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

Abstract

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 (10 5.5 M ⊙ < M * ,galaxy < 10 11.5 M ⊙ ) 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 ≈ 10 8.5 M ⊙ . In particular, we find that the nuclei tend to be bluer than their host galaxies and follow a scaling relation of M * ,nuc ∝ M * ,galaxy 0.5 for M * ,galaxy < 10 8.5 M ⊙ . In galaxies with M * ,galaxy > 10 8.5 M ⊙, 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 (∆(g ′ − r ′ )), are less asymmetric (A), and exhibit less scatter in the brightest second-order moment of light (M 20 ) than their non-nucleated counterparts at a given stellar mass. However, with the exception of ∆(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 ...

Evidence for globular cluster collapse after a dwarf-dwarf merger: A potential nuclear star cluster in formation

“…Cosmological simulations show that dwarf-dwarf mergers are undoubtedly a common phenomenon (Deason et al 2014) and dwarfs typically undergo one major and one minor merger in their evolution (Martin et al 2021). Assuming that dwarf merging is a viable channel to induce the collapse of GCs and the subsequent formation of an NSC, this would provide a natural explanation for the correlations between nucleation fraction and environmental density (e.g., Sánchez-Janssen et al 2019;Su et al 2022;Carlsten et al 2022). In a straight forward argumentation, we can assume that a higher environmental density would produce a higher probability of interactions or merging between dwarf galaxies, thus explaining this correlation.…”

“…The fraction of nucleated galaxies is strongly dependent on their mass and morphology, reaching a maximum nucleation fraction of about 80% for M = 10 9 M , decreasing for higher and lower masses (Neumayer et al 2020, and references therein). The fraction of nucleated galaxies also depends on the environment, with the fraction higher in denser environments (Sánchez-Janssen et al 2019;Su et al 2022;Carlsten et al 2022). The intrinsic properties of NSCs correlate strongly with those of their host galaxy (Balcells et al 2003(Balcells et al , 2007Côté et al 2006;Ferrarese et al 2006).…”

“…Additionally, some properties of NSCs remain to be explained under these two formation mechanisms. For instance, the correlation between the fraction of nucleated galaxies and the environment (e.g., Sánchez-Janssen et al 2019;Su et al 2022;Carlsten et al 2022) offers no easy explanation under these proposed mechanisms, as they are internal to the hosts.…”

Evidence for globular cluster collapse after a dwarf-dwarf merger: A potential nuclear star cluster in formation

“…The fraction of nucleated galaxies is strongly dependent on their mass and morphology, reaching a maximum nucleation fraction of about 80% for M = 10 9 M , decreasing for higher and lower masses (Neumayer et al 2020, and references therein). The fraction of nucleated galaxies also depends on the environment, with the fraction higher in denser environments (Sánchez-Janssen et al 2019;Su et al 2022;Carlsten et al 2022). The intrinsic properties of NSCs correlate strongly with those of their host galaxy (Balcells et al 2003;Côté et al 2006;Ferrarese et al 2006;Balcells et al 2007).…”

Nuclear star cluster formation in star-forming dwarf galaxies

“…Using a sample of 25 nucleated early-type galaxies located in the Fornax and Virgo galaxy clusters that were observed with the Multi-Unit Spectroscopic Explorer (MUSE) instrument, Fahrion et al (2021) identified a clear trend in the dominant NSC formation channel with galaxy and NSC mass. While the low-mass NSCs (M NSC < 10 7 M ) in early-type galaxies dwarf (M gal < 10 9 M ) appear to form predominantly via the inspiral of GCs (see also, e.g., Johnston et al 2020;Su et al 2022), massive NSCs in more massive galaxies have assembled most of their mass through central star formation (see also e.g. Turner et al 2012;Sánchez-Janssen et al 2019;Neumayer et al 2020).…”