Farewell to Herschel: the space infrared telescope closes its eyes

Almost four years after being launched to space, the Herschel Space Observatory ran out of helium the past April 29. This implied losing the ability to stay as close as possible to absolute zero (at about 2K, indeed) which also meant the death of the spacecraft, since this mission was designed to observe the universe in the far infrared to sub-millimeter range of the spectrum (55 to 672 μm). In any case, this has been one of the most successful missions ever performed by the European Space Agency (ESA). It is quite difficult to select a few results from its astonishing amount of discoveries (up to date, more than 650 refereed papers and still growing) but I will try to highlight some of them, probably driven by my own biased interests. However, the rest of the mission results are out there for those willing to see the universe with infrared eyes ¹.

Herschel was basically a 3,5 m diameter telescope with three operating instruments having imaging and spectroscopical capabilities. The first instrument was the Heterodyne Instrument for the Far Infrared (HIFI), a very high resolution heterodyne spectrometer. We also had PACS (Photodetector Array Camera and Spectrometer) which was both an imaging photometer and a medium resolution spectrometer. Finally, the Spectral and Photometric Imaging Receiver (SPIRE) was an imaging Fourier transform spectrometer. This set of instruments were sent to the second Lagrange point. This allowed the mission to be always at the same position relative to Sun and Earth, but L2 location did not provide a complete shadowing to the spacecraft. This lead to the iconic design of the spacecraft with a nail-shaped protection shield for preventing the telescope to be heated by our star (see Figure 1). This way we were able to stare at the cosmos using a range of the spectrum which is mostly not covered by the ground-based telescopes.

When I think of the Herschel telescope, the image displayed in Figure 2 always comes to my mind. This is part of one of the Herschel’s guaranteed time projects, called HELGA (Herschel Exploitation of Local Galaxy Andromeda) ². Andromeda (or M31) is a giant spiral galaxy which dominates our Local Group together with our home galaxy, the Milky Way. It’s only 2.5 million light-years away, so its size in the sky is even bigger than the full moon. This makes it possible to study in detail with high-spatial resolution the dust emission from our neighbor. HELGA project was unique to understand the distribution of young and old stars as well as atomic and molecular gas clouds in M31. One of the strengths of the project was the combination with other data sources, as the XMM x-rays shown in Figure 2. Not only this project provided among the best images for public outreach during Herschel mission (not always an easy task when working at far infrared wavelengths!) but also had many cosmological implications on the amount of dust that it is possible to find at high red-shift galaxies.

Being a planetary scientist, the most moving Herschel result for me was the one presented by Hartogh et al. in a 2011 paper ³. They studied the sub-millimeter water emission lines from comet 103P/Hartley 2 to determine its deuterium to hydrogen ratio. Such a value resulted pretty similar to that of Earth’s oceans, suggesting that a considerable fraction of our planet’s volatiles could come from the Kuiper belt, the place where this comet of the Jupiter-family most likely was created. Our current models of Earth’s formation and evolution point to a dry proto-Earth which was later enriched in volatiles by local accretion or impacts. However, previous measurements of D/H ratios in comets were way too high and it was commonly accepted that comets could only provide less than 10% of Earth’s water and that most of it came from asteroids. This made present water content difficult to be reached, since the amount of potential contributors was relatively low. The results by Hartogh et al. changed the picture by showing that the Jupiter family of comets is also a potential water reservoir. And this makes the current content of volatiles in our beloved planet much more likely, something that even has astrobiological implications.

However, most of the significance of Herschel results is related to cosmology and extra-galactic astrophysics. Many of the Herschel surveys were designed to statiscally study the distribution of galaxies in the older universe. Some of these, like the Herschel Astrophysical Terahertz Large Area Survey were able to detect gravitational lenses in the sub-millimeter range. Other surveys, like the Herschel Multi-tiered Extragalactic Survey (HerMES), even without detecting individual galaxies, determined the angular power spectrum of the background intensity variations and thus gave average properties of the galaxy clustering ⁴. Amblard and collaborators, for example, found that such sub-millimeter galaxies are located in dark matter haloes with less than half a billion solar masses. This is about an order of magnitude lower than what was expected from galaxy formation theories since in such cases the star formation would be inefficient due to photoionization feedback. Given that the galaxies we are observing at the far infrared are the most active star-forming ones, this also implies a profound mismatch between observations and current models of galactic formation and evolution.

This is just a glimpse of the work developed by scientists and engineers involved in the Herschel mission. There are about 25,000 observation hours accumulated and we are just scratching the surface. From this behemoth archive lots of discoveries will surely follow. What can we expect? Herschel was particularly well suited to study dust and water. Dust as a clue to star formation and galactic evolution even at high redshift. And water as a clue for understanding the chemical evolution of the Universe, eventually leading to life formation. So from comets and new-born stars to ancient galaxies, Herschel has been tracing the elements that made it possible for us to be here.