Galaxy evolution in groups: Transition galaxies in the IllustrisTNG simulations

¹ Instituto de Astronomía Teórica y Experimental, CONICET – UNC, Laprida 854, X5000BGR Córdoba, Argentina
² Facultad de Matemática, Astronomía, Física y Computación, Universidad Nacional de Córdoba, Av. Medina Allende s/n, X5000HUA Córdoba, Argentina
³ Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
⁴ Departamento de Física Teórica, Universidad Autónoma de Madrid, Cantoblanco, 28049 Madrid, Spain
⁵ Instituto de Física Teórica (IFT-UAM/CSIC), Universidad Autónoma de Madrid, 28049 Madrid, Spain
⁶ SISSA – International School for Advanced Studies, Via Bonomea 265, 34136 Trieste, Italy
⁷ INAF, Osservatorio Astronomico di Trieste, via Tiepolo 11, I-34131 Trieste, Italy
⁸ IFPU, Institute for Fundamental Physics of the Universe, via Beirut 2, 34014 Trieste, Italy

^⋆ Corresponding author.

Received: 7 March 2025
Accepted: 8 April 2025

Abstract

Context. The evolution of galaxies is significantly influenced by the environments they inhabit. While high-density regions, such as clusters of galaxies have been widely studied, the dynamics and quenching processes in intermediate environments remain less explored. These systems provide a valuable context for understanding the transition of galaxies from active star formation to quiescence.

Aims. This study aims to characterise the astrophysical properties of galaxies in intermediate-mass galaxy groups (13.5 ≤ log(M₂₀₀/M_⊙)≤13.7), with a focus on their evolutionary pathways and the key processes driving their transition through the green valley (GV) and green zone (GZ). Specifically, we explore the interplay between internal and external quenching mechanisms and their impact on galaxy evolution within groups and their surrounding environments.

Methods. Using the Illustris TNG300-1 hydrodynamical cosmological simulations, we classified galaxies based on their trajectories and environment into five categories: group galaxies (GRs), backsplash galaxies (BSs), recent infallers (RINs), infall galaxies (INs), and field galaxies (FGs). We examined their optical colours in the (u − r)-stellar mass diagram, specific star formation rates (sSFRs), gas fractions, and stellar mass evolution from z = 0.5 to z = 0.

Results. At z = 0, FGs dominate the blue cloud, while GRs show progressive reddening, especially in low-mass systems. Compared to the other classes, BSs exhibit the highest fraction of green galaxies, highlighting their transitional nature. On the other hand, RINs show a rapid quenching upon entering R₂₀₀. Notably, RINs experience greater environmental influence than BSs, due to their later entry into more massive systems.

Conclusions. Our results reveal that the timing of group entry and environmental effects, such as gas depletion, are critical to the decline in sSFR and the transition of galaxies through the GV towards quiescence. Green BS and RINs, in particular, demonstrate distinct evolutionary tracks shaped by their interactions with the group environment, with green RINs showing more rapid changes over shorter timescales. This analysis highlights the significant role of the entry time into the group in shaping galaxy evolution. BSs, having entered at an earlier stage, encounter a system that is less massive than the one RINs interact with upon arrival. Consequently, RINs experience a stronger influence from the intragroup medium than BSs do.