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

Abstract

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. 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. 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. 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. 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.

The Most Ancient Spiral Galaxy: A 2.6-Gyr-old Disk with a Tranquil Velocity Field

“…The value of f DM is in the typical range of local latetype spiral galaxies of similar V c and much larger than the massive baryon-dominated clumpy disk galaxies at z ∼ 2 (Genzel et al 2017;Lang et al 2017). However, this conclusion is subject to uncertain observational and methodological errors of f DM .…”

Merger driven star-formation activity in Cl J1449+0856 at z=1.99 as seen by ALMA and JVLA

“…There is evidence that the contribution from dark matter to the dynamical mass at these redshifts is small (Daddi et al 2010a;Wuyts et al 2016;Genzel et al 2017), and we cannot further refine the contributions from molecular gas, stellar mass and dark matter to the dynamical mass using our data. Additionally, the empirically calibrated stellar mass estimates of A1 and A2 are consistent with the stellar masses that we derive from the dynamical mass within 1σ.…”

The VIMOS Ultra Deep Survey

“…The gravitational lensing observations, however, are usually possible only for a special set of circumstances, as the objects available for exploration are limited by the geometry of lens and sources (see, e.g., Blandford & Narayan 1992;Meylan et al 2006). Other methods for studying dark-luminous matter relations are applied on the scales of individual galaxies, where e.g., studies of rotation curves (Rubin et al 1978) of stars or gas clouds within individual galaxies are used to explore the hosting dark matter halo masses and density profiles, improving the understanding of the role of dark matter haloes in galaxy formation and evolution (e.g., Genzel et al 2017;Dekel et al 2017;Katz et al 2017). On the large scales considered in this work the most effective methods make use of statistical tools.…”