LAUSR

The masses of nearby spiral galaxies are dominated by invisible 'dark matter'. Surprisingly, galaxies in the distant Universe seem to contain comparatively little of it. See Letter p.397 In the cold dark matter cosmology, the baryonic components of galaxies (stars and gas) are thought to be mixed with non-baryonic and non-relativistic dark matter, which dominates the total mass. In the local Universe, dark matter dominates the outer, baryonic regions of the disks of star-forming galaxies, leading to rotation velocities of the visible matter within the disk that are constant or increase with disk radius—an essential feature of the dark-matter model. Reinhard Genzel et al. now report rotation curves for the outer disks of six massive, high-redshift star-forming galaxies and find that the rotation velocities decrease as radius increases. They propose a combination of two causes. First, these high-redshift galaxies were strongly baryon dominated, with dark matter playing a smaller part than in the local Universe and, second, the radial pressure gradient observed in the disks slows the rotation velocity as radius increases. The effect of both factors appears to increase with redshift.