Astronomy Now Online


Planck and Herschel Exclusive Interviews

Goran Pilbratt

Herschel Project Scientist

From left to right: Tom Phillips, Hal Yorke, Pierre Encrenaz, Paul Harvey, Martin Harwit, Matt Griffin, Göran Pilbratt, Christoffel Waelkens, José Cernicharo, Albrecht Poglitsch, Jackie Fischer, Peter Barthel, and Thijs de Graauw. Missing in the picture Gerry Crone and Laurent Vigroux.

Could you explain the logistics of the “double launch” of Herschel and Planck?

The two spacecraft are two payloads on the same launcher and the launcher itself operates for 25 minutes, then two minutes later the Herschel spacecraft will be released, and a few minutes later the Planck spacecraft will be released. They are headed for similar but not the same orbits around the L2 point. Planck will inject itself into a smaller orbit which costs a lot in terms of fuel, but that is not necessary for Herschel, therefore Herschel will be in a lot bigger orbit.

Herschel and Planck will still be linked though, with Herschel destined to follow up on sources detected by Planck. What sort of sources are you expecting to follow up on?

There will be a multitude of sources, but the observations will mostly be for calibrations. They [the Planck mission team] want us to look at certain sources so they can calibrate their observations and from a more direct scientific point of view, we can provide deeper observations and observations with higher resolution for their sources. But this is not linked through the missions; Planck investigators will have to propose for Herschel observing time just like any astronomer would have to do, and we already have one such proposal.

The observing programs cover a wide range of topics. How are they selected?

Each observing program in a sense is free standing, people own a certain amount of observing time, they do their observations and they try to answers the issues they have identified in their proposals. The way the proposals are chosen is part of making sure that all aspects of what Herschel can do are addressed properly.

How far back will Herschel be able to see?
We will observe back to a few billion years after the big bang. Seeing the “first” stars and galaxies is an unfortunate term that has been used in several things that people have been writing. The very first stars and galaxies I don’t think we will have any chance to observe, but what is particularly important here, and where Herschel's contribution is particularly important, is the star formation history of the Universe. It is not flat but it had a peak; we don’t know when exactly it was, but there has to be a peak since we know it was greater in the past than it is now and of course we know there was no star formation in the beginning. We want to know where that peak is and what sort of galaxies contributed to that peak and so on. Star-formation, or star-burst galaxies, have been identified by other infrared missions but you need the wavelength coverage of Herschel to pick them up at a higher red shift because their emission peaks between 50 and 100 microns and that means that the peak is at the red shifts of a few, and this covers the spectrum of Herschel. This is one of the reasons why Herschel was built and designed to make these observations. Also, people wonder if all galaxies form in the same way. In particular, some people believe that the bulges of galaxies formed in like a big bang situation than by picking up smaller bits and pieces and adding them all together. If that is the case that probably went together with the big birth of star formation. If that is so we may be able to pick out galaxies that have not been seen yet if this is the way it happened.

Will you also make measurements of the infrared background?

The infrared background looks diffuse but in principle can be resolved into contributing sources like galaxies. We will be able to resolve more of the IR background than has ever been done before because of the simple fact we have a higher resolution at these wavelength.