As a result, star-forming regions in distant galaxies are one of the prime targets for the Atacama Large Millimetre/submillimetre Array. ALMA will consist of 66 high precision antennas, located on the Chajnantor plateau at 5000 meters altitude in northern Chile. The data from the individual antennas can be combined interferometrically, and the 15 kilometre span of the telescope provides resolution of better than a tenth of an arc-second. On its own, however, even this capability is not sufficient to make detailed pictures of young galaxies at the peak of their star formation.
"At a recent conference, ALMA scientists presented data they had used to verify the scientific capabilities of their array, among them an image of a strongly gravitationally lensed system, which immediately raised our interest", remembers Simona Vegetti, postdoctoral scientist at MPA. "Because of the lensing, the background galaxy is strongly magnified, by 17 times actually, which is why we can see it at all. Together with ALMA's unique angular resolution, this gave us the chance to try and see details in the distribution of dust in such a far-away galaxy for the first time."
Strong gravitational lensing happens when a background galaxy is closely aligned with a foreground mass concentration such as a cluster of galaxies, which bends light-rays from the source on their way to the observer. The foreground lens is, however, an imperfect optical system, leading to very large distortions (see Fig. 1). Nevertheless, from the properties of the lensed images, the mass distribution of the lensing system can be determined and a "true" (i.e. undistorted) image of the distant galaxy can be reconstructed. "Previous attempts to do this had assumed the background galaxies to be smooth and regular", explains Matus Rybak, who carried out the computer modelling at MPA. "This seems likely to be a very poor approximation to the structure of a strongly star-forming galaxy, and the raw ALMA images gave clear hints that this background source is complex. The new, more general approach we have developed is much better suited to irregular systems."