Research Highlight
February 18, 2010
Secondary anisotropy of the Cosmic Microwave Background detected with the South Pole Telescope
by The SPT Team

Secondary anisotropy of the Cosmic Microwave Background detected with the South Pole Telescope
The 10-meter South Pole Telescope is being used to obtain high sensitivity and high angular resolution images of the cosmic microwave background (CMB) radiation over a large area of the sky. In previous studies the pristine view of the early universe provided by the CMB radiation has been used by astronomers to take a snapshot of the universe as it was 13.7 billion years ago, only a few hundred thousand years after the big bang. The SPT team has now turned this around and is looking for small angular scale distortions in the background radiation caused by the scattering of the fossil light off of the ionized gas associated with the most massive structures that have formed in the universe. By determining the level of this ''secondary'' CMB anisotropy, the SPT team has in turn provided an independent determination of the level of fluctuations in the distribution of the dark matter in the present universe.

On angular scales of a few arc minutes and smaller the background radiation is expected to be distributed over the sky extremely smoothly. However, on these small angular scales the spectrum of the background radiation can be distorted by scattering off of the electrons associated with patches of gas ionized by the first population of stars in the universe, and also by the large concentrations of hot ionized gas associated with clusters of galaxies. These effects are referred to as Sunyaev-Zel'dovich (SZ) effects after the two Russian astrophysicists who postulated their existence, shortly after the CMB was discovered over 40 years ago.

The SZ effect was first detected over two decades ago toward extremely massive clusters of galaxies --- clusters which had been discovered by optical or x-ray emission. The beauty of the using the SZ effect to look for previously unknown galaxy clusters is that its brightness does not diminish with distance. It is like looking for small shadows against the smoothly distributed background radiation. The effect is very weak, however, and massive galaxy clusters are extremely rare. The SPT with its high resolution and ability to survey large regions of sky reported the first discovery of galaxy clusters via the SZ effect in 2008 (<a target='_blank' href=''>Z. Staniszewski et al, 2009</a>). Figure 1 shows a three-band image of a massive galaxy cluster discovered by the SPT.

<b>Figure 1.</b> A three-band (95/150/220 GHz) image of a galaxy cluster discovered through its SZ effect by the SPT. The color scale in each band is +/- 200 micro-Kelvin, and the images are 20 arcminutes on a side. The cluster is clearly seen as decrement of CMB photons at 95 and 150 GHz and is not seen at 220 GHz. This is exactly the predicted spectral behavior of the SZ effect from the hot gas in galaxy clusters.

Lower mass clusters are much more common, but each one makes only a tiny distortion to the background light. Instead of searching for the SZ signal from individual low mass galaxy clusters -- which would be too weak to be detected in the SPT survey -- the SPT team analyzed the statistics of the variation in the intensity of the background radiation on arc minute angular scales. This is the same technique used to measure the primary anistropy of the background radiation on much larger angular scales. The SPT secondary anisotropy results are shown in Figure 2, along with other recent CMB anisotropy measurements.

<b>Figure 2.</b> Newly published SPT measurements of anistropy in the CMB, along with other recent anisotropy measurements by the WMAP satellite and the ground-based experiments ACBAR and QUaD. The SPT measurements show clear evidence of secondary anisotropy power, caused by the interactions of CMB photons with intervening structure in the universe. (Red solid line = best-fit model of total anisotropy; red dashed line = primary CMB anisotropy; black lines = contributions from the SZ effects; orange line: contributions from point sources.)

The researchers used the frequency information provided by SPT's two observing bands to separate the weak SZ signal from other sources of anisotropy power. They report the first detection of the diffuse SZ signal from analysis of the first 100 square degrees of their survey in a paper submitted to The Astrophysical Journal on December 23, 2009 (<a target='_blank' href=''>M. Lueker et al., 2009</a>).

The SPT detection of the diffuse SZ signal marks the end of a long hunt for the weak secondary anisotropy signal. The level of the SZ signal is strongly dependent on the distribution of dark matter in the universe. The SPT team is therefore able to use the result to constrain the present day distribution of the dark matter and compare it to predictions based on the standard, dark energy dominated cosmological model.

The SPT results hint that the current models may over-predict the level of the diffuse SZ secondary anisotropy from galaxy clusters. For the level of SZ signal attributed to patchy reionization from the first stars, the SPT team also reports a stringent upper limit that already rules out some models. The analysis of the full SPT survey when completed will allow precise measurement of the history of structure formation in the universe.