Herschel loses its cool

A Cardiff instrument on Europe's Herschel Space Observatory has completed its scientific operations as the spacecraft has exhausted its supply of liquid helium coolant.

Cardiff University astronomers led the international team that designed, built and operate the SPIRE instrument on board Herschel. Using SPIRE data, astronomers from have already made ground-breaking discoveries about the formation and evolution of stars, galaxies and planets. But this has only scratched the surface, and there is far more still to come from the immense archive.

Herschel carried over 2000 litres of liquid helium when launched by the European Space Agency in May 2009, allowing its instruments to be cooled to a few degrees above absolute zero (-273 C). SPIRE, which was led by the Professor Matt Griffin from the School of Physics and Astronomy, used an additional cooler to get its detectors even colder – only a third of a degree above absolute zero. Herschel's suite of three instruments, combined with its 3.5m diameter mirror, made it the most powerful infrared observatory ever launched. SPIRE made images of the sky simultaneously in three submillimetre 'colours', and measured the spectral features of atoms and molecules in space.

"The six hundred scientific papers published so far are just the tip of the iceberg, and over the next few years there'll be many more fascinating results coming out of the Herschel archive," says Professor Griffin. "The work is far from over, and we will spend the next few years improving our analysis and ensuring that Herschel's legacy will last for years or even decades".

By observing far-infrared and sub-millimetre light, Herschel's three instruments have studied the coldest parts of the Universe. Rather than observing stars, Herschel has been observing the gas and dust grains from which stars and planets form, and which also play an important role in the evolution of galaxies. This interstellar material is generally very cold, sometimes at temperatures below -260 C, with some regions being warmed to higher temperatures by newly-formed stars.

In visible light this dust is dark and opaque, hiding from view the regions where stars are currently forming.  But in the far-infrared light which Herschel could see, the dust emits its own glow, and allows astronomers to study the birthplaces of stars in great detail. By surveying huge regions of our galaxy, Herschel showed that a filamentary network of gas and dust is threaded through the interstellar medium.

"Herschel has confirmed that stars, and planets, form in regions embedded in filaments of gas and dust threaded throughout our galaxy," says Dr Haley Gomez, School of Physics and Astronomy,. "And Herschel found that cosmic dust, which is key for the formation of planets, is mostly formed in the titanic explosions of massive stars."

Herschel has also observed the formation of stars in other galaxies, both near and far. This ranges from our nearest large neighbour, the Andromeda Galaxy, to galaxies billions of light years away. By staring at seemingly empty patches of sky, Herschel has seen hundreds of thousands of distant galaxies. These are seen at a range of distances, with the more distant galaxies seen as they were when the Universe was younger, allowing astronomers to study how galaxies have changed and evolved over the history of the cosmos.

"Our Sun formed around five billion years ago, and we can study what the Universe was like back then by studying these incredibly distant galaxies" explains Professor Steve Eales. "Galaxies were forming stars hundreds or thousands of times of faster than they are today, and so this period is hugely important for understanding the cosmic evolution of galaxies. Herschel has produced some of the largest maps ever made of these distant galaxies".

Herschel's instruments did not just produce pictures, but also carried spectrometers to allow astronomers to study the full range of wavelengths in great details.  This allows the identification of specific atoms and molecules in interstellar space and in distant galaxies. "Herschel's spectrometers cover a huge range of wavelengths, most of which has never been studies in such detail" says Professor Griffin "Not only can we study the detailed composition of gas and dust, but we can study the importance of specific types of molecules, such as water, in the formation of stars and planets."

After performing 22,000 hours of scientific observations, Herschel's archive of data is an invaluable resource for astronomers interested in a huge range of objects. "SPIRE's observations have revealed the cosmos in unprecedented detail at these wavelengths," concludes Professor Walter Gear, Head of the School of Physics and Astronomy. "And while the operational phase of the mission is over, the teams here in Cardiff and around the world will be working hard over the coming years to extract as much information as possible from the enormous trove of data."