Herschel Completes Largest Ever Dust Census in Local Universe
Astronomers using data from ESA’s Herschel Space Observatory have completed the largest-ever survey of cosmic dust, spanning 323 nearby galaxies located 50-80 million light-years away.
Collage of galaxies in the Herschel Reference Survey at infrared/submillimeter wavelengths by Herschel, left, and at visible wavelengths from the Sloan Digital Sky Survey, right. The Herschel image is colored with blue representing cold dust and red representing warm dust; the SDSS image shows young stars in blue and old stars in red. Together, the observations plot young, dust-rich spiral/irregular galaxies in the top left, with giant dust-poor elliptical galaxies in the bottom right. Image credit: ESA / Herschel / HRS-SAG2 / HeViCS Key Programmes / Sloan Digital Sky Survey / L. Cortese, Swinburne University.
Cosmic dust grains are a minor but fundamental ingredient in the recipe of gas and dust for creating stars and planets. But despite its importance, there is an incomplete picture of the dust properties in galaxies beyond our Milky Way Galaxy.
Key questions include how the dust varies with the type of galaxy, and how it might affect our understanding of how galaxies evolve.
“Cosmic dust is heated by starlight to temperatures of only a few tens of degrees above absolute zero, and can thus be only seen at far-infrared/sub-millimeter wavelengths,” said Dr Luca Cortese from Swinburne University of Technology in Melbourne, Australia, who is the lead author of a paper published in the journal Monthly Notices of the Royal Astronomical Society.
The two cameras on board the Herschel (PACS and SPIRE) allowed scientists to probe different frequencies of dust emission, which bear imprints on the physical properties of the grains and therefore were critical for this study.
“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. 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,” Dr Cortese 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,” said study co-author Dr Jacopo Fritz from Ghent University in Belgium.