Weighing the Giants

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Clusters of galaxies are the largest gravitationally bound objects in the Universe. They consist of hundreds to thousands of galaxies (each itself consisting of billions of stars), vast amounts of hot, diffuse, X-ray emitting gas, and even more mass contained in the form of elusive dark matter. As the true giants of the Universe, the properties and evolution of galaxy clusters are highly sensitive to the precise amounts and nature of dark energy and dark matter, and are thus an invaluable probe of the 'dark sector' of our Universe. Because of this, surveys of galaxy clusters are one of the most active fields of observational cosmology today. To use clusters as cosmological probes requires accurate measurements of their total mass, most of which is dark.

Gravitational lensing, a result of Albert Einstein's insight that matter bends light, provides an elegant method to accurately measure all of the mass contained in galaxy clusters. By statistically analyzing the shapes of thousands of background galaxies, the mass of a cluster in the foreground between us and the galaxies can be measured, a method known as weak lensing and a technique that will be crucial to upcoming dark energy probes such as DES and LSST. The amount of distortion is sensitive to the distance of the lensed galaxy, which is difficult to estimate for the faint galaxies used in weak lensing analyses. Accurate estimates of cluster masses therefore require not only robust shape, but also distance measurements.

A team based at KIPAC and consisting of post-doc Anja von der Linden, graduate students Patrick Kelly and Doug Applegate, former post-doc Mark Allen, and professors Steve Allen, Patricia Burchat, and David Burke, along with colleagues from several other institutions in the US and Europe, has recently measured accurate masses for a sample of 51 of the most massive clusters known. This work represents a significant step forward in the number of clusters studied, as well as in testing and verifying the methodology employed. They used the fact that the distance to a galaxy is related to its color, estimating the distance to each galaxy by observing the clusters field in at least five filters, polling galaxy colors from the near-Ultraviolet to the near-Infrared. This technique of "photometric redshift" estimation is also crucial for dark energy probes. The team developed a new method to make optimal use of these distance estimators, and showed that this method works very well for the entire sample of clusters studied.

For this method to work, they needed to measure the relative brightness of galaxies in different filters. This required precise calibration of the imaging data, a challenging task for an observing effort that spanned eight years and two telescopes. The team developed algorithms to robustly calibrate their data without requiring additional, external data. These algorithms are publicly available and should benefit other observing projects such as supernovae studies, which themselves are an important component of dark energy probes.

The team aimed for and achieved a high standard of accuracy for their measurements by rigorously testing their procedures with simulations. To protect the integrity of their results, the team 'blinded' themselves from the cosmological implications of their measurements until the end. With accurate cluster mass measurements in hand, the team is now working on incorporating them into a full cosmological analysis based on clusters detected in X-ray surveys, thereby fully utilizing these clusters to study dark energy and dark matter.

This work is described in a series of papers submitted to the Monthly Notices of the Royal Astronomical Society and available from astro-ph at arXiv:1208.0597, arXiv:1208.0602, and 1208.0605. Research at KIPAC is supported by the Department of Energy, the Kavli Foundation, the National Aeronautics and Space Administration, the National Science Foundation and Stanford University, as well as private donors. We are grateful to each of these sponsors for their continued interest and support.

Science Contact:
Anja von der Linden
KIPAC
email: anja@slac.stanford.edu

Tidbit author: Anja von der Linden, with contributions by Doug Applegate and Jack Singal