At the peak of galaxy formation ten billion years ago, the outer disks of star-forming galaxies may have been dominated by stars and gas, rather than dark matter, suggests a study published in this week’s Nature. These new measurements of the rotation curves of distant galaxies seem rather at odds with simulations of early galaxy formation.

Within galaxies, stars and gas (baryonic components) are thought to be mixed with dark (non-baryonic) matter, which dominates the total mass. The proportion of baryonic and non-baryonic matter can be calculated by measuring the rotation of galaxies. In the local Universe, dark matter dominates the outer disks of galaxies (such as the Milky Way), causing the rotation curves of these galaxies to be relatively flat. Going by the mass of the stars and gas only, the rotation velocities should fall steeply in the outer parts of the galaxies. By analysing high-quality spectra of six distant, massive, star-forming galaxies, Reinhard Genzel and colleagues find that the rotation velocities of the outer disks decrease with radius, suggesting that dark matter is less dominant than in the local Universe. This discrepancy between the components of local and distant galaxies could be because distant galaxies are gas-rich and compact, and grow by rapid accumulation of gas, which settles into disks more easily than the dark matter, the authors suggest.

The findings improve our understanding of how early galaxies form and evolve, notes Mark Swinbank in an accompanying News & Views article. He adds that the results may explain “how the star-forming, clumpy, irregular galaxies seen in the distant Universe transformed into the distinctive spiral galaxies, such as the Milky Way, that we see today.”