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 halo1. 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 model2. 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 disks3,4,5,6. 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 matter7. 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.
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Acknowledgements
We thank our colleagues at ESO-Garching and ESO-Paranal, as well as those in the 3D-HST and SINFONI/zC-SINF and KMOS/KMOS3D teams, for their support and high-quality work, which made these technically difficult observations possible. D.W. and M.F. acknowledge the support provided by DFG Projects WI 3871/1-1 and WI 3871/1-2. J.C. acknowledges the support of the Deutsche Zentrum für Luft- und Raumfahrt (DLR) via Project ID 50OR1513. T.A. and A.S. acknowledge support by the I-CORE Program of the PBC and Israel Science Foundation (Center No. 1829/12). We thank S. Lilly and A. Dekel for comments on the manuscript. This work is based on observations obtained at the Very Large Telescope (VLT) of the European Southern Observatory (ESO), Paranal, Chile (ESO programme IDs 076.A-0527, 079.A-0341, 080.A-0330, 080.A-0635, 082.A-0396, 183.A0781, 091.A-0126, 092.A-0091, 093.A-0079, 094.A-0217, 095.A-0047, 096.A-0025, 097.B-0065 and 097.A-0353).
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Drafting text, figures and Methods: R.G., N.M.F.S., H.Ü., T.N., L.J.T., O.G., D.L., A.R., R.S. and A.S.; data analysis and modelling: R.G., H.Ü., P.L., S.W. and R.D.; data acquisition and reduction: R.G., N.M.F.S., H.Ü., P.L., L.J.T., E.W., S.W., A.Be., S.B., J.C., M.F., A.G., J.T.M. and K.T.; KMOS3D and SINS/zC-SINF IFS survey design and management: N.M.F.S., R.B., E.W., C.M.C., A.R., R.S., S.T. and D.W.; 3D-HST survey analysis: N.M.F.S., S.W., G.B., I.M. and E.J.N.; theoretical interpretation: T.N., T.A., A.Bu., S.G. and O.G.
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Extended data figures and tables
Extended Data Figure 1 Location of the galaxies included in our analysis.
a, Location in stellar-mass–star-formation-rate space. The star-formation rate (SFR) is normalized to that of the ‘main sequence’37 at the redshift and stellar mass of each galaxy . b, Location in stellar-mass–size space. The size is the half-light radius measured in the observed H-band corrected to the rest-frame 5,000 Å and normalized to that of the mass–size relation for star-forming galaxies26 at the redshift and stellar mass of each source . In a and b, the greyscale image shows the distribution of the underlying galaxy population at 0.7 < z < 2.7 taken from the 3D-HST source catalogue at log(M* /M⊙) > 9.0 and KAB < 23 mag (the magnitude cut applied when selecting KMOS3D targets and corresponding roughly to the completeness limits of the parent samples for SINS/zC-SINF targets). The current 2.5-year KMOS3D sample is shown with blue circles, and the SINS/zC-SINF sample with green diamonds. The two KMOS3D and four SINS/zC-SINF galaxies with individual outer rotation curves (RCs) are plotted as yellow circles and diamonds, respectively. Similarly, the KMOS3D and SINS/zC-SINF galaxies included in the stacked rotation curve are plotted as red circles and diamonds. All 3D-HST and KMOS3D galaxies are included in a, whereas only star-forming galaxies (SFGs) are shown in b, defined as having a specific star-formation rate higher than the inverse of the Hubble time at their redshift. The galaxies with individual outer rotation curves lie on and up to a factor of four times the main-sequence (MS) in star-formation rate (with mean and median log(SFR/SFRMS) = 0.24), and have sizes 1.2–2 times the relation (‘M–R SFGs’; mean and median offset in ). In star-formation rate and , the distribution of the stacked rotation-curve sample is essentially the same as the reference 3D-HST population in mean/median offsets (approximately 0.06 dex above the main-sequence and 0.07 dex above the mass–size relation) and in their scatter about the relationships (approximately 0.3 dex in log(SFR) and 0.17 dex in ); see refs 26, 37.
Extended Data Figure 2 Quality of fit and error of parameter determinations.
The reduced chi-squared as a function of the dark-matter fraction fDM at R1/2 for the six galaxies in our sample, once the other parameters (x0, y0, the position angle of the kinematic major axis PAmaj, i, σ0, R1/2 and B/T) are fixed at their best-fit values. Global minima are marked by circles; error bars give Δχ2 = ±4 ranges, corresponding to confidence levels of 95% (2 r.m.s.) under the assumption of single-parameter Gaussian distributions. This is the most important parameter dependence for our dataset.
Extended Data Figure 3 Mean changes in fDM and for changes in the secondary parameters B/T and R1/2, for COS 01351, D3a 6397, GS4 43501 and D3a 15504.
Changes in B/T and R1/2 are labelled ‘B/T ± 0.1’ and ‘Re ± 1σ’, respectively, where 1σ is the uncertainty on R1/2 given in Table 1; is the reduced chi-squared.
Extended Data Figure 4 Cumulative mass as a function of radius for one of our studied galaxies (GS4 43501).
Solid lines show the best fit; error bars show the variations in total (black, grey), baryonic (green) and dark-matter (DM; purple) mass at the outermost projected radius constrained by our data, if deviations from B/T and R1/2 within the uncertainties are considered (only cases with are considered). Dashed lines show the best fit for a model with lower concentration parameter (c = 2 instead of c = 5); dashed-dotted lines show the best fit for a model with adiabatic contraction (AC)97. Both modifications of the dark-matter profile lead to changes in the cumulative mass that are smaller than those obtained by varying B/T and R1/2 within the above uncertainties. The grey lines encompass variations in the dark-matter fraction of fDM(R1/2) = [0.14, 0.27] (best-fit fDM(R1/2) = 0.19).
Extended Data Figure 5 Minor axis cut at Rmajor = 0.71″ of D3a 15504.
Shown are the velocities (data points, with 1 r.m.s. error bars) and disk models for different inclinations (lines): 25° (red), 30° (blue), 40° (magenta) and 50° (green). The minor-axis cut favours a low inclination. In combination with the morphology of the stellar surface density distribution (Fig. 1) and the constraint on the baryonic mass of the disk, this yields an overall inclination of 34° ± 5° (Table 1). Rmajor is the radial distance from the centre of the galaxy along the kinematic major axis.
Extended Data Figure 6 Residual maps.
a, b, Residual maps (data minus model) for velocity (a; vdata − vmodel) and velocity dispersion (b; σdata − σmodel), for the six galaxies studied here. The colour scale is the same in all maps (from −200 km s−1 (purple) to +200 km s−1 (white)). Minimum and maximum values are noted in each map, as are the median and median dispersion (‘disp’) values.
Supplementary information
Supplementary Discussion
This file contains a comparison of the results to state-of-the-art cosmological simulations of galaxy formation, and a brief comment on the connection to the ‘thick disk’-phenomenon in local spiral galaxies. (PDF 145 kb)
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Genzel, R., Schreiber, N., Übler, H. et al. Strongly baryon-dominated disk galaxies at the peak of galaxy formation ten billion years ago. Nature 543, 397–401 (2017). https://doi.org/10.1038/nature21685
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DOI: https://doi.org/10.1038/nature21685
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