Research Highlight
January 7, 2009
Galaxy clusters discovered with the South Pole Telescope
by the SPT group at KICP
The first major scientific results from the South Pole Telescope initial survey were released on October 10, 2008. A paper submitted to the Astrophysical Journal describes the detection of four distant, massive clusters of galaxies in an initial analysis of South Pole Telescope (SPT) survey data. Three of these galaxy clusters were previously unknown systems and, therefore, represent the first clusters detected using observations of the interaction between the hot gas in the cluster and Cosmic Microwave Background photons --- a process known as the Sunyaev-Zel'dovich (SZ) effect. These first four galaxy clusters are the most significant SZ detections from a subset of the ongoing SPT survey. As such, they serve as a demonstration that SZ surveys, and the SPT in particular, can be an effective means for finding galaxy clusters.

Figure: The South Pole Telescope during Austral winter. (Photo credit: Keith Vanderlinde)

The South Pole Telescope (SPT) is a 10-meter-diameter telescope operating at the National Science Foundation (NSF) South Pole research station. The telescope is designed for conducting large-area millimeter and sub-millimeter wave surveys of faint, low contrast emission, as required to map primary and secondary anisotropies in the cosmic microwave background. The SPT is a collaboration among several institutions including the University of Chicago / KICP, Cardiff University, Case Western Reserve University, Harvard-Smithsonian Astrophysical Observatory, McGill University, University of California at Berkeley, University of California at Davis, University of Colorado at Boulder, University of Illinois at Urbana-Champaign.

Figure: The SPT collaboration.

KICP members participating in the South Pole Telescope collaboration include KICP faculty John Carlstrom (PI), Wayne Hu, Andrey Kravtsov, Steve Meyer, Steve Padin (project manager), and Clem Pryke; postdocs Brad Benson, Clarence Chang, Tom Crawford, Jeff McMahon, Jared Mehl, Kathryn Schaffer, and Eric Switzer; and graduate students Lindsey Bleem, Abby Crites, Ryan Keisler, and Joaquin Vieira.

Figure: Optical pseudo-color images using data from the Blanco Cosmology Survey.

A major goal of the SPT project is to measure the abundance of massive galaxy clusters throughout the history of the universe. The SPT will detect galaxy clusters through their interaction with photons from the deepest reaches of space (the primordial Cosmic Microwave Background (CMB) photons). In this interaction --- known as the Sunyaev-Zel'dovich (SZ) effect, after the scientists that first predicted it --- photons from the CMB are scattered to slightly higher energies by electrons in the hot intracluster gas. Because this technique for finding clusters uses the CMB as a backlight, it can find clusters out to arbitrarily large distances. The SPT survey will find all clusters above a certain mass limit in the ~10% of the sky targeted by the survey, and this cluster sample will have the power to greatly increase our understanding of dark energy. Dark energy affects the expansion rate of the universe, impacting both the growth of structure and the volume of the universe, making the abundance of massive structures in the universe (such as galaxy clusters) a sensitive probe of the nature of dark energy.

Figure: Images of the first four SPT cluster detections in each of the three SPT observing bands.

Until recently, the technique of finding galaxy clusters through the SZ effect was only a promising idea; no clusters had been actually discovered this way. On October 10, 2008, the SPT team reported the first galaxy clusters discovered in an SZ survey, and images of the discovery are shown at left. The four clusters shown are the highest-significance detections in the first 40 square degrees of the SPT survey, and three of the clusters are new discoveries. These distance between these clusters and us is estimated to range from roughly one quarter to over one half of the size of the visible universe.

Figure: The SPT receiver and optics cryostats being prepared for installation at the telescope focus.

Figure: Close-up of one of the six wedges of detectors that make up the SPT focal plane. The detectors were fabricated at UC Berkeley.

The SPT project is funded primarily by the Office of Polar Programs of the National Science Foundation. Additional funding is provided by the Kavli Foundation and the Gordon and Betty Moore Foundation.

Press & paper links:

South Pole Telescope website.

Galaxy clusters discovered with a Sunyaev-Zel'dovich effect survey, Z. Staniszewski et al., 2008. ApJ, submitted.

South Pole Telescope Optics, 2008ApOpt..47.4418P.

The South Pole Telescope, Proc. SPIE, Vol. 5498, p 11-29, 2004.

Reading ripples in the cosmic microwave background, Ron Cowen, Science News. December 10, 2008.