The evolution of galaxies in the early Universe is increasingly studied in the infrared. In the near-infrared, scientists observe the redshifted light from stars in these galaxies, and in the far-infrared and submillimetre range, the light which has been swallowed by dust and then re-emitted. Many galaxies even emit up to 90 percent of their luminosity in the far-infrared. PACS operates at far-infrared wavelengths of up to 210 micrometres.
The new instrument uses supersensitive detectors that were especially developed for the application: two bolometer arrays in the camera, and two photoconductor arrays in the spectrometer. 'It works either as a quasi colour camera or as a spectrometer, depending on which sector of the optics is used,' PACS principal investigator Albrecht Poglitsch explains. While the camera is able to take pictures in three wavelength ranges, which can then be combined into a far-infrared colour photo, the spectrometer splits the light in each pixel of the studied sky section into a great many pure spectral colours.
For developing and building PACS, the MPE scientists gain about 650 hours of guaranteed observing time with Herschel. They will use this time to run two key projects: the PACS Evolutionary Probe ('PEP'), and the Survey with Herschel of the ISM in Nearby Infrared Galaxies ('SHINING'). While PEP uses the photometric features of PACS, SHINING is mainly a spectroscopic programme, so that both modes of operation of the instrument are used in the MPE projects.
In spectroscopy, the light emitted by objects in the Universe is split into its constituent colours, so that its composition can be studied in detail. From the spectral lines, the astrophysicists can infer the temperature, density and nature of the radiation source, the intensity and hardness of the radiation, and the velocity distribution. The goal of the SHINING project is to study star formation and activity in nearby galaxies in detail with high spatial resolution.
PEP is intended to resolve the cosmic infrared background - the combined emission of all distant infrared galaxies - into its constituents, and define in detail which sources emit this infrared radiation. After all, this 'background' contains as much energy as the visible starlight as a whole! Moreover, the project will explore the cosmic history of the development of star formation, and will examine how stellar evolution is influenced by the number of galaxies in a volume. Another goal of PEP is to measure the infrared radiation and energetics of known galaxy populations.
In addition to these two projects with guaranteed observing time, the MPE participates with about 370 hours of open observing time in a project on the trans-Neptunian region. The trans-Neptunian objects are remnants of the dust out of which our planets formed. Important physical attributes like the dimensions, density, and surface characteristics will be defined for more than 100 trans-Neptunian objects, in order to infer statistical information on the totality of these objects.
After the completion of additional tests on the Ariane 5 ECA in recent days, Herschel and the Planck satellite, which will be carried into space together, are now waiting on the carrier rocket ready for launch. 'After ten years of developing PACS, which were characterised by hard work, often unexpected challenges, but also the will to cope with them, the MPE Infrared Group has achieved an important aim,' says Eckhard Sturm (MPE). 'The PACS team of the MPE is looking forward to the launch of Herschel/PACS and the great data to come soon,' Sturm's colleague Dieter Lutz adds. 'The start of this satellite will be a special moment for the MPE.'