Simulations 1 : small-scale

Before we get started, it should be pointed out that what is meant here by “small-scale” is something on the scale of our galaxy’s dark matter halo. This scale is of the order of around 1.6 million light-years or so in size- quite large on the everyday scale, but small on a cosmological scale.

We’ll have a brief look at the Aquarius Project simulations.

These are a set of simulations of the evolution of the structure of galactic dark matter haloes, where the total mass involved is comparable to the mass of our Milky Way galaxy. Thus, they can be used to gain insight into the development of our own galaxy, including the comparison of observed structures in the vicinity of our galaxy (such as satellite galaxies and remnant stellar streams) with the various models’ predictions.

Broadly speaking, each model consists of up to around 200 million interacting particles (which represent dark matter, but not individual dark matter particles such as the hypothesised WIMPs- the scale is larger than that), and their motions and interactions are followed over a time equivalent to the age of the universe.

The kind of filamentary structure that is seen on the larger scales (say, in galaxy surveys as well as large-scale dark matter cosmological simulations) is seen also on the galactic-halo scale. As time progresses, however, a denser region evolves, attracting more particles via gravity- thus growing in mass and density, and becoming more attractive, and so on.

Aquarius tryptich : (left) 200 million years after the Big Bang, filamentary structure is evident, with small local condensations; (centre) after 850 million years, a central halo is forming as dark matter density increases under the influence of gravity; (right) after around 13 billion years, a halo of some 1,600,000 light-years’ diameter has formed, replete with satellite halos which may host small satellite galaxies. In each frame, the bar at the bottom of the frame represents a distance of 500 kiloparsecs (kpc), or about 1.6 million light-years.

Image source :

Eventually a large, roughly spheroidal halo is formed, attracting swarms of much smaller haloes (which might become satellite galaxies), as well as absorbing some not-so-small haloes; these are cannibalised dwarf galaxies which enrich the Milky Way’s stellar population with non-indigenous stars. The scale of this halo is much larger than the optically-visible size of our galaxy, some 1,600,000 light-years as compared with 100,000 light-years.

Our visible galaxy is but the central condensation in a much larger halo of dark matter… this agrees with the observed distribution of stellar radial velocities in our galaxy, which indicates a distribution of mass that extends well beyond the visible structure of our galaxy.

The Aquarius simulations are being used also to examine various aspects of galactic and near-galactic phenomena, such as satellite dwarf galaxies, remnant stellar streams from disrupted dwarf galaxies, dark matter self-annihilation signatures, for example.

TheoryPadric McGee