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last change 2003 July 8, R. Arlt
Galaxy research
PMAS Observes Gravitational Lens

During September 2002, a team from AIP used the new Potsdam Multi-Aperture Spectrophotometer (PMAS) to observe a spectacular gravitational lens. The PMAS instrument, attached to the 3.5-m telescope on Calar Alto / Spain, allows the observer to obtain images and spectra simultaneously. This is done by catching the light at the telescope focal plane in thin glas fibres, which are then coupled to a high-performance spectrograph. Click on the image of the instrument below to learn more about PMAS.

A gravitational lens is a phenomenon that occurs when a background source and a foreground object show nearly perfect alignment with the observer. The light from the background source is then bent due to the gravitational pull by the foreground object, and the background source appears distorted or split into multiple components. In this case, the background source is a distant luminous quasar (called HE 0435-1223), and the foreground object is a single, very massive galaxy. Due to the very special alignment, it happens that we see the quasar four times - each quasar image is tweaked around the lensing galaxy on a slightly different light path.

At least, this was the expectation when the team went to the telescope to put the recently discovered quasar HE 0435-1223 under closer scrutiny. The test is really quite simple: If the four images are indeed just manifestations of one and the same background object, they should show identical spectra (within measurement accuracy).

However, classical spectrographs gathering their light through a simple slit have a hard time to deblend the light coming from such a complicated object. Not so for PMAS, which was designed to record full spectral information of everything within its field of view, simultaneously. Click on any of the images, and confirm that the four spectra are indeed very nearly identical. Notice in particular that the strong broad emission lines correspond to exactly the same quasar redshift (i.e. distance).

The reddish blob in the centre between the four quasar images is the foreground galaxy whose mass distorts the light rays of the quasar behind it. It is itself quite distant, and therefore faint. Nevertheless, PMAS has also caught enough light from this object that - for the first time - its redshift and therefore distance could be measured.

   The quasar HE 0435-1223 was discovered to be a gravitational lens in December 2001, on the basis of a high-resolution image taken with the 6.5-m Magellan telescope in Las Campanas, Chile. While this picture is somewhat superior in quality to the PMAS image shown above, it does not allow to obtain any spectroscopic information.

Credits: Lutz Wisotzki.

The results of this investigation have recently been accepted for publication in the scientific journal "Astronomy and Astrophysics".

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