University of Oulu

Teymoor Saifollahi, Joachim Janz, Reynier F Peletier, Michele Cantiello, Michael Hilker, Steffen Mieske, Edwin A Valentijn, Aku Venhola, Gijs Verdoes Kleijn, Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster: hints of UCD formation in low-density environments, Monthly Notices of the Royal Astronomical Society, Volume 504, Issue 3, July 2021, Pages 3580–3609,

Ultra-compact dwarfs beyond the centre of the Fornax galaxy cluster : hints of UCD formation in low-density environments

Saved in:
Author: Saifollahi, Teymoor1; Janz, Joachim2,3; Peletier, Reynier F.1;
Organizations: 1Kapteyn Astronomical Institute, University of Groningen, PO Box 800, NL-9700 AV Groningen, the Netherlands
2Finnish Centre of Astronomy with ESO (FINCA), University of Turku, Vesilinnantie 5, FI-20014 Turku, Finland
3Space Physics and Astronomy Research Unit, University of Oulu, P.O. Box 3000, FI-90014 Oulu, Finland
4INAF Osservatorio Astronomico d’Abruzzo, via Magrini snc, I-64100 Teramo, Italy
5European Southern Observatory, Karl-Schwarzschild-Str 2, D-85748 Garching bei M ̈unchen, Germany
6European Southern Observatory, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago, Chile
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 24.7 MB)
Persistent link:
Language: English
Published: Oxford University Press, 2021
Publish Date: 2021-07-08


Ultra-compact dwarf galaxies (UCDs) were serendipitously discovered by spectroscopic surveys in the Fornax cluster 20 yr ago. Nowadays, it is commonly accepted that many bright UCDs are the nuclei of galaxies that have been stripped. However, this conclusion might be driven by biased samples of UCDs in high-density environments, on which most searches are based. With the deep optical images of the Fornax Deep Survey, combined with public near-infrared data, we revisit the UCD population of the Fornax cluster and search for UCD candidates, for the first time, systematically out to the virial radius of the galaxy cluster. Our search is complete down to magnitude mg = 21 mag or Mg∼ −10.5 mag at the distance of the Fornax cluster. The UCD candidates are identified and separated from foreground stars and background galaxies by their optical and near-infrared colours. This primarily utilizes the u−i/i−Ks diagram and a machine learning technique is employed to incorporate other colour combinations to reduce the number of contaminants. The newly identified candidates (44) in addition to the spectroscopically confirmed UCDs (61), increase the number of known Fornax UCD considerably (105). Almost all of the new UCD candidates are located outside the Fornax cluster core (360 kpc), where all of the known UCDs were found. The distribution of UCDs within the Fornax cluster shows that a population of UCDs may form in low-density environments. This most likely challenges the current models of UCD formation.

see all

Series: Monthly notices of the Royal Astronomical Society
ISSN: 0035-8711
ISSN-E: 1365-8711
ISSN-L: 0035-8711
Volume: 504
Issue: 3
Pages: 3580 - 3609
DOI: 10.1093/mnras/stab1118
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: TS and RFP acknowledge financial support from the European Union’s Horizon 2020 research and innovation programme under Marie Skłodowska-Curie grant agreement no. 721463 to the SUNDIAL ITN network. AV would like to thank Eemil Aaltonen Foundation for funding during this project. MC acknowledges support from MIUR, PRIN 2017 (grant 20179ZF5KS).
Copyright information: © The Author(s) 2021. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.