Space-Based Telescope Zooms in on ‘Invisible Galaxies’

070214_darkGalaxy_hmed5p.hmedium Five ancient galaxies so choked with dust that they are completely invisible at optical wavelengths have been spotted at submillimetre wavelengths by the European Space Agency's Herschel space telescope that  can detect bands of submillimetre light that are blocked by Earth's atmosphere. Observations of the galaxies' spectra suggest they are very distant, appearing as they were when the universe was just 2 to 4 billion years old, less than a third its present age when stars formed at about 100 times their current rate.

Young stars in the galaxies shed dust that blocked visible light from escaping into space, heating up the dust, causing it to radiate at infrared wavelengths. This radiation was stretched to longer wavelengths as space itself expanded, and by the time it reached the Herschel it was in the far-infrared and submillimetre range.


Ground-based observatories then snapped pictures of the sky around Herschel's finds. In each image, a galaxy lying much closer to Earth appeared in the same region of the sky, which suggested that the gravity of the nearby galaxies had bent and magnified the light from the more distant, dusty galaxies.

Team member Asantha Cooray of the University of California, Irvine, notes that there are fairly few bright sources of submillimetre light nearby, so "the brightest submillimetre sources are all gravitationally lensed". He estimates that a few hundred of these magnified galaxies could turn up by March 2011, when Herschel will complete a deep-space map of 1.5 per cent of the sky.

Because the lensed objects are magnified in size, they can be used to study how stars formed during this active period in the universe's history, Cooray says. They way their light has been bent can also be used to study the dark matter content of the galaxies responsible for the lensing.

Jason McManus via Science

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