March 16, 2017
New observations of rotating galaxies at the peak epoch of galaxy formation, 10 billion years ago, surprisingly show that these massive, star-forming galaxies are completely dominated by baryonic or “normal” mass with dark matter playing a much smaller role in comparable regions of their outer disks than in the local universe. The international group of researchers led by the Max Planck Institute for Extraterrestrial Physics mapped the rotation curves of six galaxies to distances of ~65,000 light years from their centres and found that their rotation velocities are not constant but drop with radius. These findings are supported by observations of more than 200 further galaxies, where different estimates of their dynamical state also indicate a high baryonic mass fraction. In addition, the analysis shows that these early galaxies had a much thicker disk with turbulent motion accounting for part of the dynamical support. These findings are published in a paper in the journal Nature as well as three accompanying papers in the Astrophysical Journal.