8 Jul 2011

Revealed: the origin of primordial dust

Thanks to observations made with the Herschel space telescope of the supernova 1987A, it seems that an explanation may have been found as to why very distant, and so very young, galaxies are found to be particularly rich in dust. This dust is actually made up of tiny grains that are typically much smaller than a familiar terrestrial speck of dust, being closer to the size of a smoke particle, and are composed of heavy elements such as carbon, silicon, oxygen and iron. These are released in the supernova explosions of dying, massive stars. However, until now it appeared that the quantity of dust produced by supernovae was insufficient to explain the amounts of dust observed in the primordial galaxies. This was largely due to limitations in instrumentation. In fact, using Herschel, that has better resolution and sensitivity than any previous far-infrared telescope, a group of astronomers led by Mikako Matsuura of University College London, has discovered a surprisingly large amount of dust associated with SN 1987A (at the centre of the above image). This supernova occurred 24 years ago in the Large Magellanic cloud. The amount of dust in SN 1987A, about one solar mass, is up to 1000 times the mass estimated by previous studies, and demonstrates that the amount of dust observed in primordial galaxies is consistent with the dust produced by the supernova explosions of the first generation of giant stars (over 10 solar masses), that ended their lives just a few hundred million years after the Big Bang. The data collected by Herschel are also surprising because of the estimated temperature of the dust, between 16 and 23 Kelvin, or rather, about 20 times lower than the temperatures derived from previous data at shorter wavelengths. This implies that the dust plays a key role in the formation of new stars, illustrating its abilities to cool clouds of interstellar gas, thus allowing them to collapse under gravity. In the absence of such cooling, a cloud which is undergoing gravitational contraction heats up, and the additional pressure halts further collapse. The formation of a dense nucleus that will become a star would be inhibited. In general then, it is very clear that the presence of dust generated in supernovae has influenced the evolution of galaxies from the very onset of their existence.