the hot, early Universe; traces of these differences can still be seen as minute temperature differences in the cosmic microwave background (CMB).

"The variations we observe in the ancient microwave sky represent the imprints that developed over time into the cosmic dark-matter scaffolding for the galaxies we see today," says George Smoot, director of the Berkeley Center for Cosmological Physics (BCCP), a professor of physics at the University of California at Berkeley, and a member of Berkeley Lab's Physics Division.

Smoot shared the 2006 Nobel Prize in Physics for measuring anisotropies in the CMB and is one of the authors of the ApJ paper. "It is very exciting that we can actually measure with gravitational lensing how the dark matter has collapsed and evolved since the beginning."

One goal in studying the evolution of structure is to understand dark matter itself, and how it interacts with the ordinary matter we can see. Another goal is to learn more about dark energy, the mysterious phenomenon that is pushing matter apart and causing the Universe to expand at an accelerating rate. Many questions remain unanswered: Is dark energy constant, or is it dynamic? Or is it merely an illusion caused by a limitation in Einstein's General Theory of Relativity?

The tools provided by the extended mass-luminosity relationship will do much to answer these questions about the opposing roles of gravity and dark energy in shaping the Universe, now and in the future.

Reference publication - Leauthaud, A., Finoguenov, A., Kneib, J.-P., et al., A Weak Lensing Study of X-Ray Groups in the COSMOS Survey: Form and Evolution of the Mass-Luminosity Relation, The Astrophysical Journal, Vol. 709, Number 1, 2010 January 20, pp. 97-114 DOI: 10.1088/0004-637X/709/1/97