Kinematic unrest of low mass galaxy groups

¹ Finnish centre for Astronomy with ESO (FINCA), University of Turku, Quantum, Vesilinnantie 5, 20014 Turku, Finland
² Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
e-mail: ghassem.gozaliasl@helsinki.fi
³ Helsinki Institute of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
⁴ School of Astronomy, Institute for Research in Fundamental Sciences (IPM), Tehran, Iran
⁵ CNRS, UMR 7095 & UPMC, Institut ďAstrophysique de Paris, 98bis boulevard Arago, 75014 Paris, France
⁶ Sub-department of Astrophysics, University of Oxford, Keble Road, Oxford OX1 3RH, UK

Received: 20 September 2019
Accepted: 16 January 2020

Abstract

In an effort to better understand the formation of galaxy groups, we examine the kinematics of a large sample of spectroscopically confirmed X-ray galaxy groups in the Cosmic Evolution Survey with a high sampling of galaxy group members up to z = 1. We compare our results with predictions from the cosmological hydrodynamical simulation of HORIZON-AGN. Using a phase-space analysis of dynamics of groups with halo masses of M_200c ∼ 10^12.6 − 10^14.50 M_⊙, we show that the brightest group galaxies (BGG) in low mass galaxy groups (M_200c <  2 × 10¹³ M_⊙) have larger proper motions relative to the group velocity dispersion than high mass groups. The dispersion in the ratio of the BGG proper velocity to the velocity dispersion of the group, σ_BGG/σ_group, is on average 1.48 ± 0.13 for low mass groups and 1.01 ± 0.09 for high mass groups. A comparative analysis of the HORIZON-AGN simulation reveals a similar increase in the spread of peculiar velocities of BGGs with decreasing group mass, though consistency in the amplitude, shape, and mode of the BGG peculiar velocity distribution is only achieved for high mass groups. The groups hosting a BGG with a large peculiar velocity are more likely to be offset from the L_x − σ_v relation; this is probably because the peculiar motion of the BGG is influenced by the accretion of new members.