Fossil group origins

XI. The dependence of galaxy orbits on the magnitude gap

¹ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
e-mail: ste.zarattini@gmail.com
² AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
³ INAF-Osservatorio Astronomico di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁴ IFPU Institute for Fundamental Physics of the Universe, Via Beirut 2, 34014 Trieste, Italy
⁵ Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
⁶ Departamento de Astrofísica, Universidad de La Laguna, Avenida Astrofísico Francisco Sánchez s/n, 38206 La Laguna, Spain
⁷ Dipartimento di Fisica, Università degli Studi di Trieste, Via Tiepolo 11, 34143 Trieste, Italy
⁸ Department of Astronomy, University of Wisconsin – Madison, 475 North Charter Street, Madison, WI 53706, USA

Received: 23 June 2020
Accepted: 12 July 2021

Abstract

Aims. We aim to study how the orbits of galaxies in clusters depend on the prominence of the corresponding central galaxies.

Methods. We divided our data set of ∼100 clusters and groups into four samples based on their magnitude gap between the two brightest members, Δm₁₂. 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.

Results. In systems with the largest Δm₁₂, galaxy orbits are generally 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 Δm₁₂, galaxy orbits are isotropic or only mildly radial.

Conclusions. Our study supports the results of numerical simulations that identify radial orbits of galaxies as the cause of an increasing Δm₁₂ in groups.