A census of stellar structures in the Fornax cluster
|Su, Alan Hung-Shuo1,2
1University of Oulu Graduate School
2University of Oulu, Faculty of Science, Physics, Space physics and astronomy (SpaceAstro)
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Oulu : University of Oulu,
|Academic Dissertation to be presented with the assent of the Faculty of Science,
University of Oulu, for public discussion in the Auditorium L6, on
September 23rd, 2022, at 12 o’clock noon
Professor Heikki Salo
Doctor Pierre-Alain Duc
Doctor Ignacio Trujillo
Professor Sven De Rijcke
Doctor Joachim Janz
It is well known that galaxies in the Universe tend to be found in either clusters, groups, or in the field, and that the hierarchical growth of structure dictates that galaxies can first aggregate in groups before falling into cluster. The difference in properties between environments (e.g. mass, velocity dispersion, gravitational potential) means that some mechanisms (e.g. ram pressure stripping, harassment, galaxy–cluster tidal interactions) dominate in certain environments. The main aim of this thesis is to study the effects of the environment on the properties of galaxies, particularly between the group and cluster environment, where galaxies can be transformed in groups before entering clusters. To this end, in this thesis I present a detailed view of the stellar structures hosted by galaxies in the Fornax cluster.
To study the structures in detail, we use data from the Fornax Deep Survey (FDS), a deep multi-band imaging survey covering 26 deg2 around the Fornax cluster. We use the catalogue of likely Fornax members from Venhola et al. (2018) and split the sample into the Fornax main cluster and the Fornax A group. To extract and quantify the stellar structures of galaxies, we conduct structural decompositions with models of varying complexity (single Sérsic, Sérsic+PSF, and multi-component) and calculate non-parametric morphological indices, which provide a global measure of morphology. Additionally, we apply aperture photometry to derive surface brightness profiles in order to study the faint outskirts of galaxies, as well as to measure the integrated and radial colour properties.
From the quantities derived from structural decompositions: g − r and r − i colours; effective radius (re); Sérsic index (n); mean effective surface brightness (μe,r), and non-parametric morphological indices: concentration (C); asymmetry (A); clumpiness (S); Gini (G); brightest second order moment of light (M20), we find significantly (KS test p-value < 0.05) different distributions between galaxies in the Fornax main cluster and the Fornax A group. Specifically, galaxies of a given stellar mass in the Fornax main cluster tend to be redder in g − r, bluer in r − i, larger in re, fainter in μe,r, less asymmetric (lower A), and less clumpy (lower S) than their Fornax A group counterparts. Furthermore, we find significant correlations (Spearman’s ρ p-value < 0.05) between the structural quantities and projected cluster-centric distance for the Fornax main cluster galaxies, namely in r − i, re, μe,r, A, S, G, and M20. This implies that as galaxies fall deeper towards the Fornax main cluster centre, they become bluer in r − i, more extended ((re), fainter (μe,r), less asymmetric (lower A), less clumpy (lower S), possess higher equality in terms of the light distribution (lower G), and have larger spatial distribution in the brightest 20% of pixels (higher M20). In contrast, we did not find significant group-centric trends for the Fornax A group galaxies, which is potentially due to the low sample size.
In terms of hosting large scale stellar structures (i.e. bulges, bars), we find that the vast majority of structures are hosted by massive galaxies (M∗ ≳ 109M⊙). As such, the fraction of galaxies hosting these stellar structures closely follows the galaxy stellar ass. This relation appears to be consistent across different environments, as we find similar fractions at a fixed galaxy stellar mass in the Virgo cluster and in the field. This suggests that the formation and/or maintenance of stellar structures is primarily reliant on the galaxy (stellar) mass, rather than from environmental influences.
The most prevalent structure is the nucleus, which corresponds to the unresolved nuclear star cluster (NSC). We use a combination of visual inspection and the model selection statistic Bayesian information criterion (BIC) to detect nuclei, and also explore the use of the BIC as an unsupervised method for nucleus detection. We find that the overall nucleation fraction for the Fornax main cluster is much higher (0.29) than in the Fornax A group (0.14), which alludes to the importance of the environment in the nucleation of galaxies. Overall we find a dichotomy in the nucleus properties for host galaxies with stellar masses above and below M∗,galaxy ≈ 108.5M⊙: the nuclei of lower mass galaxies tend to be bluer than their host and follow a mass scaling relation of M∗,nuc ∝ M∗,galaxy0.5, whereas higher mass galaxies tend to have redder nuclei compared to their host and follow M∗,nuc ∝ M∗,galaxy. Comparing the host properties of the nucleated and non-nucleated galaxies, we find significant difference in the distributions such that nucleated galaxies tend to be redder in g – r, less asymmetric (lower A), have redder outer regions (relative to itself; Δ(g – r)), and show less scatter in M20 than their non-nucleated counterparts.
To test whether cluster galaxies show signs of quenching (e.g. due to the removal of gas via ram pressure stripping), we compare the colours and mean effective surface brightnesses of galaxies in the Fornax and Virgo cluster to predictions from stellar population models. We find that late-type dwarfs show properties consistent with a fading and reddening stellar population, but not for the early-type dwarfs, even though they must be passively evolving. This is possibly due to a quenching of star formation very early on, which is supported by the fact that ETGs which appear to be recently additions to the Virgo cluster show signs of fading and reddening. Additionally, the sizes of ETGs and LTGs of a given stellar mass below M∗ ≲ 108M⊙ are consistent with each other, which further supports the fading and reddening scenario.
We also investigated the bright (mB < 16 mag) LTGs within the virial radius of the Fornax main cluster and Fornax A group in terms of disk breaks. Overall we find clear signs of differences in the disk breaks of LTGs between the Fornax main cluster and Fornax A group. For example, the Fornax main cluster LTGs follows a trend between their morphological Hubble type and projected cluster-centric distance, whereas no such trend can be seen for Fornax A group LTGs. Additionally, Type II and Type III disk breaks are observed in the Fornax main cluster, with Type II disk breaks showing bluer outskirts (relative to their own inner regions) and Type III disk breaks showing redder outskirts. In contrast, these is an absence of Type II disk breaks in the Fornax A group sample, and the Type III disk breaks show a mix of inner–outer colours. Lastly, the majority of Fornax main cluster LTGs have their break radius within 1re, whereas Fornax A group LTGs tend to have break radius beyond 1re. The dichotomy of disk break properties likely reflects the higher efficiency of environmental mechanisms (e.g. ram pressure stripping, tidal effects) and the longer time spent in the Fornax main cluster than the Fornax A group.
Osajulkaisut / Original papers
Osajulkaisut eivät sisälly väitöskirjan elektroniseen versioon. / Original papers are not included in the electronic version of the dissertation.
Report series in physical sciences
|Type of Publication:
G5 Doctoral dissertation (articles)
|Field of Science:
115 Astronomy and space science
I acknowledge financial support from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 721463 to the SUNDIAL ITN network. I also wish to thank the Space physics and Astronomy research unit for the financial and administrative support.
|EU Grant Number:
(721463) SUNDIAL - SUrvey Network for Deep Imaging Analysis and Learning
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