What is a galaxy?


Science tends to grow organically, branching out in new directions driven by new discoveries and new technology. But every once in a while, scientists feel the need to take stock of where their subject is going, and make some changes to keep things consistent. The most famous example of this is when the International Astronomical Union held a vote amongst its members over a new definition of the term “planet” – a controversial move that ended up with beloved oddball Pluto getting booted out of the solar system’s elite. Duncan Forbes and Pavel Kroupa posted a paper to astro-ph yesterday (to be published in PASA) suggesting that the concept of  “galaxy” similarly needs revisiting, and in proper 21st century style, they’ve created a web poll.

The family of galaxies used to be quite simple – there were spirals (disky, with arms), ellipticals (fuzzy blobs), lenticulars (in between) and irregulars (all the rest). But in recent decades, large surveys have allowed us to discover millions of galaxies all over the Universe in a whole range of shapes and sizes. Particularly in our own galactic backyard, we’ve been able to observe galaxies (ultra compact dwarfs) that are smaller, fainter or less massive than some known star clusters. Forbes & Kroupa argue that with the traditional distinction between galaxies and clusters (bigger in size, more stars) no longer clear, perhaps it’s time for a proper definition of what makes a galaxy.

The criteria they suggest are:

  • a relaxation time longer than a Hubble time (~ the expansion time of the Universe)
  • a half-light radius larger than 100 parsec
  • the presence of complex stellar populations (of different ages and abundances)
  • the presence of non-baryonic dark matter
  • the presence of a satellite system

and they talk in more detail about some definition-challenging objects, such as the supersized globular cluster Omega Cen.

If you’ve read the paper and have an opinion, you can take the authors’ poll at this website. Some good discussion is taking place on Pavel Kroupa’s own blog, the Dark Matter Crisis. Although I understand the issues, I’m not sure I have a strong opinion about what should and shouldn’t be classified as a galaxy;  and like Peter Coles, you could even argue whether the discussion is even relevant. But if galaxies are your thing, join the debate!

Duncan Forbes, & Pavel Kroupa (2011). What is a Galaxy? Cast your vote here… PASA arXiv: 1101.3309v1


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