[Paper Review] The ESO Nearby Abell Cluster Survey. XI. Segregation of cluster galaxies and subclustering

A. Biviano, P. Katgert+2 more|ArXiv.org|Jan 31, 2002

Galaxies: Formation, Evolution, Phenomena64 references86 citations

TL;DR

This study analyzes luminosity and morphology segregation in 59 nearby galaxy clusters from the ESO Nearby Abell Cluster Survey (ENACS), using 3,056 member galaxies with positions, velocities, and magnitudes. It finds that luminosity segregation is significant only for bright ellipticals ($M_R \leq -22.0$) outside substructures, while early- and late-type galaxies show distinct spatial and kinematic distributions; substructure membership further differentiates these patterns, revealing complex dynamical segregation beyond simple morphology-density relations.

ABSTRACT

We study luminosity and morphology segregation of over 3000 cluster galaxies in an ensemble cluster of 59 rich, nearby galaxy clusters observed in the ESO Nearby Cluster Survey (ENACS). Within this sample we identify those galaxies that are in substructures. We compare the distribution of projected clustercentric distance, R, and 'normalized' relative line-of-sight velocity, v, of several subsamples, using the 2D Kolmogorov-Smirnov test. We find evidence of luminosity segregation only for the ellipticals outside substructure and brighter than M_R = -22.0 +/- 0.1 (h0=100 km/s/Mpc). We confirm the well-known segregation of early- and late-type galaxies. The early and late spirals (Sa--Sb and Sbc--Ir respectively) outside substructure also appear to have different (R,v)-distributions. In each morphological class, the galaxies within substructure have different (R,v)-distributions from the galaxies that are not in substructure. Among the samples of galaxies that are not in substructure, at least 3 ensembles can and must be distinguished: [E+S0], S_early, and [S_late+ELG]. The [E+S0] ensemble is most centrally concentrated and has a fairly low velocity dispersion that hardly varies with radius. The [S_late+ELG] ensemble is least concentrated and has the highest velocity dispersion, which increases significantly towards the centre. The (R,v)-distribution of S_early galaxies is intermediate between those of the two other ensembles. Among the galaxies within substructure the S0 and [S_late+ELG] galaxies have different (R,v) distributions. We discuss briefly the implications of our results for processes of galaxy destruction and transformation within clusters. [ABRIDGED]

Motivation & Objective

• To investigate luminosity and morphology segregation in a large ensemble of nearby galaxy clusters.
• To determine how substructure membership affects the spatial and kinematic distributions of galaxies.
• To test whether different galaxy types (E, S0, S, ELG) exhibit distinct dynamical behavior based on location and velocity.
• To constrain physical processes such as substructure destruction, spiral galaxy disruption, and S0 formation through observed segregation patterns.

Proposed method

• Constructs an ensemble cluster from 59 rich, nearby Abell clusters observed in the ENACS survey, totaling 3,056 member galaxies with redshifts, magnitudes, and morphologies.
• Identifies substructures using a 2D velocity and position criterion: galaxies are in substructure if their velocity dispersion is significantly lower than the cluster mean or if their mean velocity differs significantly from the cluster's bulk velocity.
• Applies a 2D Kolmogorov-Smirnov (KS2D) test to compare the projected clustercentric distance ($R$) and line-of-sight velocity ($v$) distributions of galaxy subsamples.
• Classifies galaxies into 10 subsamples: E, S0, S_e, S_l, and ELG, both within and outside substructures.
• Uses a limiting parameter $\delta_{\text{lim}}$ to define substructure membership, testing robustness across different thresholds to assess contamination and statistical significance.
• Analyzes the significance of differences in $R$-$v$ distributions to infer dynamical segregation and constrain physical processes in clusters.

Experimental results

Research questions

• RQ1 Is there significant luminosity segregation in cluster galaxies, and under what conditions does it emerge?
• RQ2 How does substructure membership affect the spatial and kinematic distributions of early-type and late-type galaxies?
• RQ3 Do different galaxy types (E, S0, S, ELG) exhibit distinct dynamical behavior in terms of concentration and velocity dispersion?
• RQ4 Can observed segregation patterns constrain physical processes such as substructure dissolution, spiral galaxy destruction, and S0 formation?

Key findings

• Luminosity segregation is only evident for bright ellipticals ($M_R \leq -22.0$) located outside substructures, primarily due to the central concentration of the brightest cluster galaxies.
• Early- and late-type galaxies show strong morphology segregation in both position and velocity space, with early spirals (Sa–Sb) and late spirals (Sbc–Ir) having distinct $R$-$v$ distributions.
• Galaxies within substructures do not share identical $R$-$v$ distributions; S0s and late-type galaxies (S_l + ELG) in substructures show different spatial and kinematic patterns.
• The [E+S0] ensemble is most centrally concentrated with low, radius-independent velocity dispersion, while [S_l + ELG] is least concentrated and has a velocity dispersion that increases toward the cluster center.
• The S_e class shows intermediate behavior: velocity dispersion matches [E+S0] in outer regions but increases toward the center, indicating a transitional dynamical state.
• Substructure membership significantly alters the observed segregation: for example, S_l,nosub and S_l,sub show different distributions, and the S_l,sub sample is small but shows a statistically significant difference from ELG nosub, suggesting real dynamical distinction.