Space Dust

The universe is a very dusty place. Cosmic dust is made up of tiny particles of solid material floating around in the space between the stars. It is not the same as the dust you find in your house but more like cigarette smoke, with sizes around a few molecules thick to 0.1 mm. Unfortunately for astronomers, a dusty universe is similar to a smoky universe; indeed dust is responsible for blocking out a significant amount of light from distant stars and galaxies. Luckily this ‘dusty’ cloud has a silver lining: the grains absorb visible light from stars, this stolen starlight is recycled to longer wavelengths and consequently dust shines in the infra-red/sub-millimetre parts of the electromagnetic spectrum. If we are to understand the nature of dust, we need to use cameras purposefully built to “see” this longer-wavelength light, but the technological limitations has meant that this has only been possible during the last few decades with space missions such as NASA’s IRAS, ISO and Spitzer and the ground-based SCUBA camera operated by UK, Canada and the Netherlands.

After analyzing the observations from these special infra-red/sub-millimetre cameras, astronomers have realised that rather than simply being a nuisance, dust appears to be very important, for instance, we find lots of dust around young stars suggesting it is an important part of star formation and tiny dust grains provide the universe with the building blocks of rocky planets and indeed, life. Our knowledge about the importance of dust has literally exploded in the last eight years or so thanks to the advent of space telescopes that look directly at the light shining from dust grains. Not surprisingly, these new infra-red and sub-millimetre ‘eyes’ have thrown up a few surprises.

One major surprise was discovered when astronomers began to ask, “how much light in total can we see from objects in the universe and in what part of the electromagnetic spectrum do they shine in?” The total light from all sources beyond our own Milky Way is known as the Extragalactic Background and only recently have we been able to ask this question for objects shining in the infra-red and sub-millimetre. Figure 1 shows the Extragalactic Background from objects shining in the visible to infra-red, right through to sub-millimetre wavelengths. All the visible light from stars is shown in blue and the total infrared glow produced by cosmic dust, which has been warmed by all the stars since the beginning of time is shown in red.

Figure 1. The background light from stars and dust. Adapted from Dole et al. 2006.

The unexpected result revealed in Figure 1 is that the amount of energy shining in our universe from stars and dust is almost the same, implying that understanding the dusty universe is as important as understanding the visible universe. Since the radiation from dust is due to recycled starlight, we can conclude that cosmic dust is responsible for blocking almost half of all the optical light from stars and galaxies since the Big Bang!

Yet there are many questions relating to the nature of cosmic dust, in particular, where exactly does all the dust come from? Figure 2 shows a schematic of the life cycle for a cosmic dust grain. Previous observational evidence suggests that almost all of the dust we observe in galaxies is formed in low-intermediate mass stars (with mass less than five times the mass of our Sun) and is then blown off the star in a slow wind at the end of its lifetime. The ejected dust is mixed in the clouds of gas between stars and some of this will be used up in the next generation of star formation. According to this scenario, the dust grains are then destroyed by the hot blast waves from the energetic explosions of massive stars.

Figure 2. The lifecycle of dust in galaxies. Credit: E.L.Gomez.

The life cycle of dust was a well-established theory but with the recent observations made by infrared/sub-millimetre telescopes, it soon became clear that this theory had a few problems.

The 10 billion year old mystery

We soon discovered another surprise; observations show that large quantities of dust existed in the earliest galaxies, when the Universe was very young at only 1/10th of its present age. This discovery led to the question – what process is creating such a huge amount of dust so quickly? To form any dust using the current life cycle idea (Figure 2), stars require more than five billion years of their lifetime before they begin to eject dust into the surroundings. We demonstrated in 2003, and then later using a large observational dataset in 2014, that there is simply not enough time for these stars to produce all the dust we see in the very early universe (Morgan et al. 2003a, Dwek et al. 2007, Rowlands et al. 2014a).

Figure 3. The build up of dust over time for a hypothetical galaxy taken from Morgan &amp; Edmunds 2003. If there is no dust made in supernovae or massive star winds (blue line), then the winds of low mass evolved stars (black line) take to long to make the dust seen at redshifts < 6 (red line).

This was a major problem for astronomers; a new source of dust was needed and quick! Massive stars were suggested as the perfect candidate: not only do they produce many of the elements required for cosmic dust during their lifetimes (creating every element in the periodic table beyond carbon, nitrogen and oxygen), but they also live fast and die young, culminating in a supernova explosion which literally tears the star apart. This would be the ideal mechanism for injecting lots of dust into the surrounding gas in a blink of an eye compared to astronomical standards. However, until recently (Dunne et al 2003), no-one had seen enough dust in the aftermath of exploding stars to corroborate this theory. We’ve worked hard at investigating whether supernovae are important dust factories and find that massive-star supernovae do form lots of dust, whereas white-dwarf supernovae do not.