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Astronomers wave big thermometer at universe

Tracking forming galaxies

The world's largest thermometer has just entered active service. This isn't a cue for all sorts of "ooh, Matron" remarks, since the thermometer in question is taking the temperature of billion-year-old clouds of gas, way "out there" in the universe.

the Galactic HII region RCW 120 in the visible and in the submillimetre

And although it is essentially a big thermometer, it should technically be referred to as a Bolometer. And the aim is to use the new camera to find out more about the formation of the very first stars and galaxies.

"A large fraction of all the gas in the Universe has extremely cold temperatures of around minus 250 degrees Celsius, a mere 20 degrees above absolute zero," says Karl Menten, director at the Max Planck Institute for Radioastronomy (MPIfR) in Germany. "Studying these cold clouds requires looking at the light they radiate in the submillimetre range, with very sophisticated detectors."

Broadly, a bolometer works by detecting tiny changes in temperature caused by radiation hitting a foil sheet. To be sensitive to the tiny changes it is looking for, the camera needs to be kept extremely cold: less than 0.3 degrees above absolute zero.

That kind of temperature takes liquid helium, a tricky enough substance to work with at sea level, let alone at the 5,100m altitude of the 12m APEX telescope that houses the camera.

But if the telescope were not high on the Chajnantor plateau in the Chilean Andes, in the dry clean air, the signal detection would be all but impossible because the signals the researchers are looking for are readily absorbed by water vapour. One of the researchers describes it as "trying to see stars during the day".

The European Southern Observatory, which runs the site, has details of the specification: A bolometer camera combines many tiny bolometer units into a matrix, much like the pixels are combined in a digital camera. LABOCA observes at the submillimetric wavelength of 0.87 mm, and consists of 295 channels, which are arranged in 9 concentric hexagons around a central channel. The angular resolution is 18.6 arcsec, and the total field of view is 11.4 arcmin, a remarkable size for instruments of this kind.

The team is pleased with the performance of the kit so far, saying that the first observations have revealed the camera's great potential. ®

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