In September 2002, the Potsdam Multi-Aperture Spectrophotometer (PMAS) was used to observe a galaxy at redshift z=3.15 associated with an absorption system in the quasar called Q2233+131. As the light from the QSO travels towards us some of it is absorbed in clouds of neutral hydrogen. If a cloud has a column density larger than 2*10¹⁰ cm^-2, the profile of the absorption line shows broad damped wings, and is termed a Damped Lyman-Alpha (DLA) line. Spatially close to the line of sight towards the QSO, a galaxy was known previously to be responsible for the absorption cloud, i.e. it had the same redshift as the DLA line in the QSO.

The PMAS instrument, attached to the 3.5-m telescope on Calar Alto /Spain, allows the observer to obtain images and spectra simultaneously.

The left hand figure shows an image of the Q2233+131 created from the spectra and the right hand plot shows the corresponding spectrum created from the image. Confused now?
The PMAS data consists of 256 spectra, each of which represents an area of the sky of the size 0.5*0.5 arcsec. From these spectra it is possible to select any given wavelength interval, and create an image such as seen above. Each pixel in this image represents the location of one spectrum. Adding the spectra from any of the selected pixels in this image can result in a one-dimensional spectrum such as shown above. One can see the broad absoption line which is denoted by the "DLA".

Selecting this way the wavelengths corresponding to the DLA line in the above spectrum, shows an extended object having a size of 3*5 arcsec. At these wavelengths the light from the background QSO has been totally absorbed by the cloud in front of it. The emission seen here is caused by Lyman-alpha photons emitted by the galaxy, which position is marked by the black star in the image. Adding all the pixels associated with the object results in the spectrum shown above, and the location of the emission line is at the exact location of the DLA line in the quasar spectrum.

Why is this so interesting?
Having a number of spectra available from the integral field spectra, we can evaluate the velocity structure of the nebula, we can determine the origin of it, and investigate its association with the parent galaxy. At the redshift of the system, the age of the Universe was only 2 giga years, but the parent galaxy is much younger at that time - possibly only a few times 10⁷ years. Specifically, the study of DLA systems can reveal how the major part of galaxy formation proceeded in the high redshift Universe. Such a study as presented here would have been extremely time-consuming with other instruments than integral field spectrographs.

Credits: Lise Christensen.

The results of this investigation will soon be submitted to the scientific journal "Astronomy and Astrophysics".