Revisiting the original Morphology-Density Relation

TL;DR

This paper revisits the original morphology-density relation, refining it by considering local galaxy densities and their impact on elliptical and early-type galaxy distributions within clusters.

Contribution

It provides new insights into how local and average densities influence galaxy morphology distributions, extending Dressler's original relation with refined analysis.

Findings

Elliptical fraction depends on local density and cluster average density.

The overall elliptical to early-type ratio remains around 30%, independent of average density.

Clusters with higher elliptical fractions tend to have lower average local densities.

Abstract

In light of recent findings from the kinematic morphology-density relation, we investigate whether the same trends exist in the original morphology density relation, using the same data as Dressler. In addition to Dressler's canonical relations, we find that further refinements are possible when considering the average local projected density of galaxies in a cluster. Firstly, the distribution of ellipticals in a cluster depends on the relative local density of galaxies in that cluster: equivalent rises in the elliptical fraction occur at higher local densities for clusters with higher average local densities. This is not true for the late-type fraction, where the variation with local density within a cluster is independent of the average local density of galaxies in that cluster, and is as Dressler originally found. Furthermore, the overall ratio of ellipticals to early-types in a cluster does not depend on the average density of galaxies in that cluster (unlike the ratio of lenticulars to disk systems), and is fixed at around 30%. In the paradigm of fast and slow rotators, we show that such an elliptical fraction in the early-type population is consistent with a slow rotator fraction of 15% in the early-type population, using the statistics of the ATLAS3D survey. We also find the scatter in the overall ratio of ellipticals to early-types is greatest for clusters with lower average densities, such that clusters with the highest elliptical fractions have the lowest average local densities. Finally, we show that average local projected density correlates well with global projected density, but the latter has difficulty in accurately characterising the density of irregular cluster morphologies.