Defining a galaxy sounds so simple. We all know what a galaxy is, right? Well, not really. Surprisingly, there is no universally agreed upon definition and the ones generally bandied around leave a great deal of wriggle room.
All this has been thrown into stark relief in recent years by the discovery of a growing number of small, faint, galaxy-like objects that were entirely unknown until now. These have been given various names such as ultra compact dwarfs, ultra-faint dwarf spheroidal galaxies and dwarf elliptical galaxies.
But it isn’t entirely clear whether they have more in common with galaxies like our own or globular clusters, which astronomers generally do not think of as galaxies.
That makes the problem of defining a galaxy a growing concern.
So what to do? Today, Duncan Forbes at Swinburne University in Australia and Pavel Kroup at the University of Bonn in Germany put forward a novel solution. They outline the various characteristics that astronomers think about when classifying galaxies.
These include factors such as the presence of stars, so gas clouds can’t be defined as galaxies; being gravitationally bound, so materials that has been stripped away by another galaxy wouldn’t count; whether the system is stable or not; whether it hosts a good variety of different types of star, which excludes globular clusters which contain only similar stars; and whether it is held together by dark matter, which many galaxies seem to be.
There are other factors too, of course. (Although they do not include the presence of a black hole at the centre of a galaxy as a defining characteristic , which is odd given the growing interest in the link between galactic evolutoin and black holes.)
Forbes and Kroup go on to suggest that the best way to achieve consensus is to crowdsource the problem. In other works, they want to use the wisdom of the crowd to determine what factors are important what aren’t.
They’ve even set up website to solicit views by means of a survey. To take part, simply read their paper and answer their questions.
It may help to consider the current definitions of a galaxy, which are surprisingly vague.
Here’s the definition of a galaxy from the Oxford English Dictionary: “Any of the numerous large groups of stars and other matter that exist in space as independent systems.”
That measure includes various objects that many astronomers do not think of as galaxies, such as globular clusters, which do not have huge varieties of stars, or smaller objects such as ultra compact dwarfs which do not seem to sit in a region of dark matter, like our own galaxy .
The definition from Wikipedia is a little more comprehensive: “A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter.”
But even this misses out various characteristics that astronomers think are important, such as dominating the local environment, in the way the Milky Way dominates the globular clusters nearby.
By crowdsourcing opinion, Forbes and Kroup hope to avoid the controversy that surrounded the change in the definition of a planet that the International Astronomical Union adopted a few years ago and which resulted in the demotion of Pluto. They say this was hugely unpopular with certain astronomers and with the public in general.
Whether popularity is good measure of the success of a definition is a moot point. But it’ll be interesting to see what kind of definition emerges from this approach.
Ref: arxiv.org/abs/1101.3309: What Is A Galaxy? Cast Your Vote Here…