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
References
Bailin, J. and Harris, W., 2009, ApJ, 695, 1082| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1MXlvVWju7Y%3D&md5=a7e5aa591e84c012bf7f97fe949028adCAS |
Barnes, J. and Hernquist, L., 1992, Nature, 360, 715
| Crossref | GoogleScholarGoogle Scholar |
Baumgardt, H. and Mieske, S., 2008, MNRAS, 391, 942
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1MXmtlaltw%3D%3D&md5=5f2055da3a0d706a3ddafb0251c37b88CAS |
Bekki, K., Couch, W. and Drinkwater, M., 2001, ApJ, 552, 105
| Crossref | GoogleScholarGoogle Scholar |
Bekki, K. and Freeman, K., 2003, MNRAS, 346, 11
| Crossref | GoogleScholarGoogle Scholar |
Bekki, K. and Chiba, M., 2004, A&A, 417, 437
| Crossref | GoogleScholarGoogle Scholar |
Belokurov, V. et al., 2007, ApJ, 654, 897
| Crossref | GoogleScholarGoogle Scholar |
Belokurov, V. et al., 2009, MNRAS, 397, 1748
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1MXhtFGmurvN&md5=a0f0839ceae07473008e70152575f0a2CAS |
Binney, J. & Treamine, S., 1987, Galactic Dynamics, Princeton University Press
Brada, R. and Milgrom, M., 2000, ApJ, 541, 556
| Crossref | GoogleScholarGoogle Scholar |
Bromm, V. and Clarke, C., 2002, ApJ, 566, L1
| Crossref | GoogleScholarGoogle Scholar |
Carlin, J. et al., 2009, ApJ, 702, 9
| Crossref | GoogleScholarGoogle Scholar |
Chiboucas, K. et al., 2010, arXiv:1009.3950
Chilingarian, I. and Mamon, G., 2008, MNRAS, 385, 83
Chilingarian, I., Mieske, M., Hilker, M. & Infante, L., MNRAS, submitted
Coleman, M. et al., 2007, ApJ, 668, L43
| Crossref | GoogleScholarGoogle Scholar |
Conroy, C., Loeb, A. & Spergel, D., 2010, ApJ, in press
Dabringhausen, J., Hilker, M. and Kroupa, P., 2008, MNRAS, 386, 864
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXntFyku7o%3D&md5=bf91cb97e298357fca054847ffb89dc9CAS |
Dabringhausen, J., Kroupa, P. and Baumgardt, H., 2009, MNRAS, 394, 1529
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1MXls1Sns74%3D&md5=acea84fefe5f1e8124d24e6a82d704beCAS |
Da Rocha, C., Mieske, S., Georgiev, I. Y., Hilker, M., Ziegler, B. L. & Mendes de Oliveira, C., arXiv:1009.3010
Diemand, J. et al., 2008, Nature, 454, 735
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXps1Snsbg%3D&md5=a19ac6aec95e30c7c2da30a6734f3cafCAS | 18685701PubMed |
Doyle, M. et al., 2005, MNRAS, 361, 34
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD2MXpsVCjsLg%3D&md5=98028fd76befb3c0b05f0ba885cd953aCAS |
Drinkwater, M. et al., 2000, PASA, 17, 227
| Crossref | GoogleScholarGoogle Scholar |
Drinkwater, M. et al., 2004, PASA, 21, 375
| Crossref | GoogleScholarGoogle Scholar |
Duc, P. et al., 2007, A&A, 475, 187
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXnt1Kntw%3D%3D&md5=6b33ace77f8928c231d9b0aaf706b6b5CAS |
Evstigneeva, E., Gregg, M., Drinkwater, M. and Hilker, M., 2007, AJ, 133, 1722
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD2sXkvFeltr4%3D&md5=7597464b8775c8ea7bb7397faeaa4622CAS |
Fellhauer, M. and Kroupa, P., 2002, MNRAS, 330, 642
| Crossref | GoogleScholarGoogle Scholar |
Forbes, D., 2005, ApJ, 635, 137
| Crossref | GoogleScholarGoogle Scholar |
Forbes, D. A., Lasky, P., Graham, A. W. and Spitler, L., 2008, MNRAS, 389, 1924
| Crossref | GoogleScholarGoogle Scholar |
Forbes, D., Spitler, L., Graham, A., Foster, C., Hau, G. & Benson, A., 2010, MNRAS, submitted
Freeman, K., 1993, ASPC, 48, 608
Geha, M. et al., 2009, ApJ, 692, 1464
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1MXmtFansL0%3D&md5=6a16d9639876f8be5539c2f675c7c6a7CAS |
Geha, M. et al., 2010, ApJ, 711, 361
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3cXktlSksb8%3D&md5=cd0741172a6c56f94791b329afa1da95CAS |
Gentile, G., Famaey, B., Combes, F., Kroupa, P., Zhao, H. S. and Tiret, O., 2007, A&A, 472, L25
| Crossref | GoogleScholarGoogle Scholar |
Gilmore, G. et al., 2007, ApJ, 663, 948
| Crossref | GoogleScholarGoogle Scholar |
Griffen, B. et al., 2010, MNRAS, 405, 375
Hilker, M. et al., 1999, A&A, 134, 75
| Crossref | GoogleScholarGoogle Scholar |
Hilker, M., 2009, ariX:0906.0776
Hubble, E., 1926, ApJ, 64, 321
| Crossref | GoogleScholarGoogle Scholar |
Kilborn, V. et al., 2005, PASA, 22, 326
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD28XitlKjtL8%3D&md5=ca2054b063a0e2426ca9b15e8a09aff3CAS |
Kirby, E., Simon, J., Geha, M., Guhathakurta, P. and Frebel, A., 2008, ApJ, 685, 43L
| Crossref | GoogleScholarGoogle Scholar |
Kroupa, P., 1997, New Astronomy, 2, 139
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DyaK2sXmsVersb0%3D&md5=13162e5f09e7cc595448c350e412c300CAS |
Kroupa, P., 1998, MNRAS, 300, 200
| Crossref | GoogleScholarGoogle Scholar |
Kroupa, P., 2008, in Dynamical Evolution of Dense Stellar Systems, IAU Symposium 246, 13
Kroupa, P. et al., 2010, A&A, 523, 32
| Crossref | GoogleScholarGoogle Scholar |
Lane, R. et al., 2010, MNRAS, 406, 2732
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3cXhtFKjsLfO&md5=366465d8a84f4db03c90fe8ddb0375c2CAS |
Madrid, J. et al., 2010, MNRAS, submitted
Mashchenko, S. and Sills, A., 2005, ApJ, 619, 258
| Crossref | GoogleScholarGoogle Scholar |
McGaugh, S. & Wolf, J., 2010, arXiv:1003.3448
Meylan, G., Sarajedini, A., Jablonka, P., Djorgovski, S. G., Bridges, T. and Rich, R. M., 2001, AJ, 122, 830
| Crossref | GoogleScholarGoogle Scholar |
Metz, M., Kroupa, P., Theis, C., Hensler, G. and Jerjen, H., 2009, ApJ, 697, 269
| Crossref | GoogleScholarGoogle Scholar |
Mieske, S., Hilker, M. and Infante, L., 2004, A&A, 418, 445
| Crossref | GoogleScholarGoogle Scholar |
Mieske, S. et al., 2008, A&A, 487, 921
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXhtFKit7nI&md5=a146ff49b3b78ce58530c8d3db6fd431CAS |
Moore, B., 1996, ApJ, 461, L13
| Crossref | GoogleScholarGoogle Scholar |
Munoz, R., Geha, M. and Willman, B., 2010, AJ, 140, 138
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BC3cXps1akt7o%3D&md5=58727079ff8136f1b05c5f4c144c02b0CAS |
Niederste-Ostholt, M., Belokurov, V., Evans, N., Gilmore, G., Wyse, R. and Norris, J., 2009, MNRAS, 398, 1771
| Crossref | GoogleScholarGoogle Scholar |
Norris, M. & Kannappan, S., arXiv:1009.2489
Parmentier, G., 2004, MNRAS, 351, 585
| Crossref | GoogleScholarGoogle Scholar |
Phillipps, S., Drinkwater, M. J., Gregg, M. D. and Jones, J. B., 2001, ApJ, 560, 201
| Crossref | GoogleScholarGoogle Scholar |
Piotto, G., 2009, arXiv:0902.1422
Saitoh, T., Koda, J., Okamoto, T., Wada, K. and Habe, A., 2006, ApJ, 640, 22
| Crossref | GoogleScholarGoogle Scholar |
Simon, J. and Geha, M., 2007, ApJ, 670, 313
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD2sXhsVCnu7nE&md5=5c06a9a8bfc6ced76cbe753c9248f0c0CAS |
Simon, J. et al., 2010, arXiv:1007.4198
Soter, S., 2006, AJ, 132, 2513
| Crossref | GoogleScholarGoogle Scholar |
Strader, J. and Smith, G., 2008, AJ, 136, 1828
| Crossref | GoogleScholarGoogle Scholar | 1:CAS:528:DC%2BD1cXhsVaitrzJ&md5=5db167646cfa73a5d8cf4be1285bf4beCAS |
Tammann, G., 1994, ESOC, 49, 3
Taylor, M. A., Puzia, T. H., Harris, G. L., Harris, W. E., Kissler-Patig, M. and Hilker, M., 2010, ApJ, 712, 1191
| Crossref | GoogleScholarGoogle Scholar |
Toloba, E. et al., 2010, A&A, submitted
Verde, L., Oh, S. and Jimenez, R., 2002, MNRAS, 336, 541
| Crossref | GoogleScholarGoogle Scholar |
Walsh, S. M., Jerjen, H. and Willman, B., 2007, ApJ, 662, L83
| Crossref | GoogleScholarGoogle Scholar |
White, S. and Rees, M., 1978, MNRAS, 183, 341
Willman, B. et al., 2005, ApJ, 626, 85
| Crossref | GoogleScholarGoogle Scholar |
