The evolution of a satellite galaxy (stars in red) as it orbits around the Milky Way and gets fully disrupted (for more information, see Helmi et al. 2017, A&A 598, A58. A version without subtitles can be found here).

leading to a thick disk

Simulation of a minor merger (5:1 mass ratio) between a disk and a satellite, with initial conditions resembling the present-day Universe (z=0). For more information see Villalobos & Helmi (2008, 2009).

Face-on view of simulation of a minor merger (5:1 mass ratio) between a disk (in white) and a satellite (in red), with initial conditions resembling redshift z=1 conditions (Villalobos & Helmi, 2008, 2009).

Edge-on view of simulation of a minor merger (5:1 mass ratio) between a disk (in white) and a satellite (in red), with initial conditions resembling redshift z=1 conditions (Villalobos & Helmi, 2008, 2009).

Merger between a disk dwarf galaxy and a dark satellite

The satellites of dwarf galaxies may well be invisible to the eye (and not contain any stars or gas, only dark matter), but they do interact gravitational with their host and this can sometimes have a large impact (see T. Starkenburg et al. 2016).

Cosmological formation of a stellar halo

In the concordance cosmological model, the stellar halos of galaxies form hierarchically, and grow via mergers, as shown here using the Aquarius simulations coupled to a semi-analytic model of galaxy formation (Cooper et al. 2010, Helmi et al. 2011).

A dark subhalo and a narrow stream orbiting a host galaxy

This movie shows the effect of an encounter that a compact subhalo has on a cold stream and which leads to the formation of a visible gap (see e.g. Helmi and Koppelman 2016).

The motions of satellites around the Milky Way

This movie shows the motions and orbits of the classical dwarf spheroidal galaxies around the Milky Way, made possible with data from Gaia DR2 (see Helmi, van Leeuwen, Mc Millan et al, 2018 and visit this website). Credit: M. Breddels/ A. Helmi and Gaia/DPAC/ESA.