IDILICO⁺

Despite the ubiquity of poor groups of galaxies in the local universe, we know little, outside of the Local Group (LG), about the physics of these systems that typically contain fewer than five bright galaxies (L ~ L*), as other environments (e.g., clusters of galaxies) with more conspicuous characteristics have traditionally attracted more attention to the studies of the environmental dependence of galaxy evolution.

The IDILICO (Investigation of the DIffuse LIght COmponent in compact groups of galaxies) project is an international collaboration aimed at creating a large batch of numerical simulations of forming galaxy groups using high-performance computing resources. Born initially with the goal of sheding light on the origin of the extraordinarily large fractions of diffuse intragroup light found in some of the compact groups cataloged by Hickson, the original IDILICO project, under its new name IDILICO⁺, has now expanded its focus to investigate other relevant aspects of galaxy evolution, especially those driven by major mergers and the multiple collisions that galaxies experience in the group environment.

We investigate the dynamics of galaxy pairs and groups by means of controlled simulations, which facilitate an exhaustive and efficient exploration of the parameter space. Our groups are evolved from an initial epoch (z = 3) in which they are still expanding and about to abandon the linear regime until the end of the first collapse phase, when they adopt a compact configuration. This means that the role of secondary infall and, hence, of the large-scale environment of the groups, can be safely neglected. Otherwise, we have dessigned a realistic quasi-cosmological setting in which galaxies are always out of equilibrium as a result of mass accretion from their surroundings and frequent minor and hierarchical merging events with other companion galaxies continually forming new aggregations. On the other hand, we have also performed nearly 600 high-resolution N-body simulations of collisions of isolated pairs of similarly massive galaxies on bound orbits. Our dissipationless experiments follow the merging of live models of late- and early-type objects, composed of a rotating NFW CDM halo and a central stellar core, with mass ratios ranging from 1:1 to 3:1, and with orbital parameters and global properties broadly consistent with the predictions of the current cosmological model. All our simulations assume a standard spatially-flat ΛCDM concordant cosmology.

To be sure, our gas-free experiments provide only an approximate description of the very complex process of the evolution of galaxies. However, by concentrating our attention on the gravitational dynamics of the dark matter and the old stellar component of galaxies, we can obtain a clear understanding of the specific role of gravity in the build-up of galaxies, groups and clusters and, at the same time define a framework that allows the quantification of the contribution of gas physics to the evolution of these systems. This justifies our decision of adopting for the time being a collisionless approach. Yet, we do noy give up on the idea of incorporating, in a later stage of our investigation, the hydrodynamics of the gas into both the galaxies and the IGM through prescriptions for gas cooling, star formation, stellar feedback, and galactic winds. In this manner we should be able to study also those aspects of group evolution in which the gaseous component plays a fundamental role.

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