Fossil group origins

VII. Galaxy substructures in fossil systems^⋆

¹ Instituto de Astrofísica de Canarias, calle via Láctea S/N, 38205 La Laguna, Tenerife, Spain
e-mail: ste.zarattini@gmail.com
² Universidad de La Laguna, Dept. Astrofísica, 38206 La Laguna, Tenerife, Spain
³ Università degli Studi di Padova, Dipartimento di Fisica e Astronomia, vicolo dell’osservatorio 3, 35122 Padova, Italy
⁴ Università degli Studi di Trieste, Dipartimento di Fisica-Sezione Astronomia, via Tiepolo 11, 34143 Trieste, Italy
⁵ INAF−Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ Fundación Galileo Galilei−INAF, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, La Palma, Spain
⁷ Instituto Nacional de Astrofísica, Óptica y Electrónica, Luis Enrique Erro 1, Sta. Ma. Tonantzintla, Puebla, Mexico
⁸ Department of Astronomy, University of Wisconsin-Madison, 475 N. Charter St., Madison, WI 53706, USA
⁹ School of Physics and Astronomy, University of St Andrews, North Haugh, St Andrews, KY16 9SS, UK
¹⁰ NRC Herzberg Institute of Astrophysics, 5071 West Saanich Road, Victoria, BC, V9E 2E7, Canada

Received: 12 August 2015
Accepted: 28 October 2015

Abstract

Context. Fossil groups (FG) are expected to be the final product of galaxy merging within galaxy groups. In simulations, they are predicted to assemble their mass at high redshift. This early formation allows for the innermost M^∗ galaxies to merge into a massive central galaxy. Then, they are expected to maintain their fossil status because of the few interactions with the large-scale structure. In this context, the magnitude gap between the two brightest galaxies of the system is considered a good indicator of its dynamical status. As a consequence, the systems with the largest gaps should be dynamically relaxed.

Aims. In order to examine the dynamical status of these systems, we systematically analyze, for the first time, the presence of galaxy substructures in a sample of 12 spectroscopically-confirmed fossil systems with redshift z ≤ 0.25.

Methods. We apply a number of tests to investigate the substructure in fossil systems in the two-dimensional space of projected positions out to R₂₀₀. Moreover, for a subsample of five systems with at least 30 spectroscopically-confirmed members we also analyze the substructure in the velocity and in the three-dimensional velocity-position spaces. Additionally, we look for signs of recent mergers in the regions around the central galaxies.

Results. We find that an important fraction of fossil systems show substructure. The fraction depends critically on the adopted test, since each test is more sensitive to a particular type of substructure.

Conclusions. Our interpretation of the results is that fossil systems are not, in general, as relaxed as expected from simulations. Our sample of 12 spectroscopically-confirmed fossil systems need to be extended to compute an accurate fraction, but our conclusion is that this fraction is similar to the fraction of substructure detected in nonfossil clusters. This result points out that the magnitude gap alone is not a good indicator of the dynamical status of a system. However, the subsample of five FGs for which we were able to use velocities as a probe for substructures is dominated by high-mass FGs. These massive systems could have a different evolution compared to low-mass FGs, since they are expected to form via the merging of a fossil group with another group of galaxies. This merger would lengthen the relaxation time and it could be responsible for the substructure detected in present-time massive FGs. If this is the case, only low-mass FGs are expected to be dynamically old and relaxed.