Journal Article

Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago

Bender, R., Galametz, A., Tacchella, S., Carollo, C. M., Wilman, D., Brammer, G., Burkert, A., Momcheva, I., Naab, T., Chan, J., Belli, S., Renzini, A., Genel, S., Sternberg, A., Lutz, D., Beifiori, A., Schreiber, N. M. Förster, Alexander, T., Davies, R., Mendel, J. T., Gerhard, O., Lang, P., Saglia, R., Tacconi, L. J., Wuyts, S., Fossati, M., Tadaki, K., Übler, H., Genzel, R., Wisnioski, E., Nelson, E. J.

2017

Overview

The rotation velocities in the outer disks of six massive star-forming galaxies are shown to decrease with disk radius, owing to high baryonic mass fractions and large velocity dispersions. Early galaxies not so dark 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. In the cold dark matter cosmology, the baryonic components of galaxies—stars and gas—are thought to be mixed with and embedded in non-baryonic and non-relativistic dark matter, which dominates the total mass of the galaxy and its dark-matter halo 1 . In the local (low-redshift) Universe, the mass of dark matter within a galactic disk increases with disk radius, becoming appreciable and then dominant in the outer, baryonic regions of the disks of star-forming galaxies. This results in rotation velocities of the visible matter within the disk that are constant or increasing with disk radius—a hallmark of the dark-matter model 2 . Comparisons between the dynamical mass, inferred from these velocities in rotational equilibrium, and the sum of the stellar and cold-gas mass at the peak epoch of galaxy formation ten billion years ago, inferred from ancillary data, suggest high baryon fractions in the inner, star-forming regions of the disks 3 , 4 , 5 , 6 . Although this implied baryon fraction may be larger than in the local Universe, the systematic uncertainties (owing to the chosen stellar initial-mass function and the calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter 7 . Here we report rotation curves (showing rotation velocity as a function of disk radius) for the outer disks of six massive star-forming galaxies, and find that the rotation velocities are not constant, but decrease with radius. We propose that this trend arises because of a combination of two main factors: first, a large fraction of the massive high-redshift galaxy population was strongly baryon-dominated, with dark matter playing a smaller part than in the local Universe; and second, the large velocity dispersion in high-redshift disks introduces a substantial pressure term that leads to a decrease in rotation velocity with increasing radius. The effect of both factors appears to increase with redshift. Qualitatively, the observations suggest that baryons in the early (high-redshift) Universe efficiently condensed at the centres of dark-matter haloes when gas fractions were high and dark matter was less concentrated.

• Publisher: Nature Publishing Group UK,Nature Publishing Group
• Subject: 639/33 / 639/33/34/124 / Baryons / Cosmology / Equilibrium / Galaxies / Humanities and Social Sciences / Kinematics / letter / multidisciplinary / Natural history / Physics research / Satellites / Science / Spectrum analysis / Star & galaxy formation / Stars / Stars & galaxies / Studies / Velocity