[Paper Review] ENACSVII, Galaxy density profiles of rich clusters of galaxies

C. Adami, A. Mazure+2 more|arXiv (Cornell University)|Apr 17, 1998

Galaxies: Formation, Evolution, Phenomena1 references3 citations

TL;DR

This study analyzes galaxy density profiles in rich clusters using ENACS/COSMOS data, comparing core-like (King, Hubble) and cusp-like (NFW, de Vaucouleurs) models via maximum-likelihood fitting. It finds that faint galaxies favor core profiles (King), while brightest galaxies are equally well-fit by both, and outer profiles suggest a low Omega_0.

ABSTRACT

We have analyzed the projected galaxy distributions in a subset of the ENACS/ COSMOS cluster sample. We made Maximum-Likelihood fits to the distribution of COSMOS galaxies for 4 theoretical profiles, with `cores' (generalized King- and Hubble-profiles) and with `cusps' (generalized Navarro et al., or NFW, and de Vaucouleurs profiles). We use the Likelihood ratio to investigate whether the observations are better described by profiles with cusps or with cores. Taking the King and NFW profiles as model of either class, we find that about 75% of the clusters are better fitted by the King profile than by the NFW profile. However, for the individual clusters the preference for the King profile is rarely significant. When we limit ourselves to the central regions it appears that the significance increases drastically, with 65% of the clusters showing a strong preference for a King over an NFW profile. We constructed composite clusters from the COSMOS and ENACS data, taking special care to avoid the creation or the destruction of cusps. We scale by different ways (projected distances, core radii, r_{200}).In all three cases we find that the King profile is clearly preferred over the NFW profile. However, this preference is not shared by the brightest galaxies. We conclude that these galaxies are represented almost equally well by King and NFWprofiles, but that the distribution of the fainter galaxies clearly shows a core rather than a cusp. Finally, we compared the outer slope of the galaxy distributions in ourclusters with results for model calculations with different cosmological parameters. We conclude that the observed profile slope indicates a low value for Omega_0. This is consistent with the direct estimate of Omega_0 based on the M/L ratios of the individual clusters.

Motivation & Objective

• To determine whether galaxy density profiles in rich clusters are better described by core-like or cusp-like models.
• To assess the significance of core vs. cusp profiles in individual clusters and their central regions.
• To construct composite clusters to minimize noise and avoid artificial cusps or cores.
• To compare observed outer slope with cosmological model predictions to constrain Omega_0.
• To examine differences in fitting between brightest galaxies and fainter cluster members.

Proposed method

• Performs maximum-likelihood fits to projected galaxy distributions in 4 theoretical profiles: generalized King, Hubble, NFW, and de Vaucouleurs.
• Uses the likelihood ratio test to statistically compare fits of core (King, Hubble) vs. cusp (NFW, de Vaucouleurs) models.
• Constructs composite clusters from ENACS and COSMOS data using multiple scaling methods: projected distance, core radius, and r_200.
• Applies careful selection to avoid introducing or removing artificial cusps during composite construction.
• Compares observed outer slope of galaxy profiles with predictions from cosmological simulations under different Omega_0 values.
• Analyzes fitting performance separately for brightest galaxies and fainter galaxies within clusters.

Experimental results

Research questions

• RQ1 Which model—core-like (King) or cusp-like (NFW)—provides a better fit to the galaxy density distribution in rich clusters?
• RQ2 How significant is the preference for a core profile over a cusp profile in individual clusters, especially in the central regions?
• RQ3 Do composite cluster profiles show a consistent preference for core or cusp models, and does this preference depend on scaling method?
• RQ4 How do the fitting results differ between the brightest cluster galaxies and the general population of fainter galaxies?
• RQ5 What constraints on Omega_0 can be derived from the observed outer slope of galaxy density profiles?

Key findings

• About 75% of individual clusters are better fitted by the King profile than by the NFW profile, though the preference is rarely statistically significant.
• In the central regions, 65% of clusters show a strong preference for the King profile over the NFW profile, indicating a core structure in dense cluster centers.
• Composite cluster profiles constructed from ENACS and COSMOS data consistently favor the King profile over the NFW profile across all three scaling methods (projected distance, core radius, r_200).
• The distribution of fainter galaxies clearly shows a core, while the brightest galaxies are fitted almost equally well by both King and NFW profiles.
• The observed outer slope of galaxy density profiles is consistent with a low value of Omega_0, supporting direct estimates from M/L ratios in individual clusters.
• The preference for core profiles in composite clusters is robust and not an artifact of scaling or data combination.