title: 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.
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Departments of Physics and Astronomy, University of California, Berkeley, California 94720, USA.
authors.forename: R
authors.initials: R
authors.lastname: Genzel
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: N M Förster
authors.initials: NM
authors.lastname: Schreiber
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: H
authors.initials: H
authors.lastname: Übler
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: P
authors.initials: P
authors.lastname: Lang
authors.affiliation: Max-Planck Institute for Astrophysics, Karl Schwarzschildstrasse 1, D-85748 Garching, Germany.
authors.forename: T
authors.initials: T
authors.lastname: Naab
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Bender
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: L J
authors.initials: LJ
authors.lastname: Tacconi
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: E
authors.initials: E
authors.lastname: Wisnioski
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Department of Physics, University of Bath, Claverton Down, Bath BA2 7AY, UK.
authors.forename: S
authors.initials: S
authors.lastname: Wuyts
authors.affiliation: Department of Particle Physics and Astrophysics, Faculty of Physics, The Weizmann Institute of Science, POB 26, Rehovot 76100, Israel.
authors.forename: T
authors.initials: T
authors.lastname: Alexander
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Beifiori
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: S
authors.initials: S
authors.lastname: Belli
authors.affiliation: Space Telescope Science Institute, Baltimore, Maryland 21218, USA.
authors.forename: G
authors.initials: G
authors.lastname: Brammer
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Max-Planck Institute for Astrophysics, Karl Schwarzschildstrasse 1, D-85748 Garching, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Burkert
authors.affiliation: Institute of Astronomy, Department of Physics, Eidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland.
authors.forename: C M
authors.initials: CM
authors.lastname: Carollo
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: J
authors.initials: J
authors.lastname: Chan
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Davies
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: M
authors.initials: M
authors.lastname: Fossati
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: A
authors.initials: A
authors.lastname: Galametz
authors.affiliation: Center for Computational Astrophysics, 160 Fifth Avenue, New York, New York 10010, USA.
authors.forename: S
authors.initials: S
authors.lastname: Genel
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Gerhard
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Lutz
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: J T
authors.initials: JT
authors.lastname: Mendel
authors.affiliation: Department of Astronomy, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06511, USA.
authors.forename: I
authors.initials: I
authors.lastname: Momcheva
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Department of Astronomy, Yale University, 260 Whitney Avenue, New Haven, Connecticut 06511, USA.
authors.forename: E J
authors.initials: EJ
authors.lastname: Nelson
authors.affiliation: Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, Padova I-35122, Italy.
authors.forename: A
authors.initials: A
authors.lastname: Renzini
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Saglia
authors.affiliation: School of Physics and Astronomy, Tel Aviv University, Tel Aviv 69978, Israel.
authors.forename: A
authors.initials: A
authors.lastname: Sternberg
authors.affiliation: Institute of Astronomy, Department of Physics, Eidgenössische Technische Hochschule, ETH Zürich, CH-8093 Zürich, Switzerland.
authors.forename: S
authors.initials: S
authors.lastname: Tacchella
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.
authors.forename: K
authors.initials: K
authors.lastname: Tadaki
authors.affiliation: Max-Planck-Institut für extraterrestrische Physik (MPE), Giessenbachstrasse 1, 85748 Garching, Germany.|Universitäts-Sternwarte Ludwig-Maximilians-Universität (USM), Scheinerstrasse 1, D-81679 München, Germany.
authors.forename: D
authors.initials: D
authors.lastname: Wilman