ABSTRACT
The Fornax cluster is the nearest large cluster in the southern sky, and is currently experiencing active assembly of mass. It is thus the target of a number of ongoing observing campaigns ...at optical, near-infrared, and radio wavelengths, using state-of-the-art facilities in the Southern hemisphere. Spectroscopic redshifts are essential not only for determining cluster membership, but also kinematics within the cluster and identifying substructures. We present a compilation of all available major spectroscopic campaigns undertaken within the Fornax region, including new and previously unpublished spectroscopy. This provides not only a comprehensive census of Fornax cluster membership as a resource for the many ongoing studies of this dynamic system, but also probes the large-scale structure in the background volume.
Context. Due to their relatively low stellar mass content and diffuse nature, the evolution of dwarf galaxies can be strongly affected by their environment. Analyzing the properties of the dwarf ...galaxies over a wide range of luminosities, sizes, morphological types, and environments, we can obtain insights about their evolution. At ∼50 Mpc, the Hydra I cluster of galaxies is among the closest cluster in the z ≃ 0 Universe, and an ideal environment to study dwarf galaxy properties in a cluster environment. Aims. We exploit deep imaging data of the Hydra I cluster to construct a new photometric catalog of dwarf galaxies in the cluster core, which is then used to derive properties of the Hydra I cluster dwarf galaxy population as well as to compare it with other clusters. Moreover, we investigate the dependency of dwarf galaxy properties on their surrounding environment. Methods. The new wide-field g- and r-band images of the Hydra I cluster obtained with the OmegaCAM camera on the VLT Survey Telescope (VST) in the context of the VST Early-type GAlaxy Survey (VEGAS) were used to study the dwarf galaxy population in the Hydra I cluster core down to r-band magnitude Mr = −11.5 mag. We used an automatic detection tool to identify dwarf galaxies from a ∼1 deg2 field centered on the Hydra I core, covering almost half of the cluster virial radius. The photometric pipeline was used to estimate the principal photometric parameters for all targets. Scaling relations and visual inspection were used to assess the cluster membership and construct a new dwarf galaxy catalog. Finally, based on the new catalog, we studied the structural (Sérsic index n, effective radius Re, and axis ratio) and photometric (colors and surface brightness) properties of the dwarf galaxies, also investigating how they vary as a function of clustercentric distance. Results. The new Hydra I dwarf catalog contains 317 galaxies with a luminosity between −18.5 < Mr < −11.5 mag, a semi-major axis larger than ∼200 pc (a = 0.84″), of which 202 are new detections, and previously unknown dwarf galaxies in the Hydra I central region. We estimate that our detection efficiency reaches 50% at the limiting magnitude Mr = −11.5 mag, and at the mean effective surface brightness μ̄e,r = 26.5 mag arcsec−2. We present the standard scaling relations for dwarf galaxies, which are color-magnitude, size-luminosity, and Sérsic n-magnitude relations, and compare them with other nearby clusters. We find that there are no observational differences for dwarfs scaling relations in clusters of different sizes. We study the spatial distribution of galaxies, finding evidence for the presence of substructures within half the virial radius. We also find that mid- and high-luminosity dwarfs (Mr < −14.5 mag) become, on average, redder toward the cluster center, and that they have a mild increase in Re with increasing clustercentric distance, similar to what is observed for the Fornax cluster. No clear clustercentric trends are reported for surface brightness and Sérsic index. Considering galaxies in the same magnitude bins, we find that for high and mid-luminosity dwarfs (Mr < −13.5 mag), the g − r color is redder for the brighter surface brightness and higher Sérsic n index objects. This finding is consistent with the effects of harassment and/or partial gas stripping.
ABSTRACT
Due to their low surface brightness, dwarf galaxies are particularly susceptible to tidal forces. The expected degree of disturbance depends on the assumed gravity law and whether they have ...a dominant dark halo. This makes dwarf galaxies useful for testing different gravity models. In this project, we use the Fornax Deep Survey (FDS) dwarf galaxy catalogue to compare the properties of dwarf galaxies in the Fornax Cluster with those predicted by the Lambda cold dark matter (ΛCDM) standard model of cosmology and Milgromian dynamics (MOND). We construct a test particle simulation of the Fornax system. We then use the Markov Chain Monte Carlo (MCMC) method to fit this to the FDS distribution of tidal susceptibility η (half-mass radius divided by theoretical tidal radius), the fraction of dwarfs that visually appear disturbed as a function of η, and the distribution of projected separation from the cluster centre. This allows us to constrain the η value at which dwarfs should get destroyed by tides. Accounting for an r′-band surface brightness limit of 27.8 magnitudes per square arcsec, the required stability threshold is $\eta _{\textrm {destr}} = 0.25^{+0.07}_{-0.03}$ in ΛCDM and $1.88^{+0.85}_{-0.53}$ in MOND. The ΛCDM value is in tension with previous N-body dwarf galaxy simulations, which indicate that ηdestr ≈ 1. Our MOND N-body simulations indicate that ηdestr = 1.70 ± 0.30, which agrees well with our MCMC analysis of the FDS. We therefore conclude that the observed deformations of dwarf galaxies in the Fornax Cluster and the lack of low surface brightness dwarfs towards its centre are incompatible with ΛCDM expectations but well consistent with MOND.
Context.
The transformation of late-type galaxies has been suggested as the origin of early-type dwarf galaxies (typically
M
⋆
≤ 10
9
M
⊙
) in galaxy clusters. Based on deep images, Venhola and ...colleagues analysed correlations between colour and surface brightness for galaxies in the Fornax cluster binned by luminosity or stellar mass. In the bins with
M
⋆
< 10
8
M
⊙
, the authors identified a correlation of redness with fainter surface brightness and interpreted it as a consequence of the quenching of star formation by ram pressure stripping in the dwarf galaxies.
Aims.
This study carries out a similar analysis for the Virgo cluster. The analysis for both clusters is then used to compare the Virgo and Fornax clusters, for which the ram pressure is expected to have different strengths. The purpose of this is to scrutinise the ram pressure interpretation from the other study and search for differences between the clusters that reflect the different ram pressure efficiencies, which would either support or weaken this interpretation. Ultimately, this could help weigh the importance of ram pressure stripping relative to other transformative processes in the shaping of the dominant early-type dwarf galaxy population.
Methods.
We extend the analysis of colour versus surface brightness binned by stellar mass to higher masses and a wider range of optical colours. The results, in particular at low stellar mass, are compared to predictions of stellar evolution models. Benefitting from larger sample sizes, we also analyse late- and early-type galaxies separately. This analysis is carried out for the Virgo and Fornax clusters, and the colour versus surface brightness relation, as well as other properties of the two clusters’ galaxy populations, are compared.
Results.
While the colour–surface brightness diagrams are remarkably similar for the two clusters, only the low-mass late-type galaxies are found to have slopes consistent with a fading and reddening following the quenching of star formation. For the early-type galaxies, there are no (or only weak) correlations between colour and surface brightness in all mass bins. Early- and late-type galaxies in both clusters have comparable sizes below a stellar mass of
M
⋆
≲ 10
8
M
⊙
. The colour and size scaling relations are very similar for the Virgo and Fornax clusters. However, Virgo features a lower fraction of early-type or red galaxies despite its higher mass.
Conclusions.
The similarity of early-type dwarfs and low-mass late types in size at the masses
M
⋆
≲ 10
8
M
⊙
as well as the overall consistency of the colour–surface brightness correlation with fading stellar populations support a scenario of transformation via the quenching of star formation, for example by gas removal. However, the lack of this imprint of an ageing stellar population on the early-type dwarfs themselves calls for some additional explanation. Finally, the Virgo cluster is an atypical cluster with a comparably low fraction of quiescent early-type galaxies at all galaxy masses despite its large cluster mass.
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_{\mathrm{*,nuc}} \propto {M_{\mathrm{*,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 (Δ(
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 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
.
Context. The transformation of late-type galaxies has been suggested as the origin of early-type dwarf galaxies (typically M⋆ ≤ 109 M⊙) in galaxy clusters. Based on deep images, Venhola and ...colleagues analysed correlations between colour and surface brightness for galaxies in the Fornax cluster binned by luminosity or stellar mass. In the bins with M⋆ < 108 M⊙, the authors identified a correlation of redness with fainter surface brightness and interpreted it as a consequence of the quenching of star formation by ram pressure stripping in the dwarf galaxies. Aims. This study carries out a similar analysis for the Virgo cluster. The analysis for both clusters is then used to compare the Virgo and Fornax clusters, for which the ram pressure is expected to have different strengths. The purpose of this is to scrutinise the ram pressure interpretation from the other study and search for differences between the clusters that reflect the different ram pressure efficiencies, which would either support or weaken this interpretation. Ultimately, this could help weigh the importance of ram pressure stripping relative to other transformative processes in the shaping of the dominant early-type dwarf galaxy population. Methods. We extend the analysis of colour versus surface brightness binned by stellar mass to higher masses and a wider range of optical colours. The results, in particular at low stellar mass, are compared to predictions of stellar evolution models. Benefitting from larger sample sizes, we also analyse late- and early-type galaxies separately. This analysis is carried out for the Virgo and Fornax clusters, and the colour versus surface brightness relation, as well as other properties of the two clusters’ galaxy populations, are compared. Results. While the colour–surface brightness diagrams are remarkably similar for the two clusters, only the low-mass late-type galaxies are found to have slopes consistent with a fading and reddening following the quenching of star formation. For the early-type galaxies, there are no (or only weak) correlations between colour and surface brightness in all mass bins. Early- and late-type galaxies in both clusters have comparable sizes below a stellar mass of M⋆ ≲ 108 M⊙. The colour and size scaling relations are very similar for the Virgo and Fornax clusters. However, Virgo features a lower fraction of early-type or red galaxies despite its higher mass. Conclusions. The similarity of early-type dwarfs and low-mass late types in size at the masses M⋆ ≲ 108 M⊙ as well as the overall consistency of the colour–surface brightness correlation with fading stellar populations support a scenario of transformation via the quenching of star formation, for example by gas removal. However, the lack of this imprint of an ageing stellar population on the early-type dwarfs themselves calls for some additional explanation. Finally, the Virgo cluster is an atypical cluster with a comparably low fraction of quiescent early-type galaxies at all galaxy masses despite its large cluster mass.
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