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What Is a Galaxy? Cast Your Vote Here

Published online by Cambridge University Press:  02 January 2013

Duncan A. Forbes  and
Pavel Kroupa
Duncan A. Forbes*
Affiliation:
Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn VIC 3122, Australia
Pavel Kroupa
Affiliation:
Argelander Institute for Astronomy, University of Bonn, Auf dem Hugel 71, D-53121 Bonn, Germany
*
CCorresponding author. Email: dforbes@swin.edu.au
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Abstract

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Although originally classified as galaxies, Ultra-Compact Dwarfs (UCDs) have 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 as 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 gravitationally bound stellar system as a minimum. In order to distinguish 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 the adoption of each of these definitions. Some special cases of objects with a ambiguous nature are also discussed. Finally, we give our favoured criteria, and in the spirit of ‘collective wisdom’, invite readers to vote on their prefered definition of a galaxy via a dedicated website.

Keywords

galaxies: dwarfgalaxies: fundamental parametersgalaxies: star clustersgalaxies: general
Type
Research Article
Information
Publications of the Astronomical Society of Australia , Volume 28 , Issue 1 , 2011 , pp. 77 - 82
Copyright
Copyright © Astronomical Society of Australia 2011

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