Scientists from the University of Durham may have solved a decades-old puzzle regarding the distribution of the eleven small satellite galaxies that surround the Milky Way. The Milky Way is not alone. It is surrounded by a retinue of small "dwarf galaxy" companions. Cosmological theory predicts that these galaxies should occupy a large, nearly spherical halo but observations show that the satellite galaxies have a bizarre flattened, pancake-like distribution. The Durham team used sophisticated supercomputer models to simulate the formation of these galaxies and have succeeded in predicting the pancake configuration. Noam Libeskind, from the Institute for Computational Cosmology in Durham, will present the team's findings at the RAS National Astronomy Meeting at the University of Birmingham on Thursday 7th April.

All galaxies have smaller satellite galaxies in orbit around them, which inhabit pockets of dark matter. Dark matter does not interact with light and the only way that we can infer its existence is by detecting the gravitational influence it exerts on normal matter, such as stars. According to cosmological theory, soon after the Big Bang, cold dark matter formed the universe's first large-scale structures, which then collapsed under their own weight to form vast halos. The gravitational pull of these halos sucked in normal matter and provided a focus for the formation of galaxies. Galaxies are built up piece-by-piece as sub-galactic fragments merge together and, theoretically, this should lead to the formation of a tightly-bound galaxy at the core surrounded by a diffuse sphere of satellite structures. Cosmologists have been puzzled by the fact that not only do the Milky Way's satellites lie on a flat circle, approximately perpendicular to the Galactic Plane, but also there are far too few satellite galaxies to fit in with predictions. This discrepancy had led some cosmologists to question the entire paradigm for the cold dark matter–driven process of galaxy formation.

The Durham team simulated the evolution of parts of the universe, randomly selected from a large cosmological volume, using a sophisticated supercomputer model. The model built up a complete history of all mergers between galactic building blocks, resulting in a family tree for each satellite galaxy formed. Using the powerful, "Cosmology Machine" supercomputer, they carried out six simulations in total and, in each case, found not only the correct number of satellites but also, surprisingly, that the eleven most massive satellite galaxies showed the same pancake-like distribution around the core galaxy that is observed in the satellites of the Milky Way. To find an explanation, the team made animations of the simulations and looked at the evolution of the dark matter halo in which the galaxy formed. The simulations show that the original dark matter halo began its collapse by forming a sheet-like structure that then wrapped up to form a web of filaments. The galaxies formed at dense knots of dark matter in this cosmic web and then moved along the spines of filaments towards the original halo's major axis. The team found that this axis is aligned with the elongated disc formed by the satellite galaxies and have concluded that it is this drift towards the backbone of the main halo that holds the key to the satellites' pancake-like configuration.

Far from challenging the current cosmological paradigm – the cold dark matter model – the findings of the Durham group represent a triumph of the model and indicate that a coherent picture of how galaxies like the Milky Way emerged from the Big Bang is now beginning to fall into place.

So far simulations have been confined to satellites located within 250 kiloparsecs of the galactic centre. The team are planning further simulations to investigate how widespread the formation of cosmic pancakes is. In particular, they plan to search for evidence of pancakes in structures even larger than the Milky Way, the so-called great clusters of galaxies, This will provide a further, stringent test of the cold dark matter paradigm.

NOTES FOR EDITORS

1 parsec = 3.26 light years

The 2005 RAS National Astronomy Meeting is hosted by the University of Birmingham and sponsored by the UK Particle Physics and Astronomy Research Council (PPARC).

CONTACTS

On 7th April, Noam Libeskind and Professor Carlos Frenk can be contacted via the NAM press office (see above).

Noam Libeskind
Institute for Computational Cosmology
Department of Physics
University of Durham
Science Laboratories
South Road, Durham DH1 3L
Tel: 07799-628673
E-mail: noam.libeskind@durham.ac.uk

Professor Carlos Frenk
Institute for Computational Cosmology
Department of Physics
University of Durham
Science Laboratories
South Road, Durham DH1 3L
Tel: 0191-3343641
E-mail: c.s.frenk@durham.ac.uk

ANIMATIONS

The collapse of a galactic halo along a filament of dark matter (~21MB) http://star-www.dur.ac.uk/~nil/satellite_pancake.gif The Milky Way is shown as the central blue dot, and the red dots indicate the satellites of the Milky Way. Only dark matter that ends up with in 250kpc is plotted. The satellites are accreted along the direction of the filament and as such the end up on a flattened pancake like distribution. The left panel shows the evolution of the halo with only the 11 most massive satellites of the Milky Way; this is the limit of what is actually observed.

The right hand panel shows the evolution of all the satellites in this simulation (around 40). Most of these satellites are too small to be observed, however their final shape distribution is also correlated with the large-scale structure. Credit: Noam Libeskind, Institute for Computational Cosmology, University of Durham

Formation of a dark matter halo

http://star-www.dur.ac.uk/~csf/movies/adrian/blue_fast.mpg (19 Mbytes)

http://star-www.dur.ac.uk/~csf/movies/mjr/halo_16small_title.mpg (19 Mbytes)

Animations showing the formation of a dark matter halo similar to that in which the Milky Way is thought to be immersed. The movies show the growth of the halo by the merging of pregalactic dark matter fragments that is characteristic of "hierarchical clustering." Credit: Adrian Jenkins, Institute for Computational Cosmology, University of Durham

IMAGES

Stills from the animations can be found at: http://star-www.dur.ac.uk/~csf/movies/adrian/stills/ Credit: Adrian Jenkins, Institute for Computational Cosmology, University of Durham