We study the formation of the Intra-Cluster Light (ICL) using a semi-analytic model of galaxy formation, coupled to merger trees extracted from N-body simulations of groups and clusters. We assume that the ICL forms by (1) stellar stripping of satellite galaxies and (2) relaxation processes that take place during galaxy mergers. The fraction of ICL in groups and clusters predicted by our models ranges between 10 and 40 per cent, with a large halo-to-halo scatter and no halo mass dependence. We note, however, that our predicted ICL fractions depend on the resolution: for a set of simulations with particle mass one order of magnitude larger than that adopted in the high resolution runs used in our study, we find that the predicted ICL fractions are 30-40 per cent larger than those found in the high resolution runs. On cluster scale, large part of the scatter is due to a range of dynamical histories, while on smaller scale it is driven by individual accretion events and stripping of very massive satellites, M * 10 10.5 M ⊙ , that we find to be the major contributors to the ICL. The ICL in our models forms very late (below z ∼ 1), and a fraction varying between 5 and 25 per cent of it has been accreted during the hierarchical growth of haloes. In agreement with recent observational measurements, we find the ICL to be made of stars covering a relatively large range of metallicity, with the bulk of them being sub-solar.

We present simulations of the formation of thick discs via the accretion of two-component satellites onto a pre-existing thin disc. Our goal is to establish the detailed characteristics of the thick discs obtained in this way, as well as their dependence on the initial orbital and internal properties of the accreted objects. We find that mergers with 10-20 per cent mass of the host lead to the formation of thick discs whose characteristics are similar, both in morphology as in kinematics, to those observed. Despite the relatively large mass ratios, the host discs are not fully destroyed by the infalling satellites: a remaining kinematically cold and thin component containing ∼15-25 per cent of the mass can be identified, which is embedded in a hotter and thicker disc. This may for example, explain the existence of a very old thin disc stars in the Milky Way. The final scaleheights of the discs depend both on the initial inclination and properties of the merger, but the fraction of satellite stellar particles at ∼4 scaleheights directly measures the mass ratio between the satellite and host galaxy. Our thick discs typically show boxy isophotes at very low surface brightness levels (>6 mag below their peak value). Kinematically, the velocity ellipsoids of the simulated thick discs are similar to that of the Galactic thick disc at the solar radius. The trend of σ Z /σ R with radius is found to be a very good discriminant of the initial inclination of the accreted satellite. In the Milky Way, the possible existence of a vertical gradient in the rotational velocity of the thick disc as well as the observed value of σ Z /σ R at the solar vicinity appear to favour the formation of the thick disc by a merger with either low or intermediate orbital inclination.

Motivated by recent studies suggesting that the Large Magellanic Cloud (LMC) could be significantly more massive than previously thought, we explore whether the approximation of an inertial Galactocentric reference frame is still valid in the presence of such a massive LMC. We find that previous estimates of the LMC's orbital period and apocentric distance derived assuming a fixed Milky Way are significantly shortened for models where the Milky Way is allowed to move freely in response to the gravitational pull of the LMC. Holding other parameters fixed, the fraction of models favoring first infall is reduced. Due to this interaction, the Milky Way center of mass within the inner 50 kpc can be significantly displaced in phase-space in a very short period of time that ranges from 0.3 to 0.5 Gyr by as much as 30 kpc and 75 km/s. Furthermore, we show that the gravitational pull of the LMC and response of the Milky Way are likely to significantly affect the orbit and phase space distribution of tidal debris from the Sagittarius dwarf galaxy (Sgr). Such effects are larger than previous estimates based on the torque of the LMC alone. As a result, Sgr deposits debris in regions of the sky that are not aligned with the present-day Sgr orbital plane. In addition, we find that properly accounting for the movement of the Milky Way around its common center of mass with the LMC significantly modifies the angular distance between apocenters and tilts its orbital pole, alleviating tensions between previous models and observations. While these models are preliminary in nature, they highlight the central importance of accounting for the mutual gravitational interaction between the MW and LMC when modeling the kinematics of objects in the Milky Way and Local Group.

We present a detailed analysis of the influence of the environment and of the environmental history on quenching star formation in central and satellite galaxies in the local Universe. We take advantage of publicly available galaxy catalogues obtained from applying a galaxy formation model to the Millennium simulation. In addition to halo mass, we consider the local density of galaxies within various fixed scales. Comparing our model predictions to observational data (SDSS), we demonstrate that the models are failing to reproduce the observed density dependence of the quiescent galaxy fraction in several aspects: for most of the stellar mass ranges and densities explored, models cannot reproduce the observed similar behaviour of centrals and satellites, they slightly under-estimate the quiescent fraction of centrals and significantly overestimate that of satellites. We show that in the models, the density dependence of the quiescent central galaxies is caused by a fraction of "backsplash" centrals which have been satellites in the past. The observed stronger density dependence on scales of 0.2 − 1 Mpc may, however, indicate additional environmental processes working on central galaxies. Turning to satellite galaxies, the density dependence of their quiescent fractions reflects a dependence on the time spent orbiting within a parent halo, correlating strongly with halo mass and distance from the halo centre. Comparisons with observational estimates suggest relatively long gas consumption time scales of roughly 5 Gyr in low mass satellite galaxies. The quenching time scales decrease with increasing satellite stellar mass. Overall, a change in modelling both internal processes and environmental processes is required for improving currently used galaxy formation models.

We study the orbital properties of stars in four (published) simulations of thick discs formed by (i) accretion from disrupted satellites, (ii) heating of a pre‐existing thin disc by a minor merger, (iii) radial migration and (iv) gas‐rich mergers. We find that the distribution of orbital eccentricities is predicted to be different for each model: a prominent peak at low eccentricity is expected for the heating, migration and gas‐rich merging scenarios, while the eccentricity distribution is broader and shifted towards higher values for the accretion model. These differences can be traced back to whether the bulk of the stars in each case is formed in situ or is accreted, and is robust to the peculiarities of each model. A simple test based on the eccentricity distribution of nearby thick‐disc stars may thus help elucidate the dominant formation mechanism of the Galactic thick disc.