How do you define a galaxy? Is the treasure of the southern sky Omega Centauri a galaxy or a globular cluster?

The inner parts of the galaxy Messier 100 as seen by the Hubble Space Telescope

The inner parts of the galaxy Messier 100 as seen by the Hubble Space Telescope. Image courtesy NASA and the Space Telescope Science Institute

The definition of a planet in the solar system was settled at a meeting of the International Astronomical Union in Prague in 2006. The definition did not make everyone happy and in particular annoyed some American astronomers. Plus it will eventually need to be rethought when discussing planets around other stars once sufficiently small ones are discovered. Still the definition exists.

There is no such accepted definition for galaxies. It has now become an issue as new types of small galaxies have been defined such as ultra compact dwarfs (UCDs) that can be hard to distinguish from a large globular cluster such as the famous southern sky object Omega Centauri.

The globular cluster Omega Centauri. Image Nick Lomb and Geoff Wyatt

The globular cluster Omega Centauri. Image Nick Lomb and Geoff Wyatt

A new scientific paper to be published in the Publications of the Astronomical Society of Australia by Duncan A. Forbes of Swinbourne University and Pavel Kroupa of the University of Bonn raises the issue of a definition of a galaxy. They take the interesting route of proposing a number of criteria, explaining the consequence of the each criterion for a number of key objects including Omega Centauri and then allowing readers to comment and vote.

Forbes and Kroupa start by quoting a few definitions of galaxies from websites including the following from Swinburne: A galaxy is a gravitationally bound entity, typically consisting of dark matter, gas, dust and stars. They then propose a number of possible criteria for an object to be a galaxy. An attempt at simplified versions of these criteria is below:

1. Gravitationally bound – its constituents cannot float off into space
2. Must contain stars
3. Relaxation time greater than the age of the Universe. This is the time for energy to be equally distributed amongst the constituent stars, gas and dark matter.
4. Effective radius greater than 326 light years
5. Has stars of varying ages
6. Contains dark matter – matter that cannot be seen but can be discerned through its gravitational interaction with normal matter
7. Has satellite galaxies

Omega Centauri, which is the most massive of the globular clusters in the Milky Way, is a special case as it has stars of varying ages, indicating that its stars formed over a long period. This and other reasons have led astronomers to suggest that it is the nucleus of a small galaxy that fell into the Milky Way and had its outer parts stripped off. It could then be regarded as a low mass example of a UCD. However, it fails almost all the proposed criteria listed above and so instead of becoming the smallest galaxy, it may remain as the king of the globulars.

This paper has created lots of interesting discussion. One good comment is that we should ‘distinguish between galaxies and “non-galaxies” using their formation and evolution mechanisms.’ To me though that would be a difficult way of separating objects as an object maybe classifed as a galaxy today, but not in a decade or two as scientists’ understanding of the formation of individual objects changes with time.

What do you think?

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