Fossil group origins. XI. The dependence of galaxy orbits on the magnitude gap

TL;DR

This study investigates how galaxy orbits in clusters depend on the magnitude gap between the brightest members, revealing that larger gaps are associated with predominantly radial orbits, especially in the outer regions.

Contribution

It provides observational evidence linking galaxy orbit anisotropy to the magnitude gap, supporting simulation predictions about cluster evolution.

Findings

Larger magnitude gaps correlate with radial galaxy orbits.

Orbits are isotropic near the center regardless of the gap.

Smaller gaps show more isotropic or mildly radial orbits.

Abstract

We aim to study how the orbits of galaxies in clusters depend on the prominence of the corresponding central galaxies. We divided our data set of $\sim$ 100 clusters and groups into four samples based on their magnitude gap between the two brightest members, $\Delta m_{12}$. We then stacked all the systems in each sample, in order to create four stacked clusters, and derive the mass and velocity anisotropy profiles for the four groups of clusters using the MAMPOSSt procedure. Once the mass profile is known, we also obtain the (non parametric) velocity anisotropy profile via the inversion of the Jeans equation. In systems with the largest $\Delta m_{12}$, galaxy orbits are prevalently radial, except near the centre, where orbits are isotropic (or tangential when also the central galaxies are considered in the analysis). In the other three samples with smaller $\Delta m_{12}$, galaxy orbits are isotropic or only mildly radial. Our study supports the results of numerical simulations that identify radial orbits of galaxies as the cause of an increasing $\Delta m_{12}$ in groups.