New Space and Tech

What is a Galaxy? Cast Your Vote Here

By Keith Cowing
April 8, 2013
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astro-ph arXiv:1101.3309: “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.” [survey website link below]

“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 eectively 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:

Duncan Forbes (Swinburne, Melbourne), Pavel Kroupa (AIfA, Bonn) Comments: PASA, in press, LaTeX, 6 pages Subjects: Galaxy Astrophysics (astro-ph.GA); Cosmology and Extragalactic Astrophysics (astro-ph.CO) Full paperadditional information

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