Australasian Science: Australia's authority on science since 1938

A Universe Less Dusty

By Stephen Luntz

An exceptional galaxy has cast doubt on our conception of the early universe and the way in which galaxies form.

The Milky Way resembles most surrounding galaxies in having substantial quantities of elements heavier than helium (known as metals) and low rates of star formation. The most distant galaxies we can see, viewed as they were billions of years ago, have the reverse. Unfortunately they are too far away to study in detail, so many of our ideas of them come from models based on the older galaxies with which we are familiar.

However, just 60 million light years away lies IZw18, a galaxy whose very low metalicity and very rapid star formation makes it look like a throwback to an earlier time.

“It’s an extreme galaxy in the local universe, but it tells us a lot about a stage that almost all galaxies have gone through, so it gives us a picture of what the first galaxies look like,” says Dr David Fisher of the Swinburne Centre for Astrophysics and Supercomputing.

How IZw18 came to be the way it is remains a mystery. “You could say it is a rare galaxy on the extreme end of a distribution of properties,” says Fisher. “It is not a huge problem because it is so rare out of hundreds of thousands of galaxies, but we don’t know why.”

Given its unusual status Fisher says we cannot be sure that IZw18 resembles its predecessors, but notes: “It is the best chance we have to study this environment, the only lab we have”.

Consequently, astronomers from around the world came together to study IZw18 and found it unexpectedly low in dust, a result published in Nature. “It’s not just that the dust mass is low. We found that the dust mass is 100 times smaller than would be expected based on commonly assumed theories,” says Fisher. Radiation within IZw18 was also 200 times stronger than that within the Milky Way, partly as a result of all the hot young stars.

“In a normal galaxy dust blocks the light, but with so little dust there becomes a feedback loop where what little there exists is strongly affected,” says Fisher. The dust that does exist is either broken down by the force of the radiation or pushed into star-forming fields.

Dust plays a crucial role in the formation of stars in developed galaxies, providing surfaces on which gas can cool down – an essential step in star formation. “If you have less dust it is hard to say how stars can form,” says Fisher. “The universe clearly knows how, but astronomers don’t.”