In The Field

Antarctica 2010: Hunting for galaxy clusters

Posted on behalf of Chaz Firestone

After two days at McMurdo Station, we leave tomorrow for the South Pole, home of the new South Pole Telescope (SPT), which saw its first light in 2007. But before we head over there, I had the chance to speak with John Carlstrom, an astrophysicist at the University of Chicago who leads a research team at the SPT. We’ll hear from Carlstrom again later, but for now he offered a taste of the work his team is doing there: carlstrominterviewsm.JPG

The story of the Big Bang — and the cosmic microwave background radiation (CMBR) that is today a relic of that fateful explosion — will be well known to many readers, but the use of this radiation to detect galaxy clusters may not be. The CMBR has a wavelength in the millimeters to centimeters range and is often called the “afterglow” of the Big Bang, as scientists can study it to learn what the universe was like in its infancy. But like any light, the CMBR is also capable of casting shadows (or, rather, being obstructed to create a shadow), which is how the SPT uses it to detect clusters of galaxies.

Most of the time, light travels about the universe unperturbed, but when it passes through a mass of high-energy electrons, it can be distorted. In the vast expanse of space, galaxy clusters are hotbeds for this scattering of light, as they are essentially breeding grounds for the plasma that contains these ionized particles. So when the CMBR passes through a galaxy cluster, intra-cluster plasma distorts it, a phenomenon known as the Sunyaev-Zel’dovich (SZ) effect.

It was hypothesized some time ago that the SZ effect could be used to detect galaxy clusters, which would show up as “shadows” in the CMBR from the point of view of an earthbound telescope. In 2008, the South Pole Telescope swept the sky for CMBR and found these shadows, becoming the first telescope to use the SZ effect to detect galaxy clusters when it found four clusters, three of which had not been previously detected.

Conditions at the South Pole are ripe for this kind of astronomy. The atmosphere is thin at the Pole, where the altitude is 3,000 metres (10,000 feet), and because the sun is absent from the sky in the winter, the atmosphere there isn’t churned up on 24-hour cycles like it is at other parts of the planet. The extremely low temperatures also limit the amount of water vapor in the air, which reduced signal disruption. And if you think you could just make a similar telescope at the North Pole, think again — that pole is over water.

The SPT has so far surveyed 800 square degrees of the sky and will have surveyed 2,000 square degrees by 2011. The ultimate goal of the current project is to survey to 4,000 square degrees, which would be about a fifth of the area visible from the South Pole (the entire sky is 40,000 square degrees, but you can only see half the sky from the South Pole). Carlstrom said they expect to find hundreds and thousands of clusters that no one has ever seen.

Hopefully we’ll get to see the SPT tomorrow.

Image: Glenn Zorpette, of IEEE, interviewing John Carlstrom


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