In Summary

An international team of astronomers has completed a benchmark study of more than 300 galaxies, producing the largest census of dust in the local Universe, the Herschel Reference Survey.

Led by Dr Luca Cortese from the Centre for Astrophysics and Supercomputing at Swinburne University of Technology, the team used the Herschel Space Observatory to observe galaxies at far-infrared and sub-millimetre wavelength, and captured the light directly emitted by dust grains.

“These dust grains are believed to be fundamental ingredients for the formation of stars and planets, but until now very little was known about their abundance and physical properties in galaxies other than our own Milky Way,” Dr Cortese said.

Cosmic dust is heated by starlight to temperatures of only a few tens of degrees above absolute zero, and can thus only be seen at far-infrared/sub-millimetre wavelengths.

The two cameras on board the Herschel satellite, SPIRE and PACS, allowed astronomers to probe different frequencies of dust emission, which bear imprints on the physical properties of the grains, and therefore were critical for this study. Although the SPIRE data were obtained three years ago, the team had to wait for the completion of the PACS survey last year.

“The long wait was worthwhile, as the combination of the PACS and SPIRE data shows that the properties of grains vary from one galaxy to another – more than we originally expected,” Dr Cortese said.

“As dust is heated by starlight, we knew that the frequencies at which grains emit should be related to a galaxy’s star formation activity. However, our results show that galaxies’ chemical history plays an equally important role.”

By knowing all of these properties astronomers can gain a thorough picture of the amount of dust in galaxies across the Universe.

Co-author of the work, Dr Jacopo Fritz, from Gent University in Belgium, said: “This affects our ability to accurately estimate how much dust is in the Universe. It is particularly an issue for the most distant galaxies, which have a star formation and chemical history significantly different to the one in our own Milky Way.”

The data obtained for the Herschel Reference Survey has been made publicly available to allow further studies of dust properties in nearby galaxies, located about 50 to 80 million light years from Earth.

Although the Herschel Space Telescope completed its mission in April 2013, the combination of data in the Herschel archive, with future observations from the newly commissioned Atacama Large Millimeter/submillimeter Array (ALMA) in Chile, will help astronomers to further unveil the mystery of cosmic dust in galaxies in the years to come.

The team included researchers from Swinburne University of Technology, European Southern Observatory, University of Gent, Arcetri Observatory, Laboratory of Astrophysics of Marseille, University of Crete, Jodrell Bank Centre for Astrophysics, University of Cambridge, Institut d'Astrophysique de Paris, Padova Observatory, University of California, Heidelberg University, Cardiff University, University of Paris VII, Max-Planck-Institute for extragalactic astronomy, INAF-Roma, University of the Western Cape, Joint ALMA Observatory.

The research is published in the Monthly Notices of the Royal Astronomical Society.

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