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

Abstract: In cold dark matter cosmology, the baryonic components of galaxies 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 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. Comparison between the dynamical mass and the sum of stellar and cold gas mass at the peak epoch of galaxy formation, inferred from ancillary data, suggest high baryon factions 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 (stellar initial mass function, calibration of gas masses) render such comparisons inconclusive in terms of the mass of dark matter. Here we report rotation curves 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 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 Universe efficiently condensed at the centres of dark matter halos when gas fractions were high, and dark matter was less concentrated. [Abridged]

• The paper analyzes VLT near-infrared spectroscopy data to derive Hα rotation curves of six high-redshift massive disk galaxies.
• It finds declining rotation curves with increasing radius, challenging the conventional dark matter-dominated paradigm observed in the local universe.
• The results imply strong baryon dominance, suggesting efficient baryon condensation at galaxy centers during the peak of galaxy formation.

Analysis of Baryon-Dominated Disk Galaxies at High Redshift

The paper "Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago" provides a critical examination of the baryon mass distribution in massive star-forming disk galaxies located approximately ten billion years ago, during the peak epoch of galaxy formation. The study investigates the rotation curves of such galaxies, challenging the conventional notion of cold dark matter-dominated galactic disks prevalent in the local universe.

Core Findings and Methodology

The authors conducted a comprehensive analysis using data obtained from the Very Large Telescope (VLT) of the European Southern Observatory. Their sample comprises six massive star-forming galaxies, spanning redshifts between 0.9 and 2.4. By employing near-infrared integral field spectroscopy (IFS) with SINFONI and KMOS instruments, the researchers extracted Hα rotation curves to deduce rotation velocities as a function of galactic radius.

A principal finding of the paper is that contrary to traditional expectations of flat or rising rotation curves derived from dark matter models, the rotation curves in these high-redshift galaxies decline with radius. This observation suggests a marked divergence from the typical galaxy dynamics observed in the local universe, where non-baryonic dark matter significantly contributes to the overall mass.

Implications of Baryon Dominance

The findings imply that a significant fraction of the massive, high-redshift galaxy population was strongly baryon dominated. Several factors support this, including the high velocity dispersion within the high-redshift disks, adding a substantial pressure term. The result is a decreasing rotation velocity with increasing radius, a trend that becomes more pronounced at higher redshifts.

The paper argues that this baryon dominance could emerge from the efficient condensation of baryons in the early universe at the centers of dark matter halos. This contrasts with the local universe, where the dark matter halo generally dominates the mass.

Theoretical and Practical Implications

The potential insight offered by this study bears considerable theoretical and observational significance. It challenges the conventional Cold Dark Matter (CDM) paradigm by suggesting that at high redshifts, many disk galaxies appear to have dynamics primarily driven by baryons rather than dark matter. This raises questions about the universality of CDM models and opens the floor to alternate galaxy formation and evolution theories at different cosmic epochs.

Moreover, the apparent high baryonic concentration could offer explanations for the formation and evolution of massive bulges and stellar components observed in elliptical galaxies today. It also aligns with the notion of 'compaction events,' articulated as pivotal moments in the evolutionary pathways of galaxies influenced by mergers or substantial accretion streams.

Future Directions

Future studies should aim to expand beyond the six galaxies considered, ideally incorporating larger, possibly more statistically significant samples to corroborate these findings. Additionally, further investigations utilizing simulations with refined feedback mechanisms might offer deeper insights into the interplay between baryonic and dark matter dynamically. Advances in computational astrophysics and high-resolution cosmological simulations will be critical in understanding the broader applicability of these outcomes to galaxy evolution models.

Furthermore, observations that explore the high-redshift universe could help delineate these characteristics, providing a bridge between the known properties of local galaxies and their high-redshift counterparts.

In summary, this study significantly contributes to understanding the mass distribution and dynamics of early galaxies. It underscores the importance of re-evaluating the dominance of dark matter in shaping large-scale structures, particularly in the context of galaxies formed during earlier cosmic times.