Most interesting matter

LAST week, while relaxing with fellow astronomy colleagues in Fukuoka, Japan, I found myself involved in an excited discussion about an amazing discovery.

At that stage it was all rather hush-hush because of a media embargo, but the next day, the news was out: astronomers had found that the galaxy named NGC 1052DF2 has no dark matter.

A surprise in astronomy is nothing new: for all the time humans have been studying the universe, it has been shown to be full of unexpected features. It was only four centuries ago that it was becoming established that the Earth and other planets revolved around the sun (instead of everything revolving around the Earth), and a little less than a century ago that our galaxy was not the only one. We now know the universe contains hundreds of billions of galaxies.

Another big surprise, in the 1990s, was the discovery that the expansion of the universe is accelerating, leading to the conclusion that more than two-thirds of the content of the universe is made up of socalled dark energy that we currently do not understand.

We’ll get back to the latest discovery shortly, but first, a little history.

The idea of dark matter dates back to the late 19th century, but it was in the 1930s that measurements of the speed of motion of individual galaxies within a cluster of galaxies called the Coma Cluster revealed that there was much more matter there than could be seen.

Later on, also very revealing were the measurements of the motions of individual stars within their galaxies. As we all know, the farther a planet is from the sun, the slower its speed of revolution, and the longer it takes to complete each orbit. The same was expected to hold true for stars in the outer reaches of a galaxy: they should have followed this wellunderstood law.

However, they did not: they were found to maintain a higher speed of revolution than expected. Astronomers realised there must be some unseen matter, helping to drag the outer stars around.

It has been called dark matter, and although it is known to exert a gravitational force, we do not yet understand its makeup.

The existence of dark matter is now well established. We do not know what it is, although it is known to exert a gravitational force. Dark matter has also helped us to understand the formation of galaxies themselves; clumps of dark matter are thought to cause accumulations of normal matter.

This is why the nature of NGC 1052-DF2 was so unexpected. Measurements of the motions of its stars show that they are not being affected by any dark matter. Not only that, but astronomers can also see right through the galaxy and view distant galaxies beyond, giving additional evidence of the absence of dark matter.

Astronomers around the world have been amazed and rather confused by this discovery, but there is a very positive result that has come from this finding: it has provided further confirmation of the existence of dark matter itself.

The logical argument goes like this. There have been theories that suggest that most galaxies behave the way they do without needing to apply the idea of dark matter. So initially, the absence of dark matter in NGC 1052-DF2 would suggest that this alternative idea was right, except for one major problem. The problem is that the alternative ideas would see the stars moving at different velocities to those observed.

Finally, one could take a far more simplistic approach to this: we can see through this galaxy to more distant objects beyond, and the velocities of the stars support the absence of dark matter. It’s mostly or completely transparent, it’s different, and it’s a puzzle. Martin George is manager of the Launceston Planetarium (QVMAG).