Astronomy and Astrophysics – Astrophysics
Scientific paper
2005-01-27
Nature437:707,2005
Astronomy and Astrophysics
Astrophysics
Letter to Nature, 13+15 pages, 2+11 figures, improved text, extended Supplementary Information added
Scientific paper
10.1038/nature03970
There is strong evidence that the mass in the Universe is dominated by dark matter, which exerts gravitational attraction but whose exact nature is unknown. In particular, all galaxies are believed to be embedded in massive haloes of dark matter. This view has recently been challenged by surprisingly low random stellar velocities in the outskirts of ordinary elliptical galaxies, which were interpreted as indicating a lack of dark matter (Mendez et al. 2001; Romanowsky et al. 2003). Here we show that the low velocities are in fact compatible with galaxy formation in dark-matter haloes. Using numerical simulations of disc-galaxy mergers, we find that the stellar orbits in the outer regions of the resulting ellipticals are very elongated. These stars were torn by tidal forces from their original galaxies during the first close passage and put on outgoing trajectories. The elongated orbits, combined with the steeply falling density profile of the observed tracers, explain the observed low velocities even in the presence of large amounts of dark matter. Projection effects when viewing a triaxial elliptical can lead to even lower observed velocities along certain lines of sight.
Cox Theodore J.
Dekel Avishai
Mamon Gary A.
Novak Gregory S.
Primack Joel R.
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