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RESEARCH ARTICLE (Open Access)
Contents Vol 28(1)

What Is a Galaxy? Cast Your Vote Here

Duncan A. Forbes A C and Pavel Kroupa B
+ Author Affiliations
- Author Affiliations

A Centre for Astrophysics & Supercomputing, Swinburne University, Hawthorn VIC 3122, Australia

B Argelander Institute for Astronomy, University of Bonn, Auf dem Hugel 71, D-53121 Bonn, Germany

C Corresponding author. Email: dforbes@swin.edu.au

Publications of the Astronomical Society of Australia 28(1) 77-82 https://doi.org/10.1071/AS10029
Submitted: 16 8 2010  Accepted: 27 12 2010   Published: 23 March 2011

Journal Compilation © Astronomical Society of Australia 2011

Abstract

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 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 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 an ambiguous nature are also discussed. Finally, we give our favoured criteria, and in the spirit of ‘collective wisdom’, invite readers to vote on their preferred definition of a galaxy via a dedicated website.

Keywords: galaxies: dwarf — galaxies: fundamental parameters — galaxies: star clusters — galaxies: general


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