Tracing the evolution of the brightest galaxies and diffuse light in galaxy groups

¹ Instituto de Astrofísica de Andalucía (IAA–CSIC), Glorieta de la Astronomía, s/n, E-18008 Granada, Spain
² Departament de Física Quàntica i Astrofísica and Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona, C. Martí i Franquès, 1, E-08028 Barcelona, Spain

^★ Corresponding author: betelehem@iaa.csic.es

Received: 1 August 2025
Accepted: 27 October 2025

Abstract

We present the second study based on a suite of 100 cosmologically motivated, controlled N-body simulations designed to advance our understanding of the role of purely gravitational dynamics in the early formation of low-mass galaxy groups (∼1–5 × 10¹³ M_⊙). In this work, we investigate the temporal evolution of key indicators of dynamical relaxation, with a particular emphasis on the secular growth of the diffuse intragroup light (IGL), the four major group galaxies, and the mass distributions of their progenitors. We also assess the diagnostic power of several magnitude gaps between top-ranked galaxies as proxies for dynamical age. As in our previous study, we compare the outcomes from three group classes defined by the number of the brightest group galaxies (BGGs) present at the end of the simulations: single-, double-, and non-BGG systems. We find that the early assembly of galaxy groups is consistent with a stochastic Poisson process at an approximately constant merger rate. Various dynamical diagnostics, including galaxy pairwise separations, velocity dispersions, and the offset of the first-ranked galaxy from the group barycentre, indicate that single-BGG groups evolve more rapidly towards virialisation than double- and (especially) non-BGG systems. We further find that first-ranked group members and the IGL, despite their intertwined origins, follow distinct growth histories, with the IGL assembled from a more numerous and systematically lower mass population than the central object. This distinction is particularly pronounced in non-BGG systems, where about one-third of the IGL originates from small galaxies, each contributing less than 5% to this component. Among the tested magnitude gaps, the difference between the first- and fourth-ranked galaxies, Δℳ₄₋₁, is proven to be a more robust indicator of dynamical age for low-mass groups than the conventional Δℳ₂₋₁ gap. The Δℳ₅₋₁ and Δℳ₆₋₁ gaps also perform well and might be preferable in certain contexts.