University of Oulu

Su, A. H., Salo, H., Janz, J., Venhola, A., & Peletier, R. F. (2022). Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster. Astronomy & Astrophysics, 664, A167.

Photometric properties of nuclear star clusters and their host galaxies in the Fornax cluster

Saved in:
Author: Su, Alan H.1; Salo, Heikki1; Janz, Joachim2,1,3;
Organizations: 1Space Physics and Astronomy Research Unit, University of Oulu, Pentti Kaiteran katu 1, 90014 Oulu, Finland
2Finnish Centre of Astronomy with ESO (FINCA), Vesilinnantie 5, University of Turku, 20014 Turku, Finland
3Specim, Spectral Imaging Ltd., Elektroniikkatie 13, 90590 Oulu, Finland
4Kapteyn Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands
Format: article
Version: published version
Access: open
Online Access: PDF Full Text (PDF, 4 MB)
Persistent link:
Language: English
Published: EDP Sciences, 2022
Publish Date: 2023-01-17


Aims: We aim to investigate the relations between nuclear star clusters (NSCs) and their host galaxies and to offer a comparison between the structural properties of nucleated and non-nucleated galaxies. We also address the environmental influences on the nucleation of galaxies in the Fornax main cluster and the Fornax A group.

Methods: We selected 557 galaxies (105.5M < M*, galaxy < 1011.5M) for which structural decomposition models and non-parametric morphological measurements are available from our previous work. We determined the nucleation of galaxies based on a combination of visual inspection of galaxy images and residuals from multi-component decomposition models, as well as using a model selection statistic, the Bayesian information criterion (BIC), to avoid missing any faint nuclei. We also tested the BIC as an unsupervised method to determine the nucleation of galaxies. We characterised the NSCs using the nucleus components from the multi-component models conducted in the g′, r′, and i′ bands.

Results: Overall, we find a dichotomy in the properties of nuclei that reside in galaxies more or less massive than M*, galaxy ≈ 108.5M. In particular, we find that the nuclei tend to be bluer than their host galaxies and follow a scaling relation of M*, nuc ∝ M*, galaxy0.5 for M*, galaxy < 108.5M. In galaxies with M*, galaxy > 108.5M,  we find redder nuclei compared to the host galaxy, which follows M*, nuc ∝ M*, galaxy. Comparing the properties of nucleated and non-nucleated early-type galaxies, we find that nucleated galaxies tend to be redder in global (g′−r′) colour, have redder outskirts relatively to their own inner regions (Δ(g′−r′)), are less asymmetric (A), and exhibit less scatter in the brightest second-order moment of light (M20) than their non-nucleated counterparts at a given stellar mass. However, with the exception of Δ(g′−r′) and the Gini coefficient (G), we do not find any significant correlations with cluster-centric distance. Yet, we find the nucleation fractions to be typically higher in the Fornax main cluster than in the Fornax A group, and that the nucleation fraction is highest towards the centre of their respective environments. Additionally, we find that the observed ultra-compact dwarf (UCD) fraction (i.e. the number of UCDs over the number of UCDs and nucleated galaxies) in Fornax and Virgo peaks at the cluster centre and is consistent with the predictions from simulations. Lastly, we find that the BIC can recover our labels of nucleation up to an accuracy of 97% without interventions.

Conclusions: The different trends in NSC properties suggest that different processes are at play at different host stellar masses. A plausible explanation is that the combination of globular cluster in-spiral and in situ star formation play a key role in the build-up of NSCs. In addition, the environment is clearly another important factor in the nucleation of galaxies, particularly at the centre of the cluster where the nucleation and UCD fractions peak. Nevertheless, the lack of significant correlations with the structures of the host galaxies is intriguing. Finally, our exploration of the BIC as a potential method of determining nucleation have applications for large-scale future surveys, such as Euclid.

see all

Series: Astronomy and astrophysics
ISSN: 0004-6361
ISSN-E: 1432-0746
ISSN-L: 0004-6361
Volume: 664
Article number: A167
DOI: 10.1051/0004-6361/202142593
Type of Publication: A1 Journal article – refereed
Field of Science: 115 Astronomy and space science
Funding: We acknowledge financial support from the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 721463 to the SUNDIAL ITN network. H.S. and A.V. are also supported by the Academy of Finland grant No. 297738.
EU Grant Number: (721463) SUNDIAL - SUrvey Network for Deep Imaging Analysis and Learning
Academy of Finland Grant Number: 297738
Detailed Information: 297738 (Academy of Finland Funding decision)
Copyright information: Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.