Starship Asterisk*

Nobody is quite sure how to define a galaxy. So astronomers are appealing to the wisdom of crowds to help

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:1101.3309: What Is A Galaxy? Cast Your Vote Here...

• arXiv.org > astro-ph > arXiv:1101.3309 > 17 Jan 2011 > (pdf)

Astract: Although originally classified as galaxies, Ultra Compact Dwarfs (UCDs) share many properties in common with globular star clusters. The debate on the origin and nature of UCDs, and the recently discovered ultra-faint dwarf spheroidal (dSph) galaxies which contain very few stars, has motivated us to ask the question `what is a galaxy?' Our aim here is to promote further discussion of how to define a galaxy and, in particular, what separates it from a star cluster. Like most previous definitions, we adopt the requirement of a gravitationally bound stellar system as a minimum. In order to separate a dwarf galaxy from a globular cluster, we discuss other possible requirements, such as a minimum size, a long two-body relaxation time, a satellite system, the presence of complex stellar populations and non-baryonic dark matter. We briefly mention the implications of each of these definitions if they are adopted. Some special cases of objects with an ambiguous nature are also discussed. Finally, we give our favoured criteria, and in the spirit of a `collective wisdom', invite readers to vote on their preferred definition of a galaxy via a dedicated website.

Introduction

Astronomers like to classify things. That classification may initially be based on appearance to the human eye (e.g. Hubble 1926), but to make progress this taxonomy may need to have some basis in the underlying nature/physics of the objects being examined. With this mind, astronomers need a working definition so as to divide objects into different categories and to explore the interesting transition cases that might share common properties. Hopefully this results in additional insight into the physical processes that are operating. ...

Conclusion

Here we have accepted the popular definition of a galaxy requiring that it be both gravitationally bound and consist of a system of stars. As such criteria would include globular (star) clusters, additional criteria are required to define a galaxy. We suggest that the next best criterion is a dynamical one, i.e. that the stars are collisionless, subject to the general gravitational field of the system. This can be usefully quantified using the two-body relaxation time. With these three criteria, globular clusters are effectively excluded from the definition of a galaxy, as are Omega Cen, Segue 1 (and similar objects) and Coma Berenices. However Ultra Compact Dwarfs (and perhaps the most massive globular clusters) would be classed as galaxies. Although this may satisfy some, a fourth criterion would be required to exclude Ultra Compact Dwarfs. We suggest a size-based criterion, e.g. half-light radius greater than 100 pc. This fourth criterion would exclude the vast bulk of known Ultra Compact Dwarfs but may still include extreme objects such as VUCD7 and F-19. Bootes III (and similar objects, assuming they are gravitationally bound) and tidal dwarfs like VCC 2062 would also be classed as galaxies.

The combining of criteria above is somewhat subjective and the opinion of two astronomers. The decision of how to define a small planet, and hence the taxonomic fate of Pluto, was decided by 424 astronomers present on the last day of the IAU General Assembly in Prague, held in August 2006. In order to capture the thoughts of a wider audience about how to define a galaxy, we invite readers to vote. This 'collective wisdom' or 'crowd-sourcing' will be captured in an online poll. The poll allows one to choose the single best criterion or multiple criteria. Results of the poll will be reported from time to time at future astronomy conferences. The website for anonymous voting is: http://www.surveymonkey.com/s/WLRJMWS