The rest-frame ultraviolet properties of galaxies during the first three billion years of cosmic time (redshift z > 4) indicate a rapid evolution in the dust obscuration of such galaxies1,2,3. This evolution implies a change in the average properties of the interstellar medium, but the measurements are systematically uncertain owing to untested assumptions4,5 and the inability to detect heavily obscured regions of the galaxies. Previous attempts to measure the interstellar medium directly in normal galaxies at these redshifts have failed for a number of reasons6,7,8,9, with two notable exceptions10,11. Here we report measurements of the forbidden C ii emission (that is, [C ii]) from gas, and the far-infrared emission from dust, in nine typical star-forming galaxies about one billion years after the Big Bang (z ≈ 5–6). We find that these galaxies have thermal emission that is less than 1/12 that of similar systems about two billion years later, and enhanced [C ii] emission relative to the far-infrared continuum, confirming a strong evolution in the properties of the interstellar medium in the early Universe. The gas is distributed over scales of one to eight kiloparsecs, and shows diverse dynamics within the sample. These results are consistent with early galaxies having significantly less dust than typical galaxies seen at z < 3 and being comparable in dust content to local low-metallicity systems12.
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Support for this work was provided by NASA through an award issued by JPL/Caltech. We thank the ALMA staff for facilitating the observations and aiding in the calibration and reduction process. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This work is based in part on observations made with the Spitzer Space Telescope and the W.M. Keck Observatory, along with archival data from the NASA/ESA Hubble Space Telescope, the Subaru Telescope, the Canada-France-Hawaii-Telescope and the ESO Vista telescope obtained from the NASA/IPAC Infrared Science Archive. V.S. acknowledges funding by the European Union’s Seventh Framework programme under grant agreement 337595 (ERC Starting Grant, ‘CoSMass’).
The authors declare no competing financial interests.
This paper makes use of ALMA data: ADS/JAO.ALMA#2012.1.00523.S. ALMA.
Extended data figures and tables
Our sample is consistent with the low-z SFR–L[C ii] relation from the literature at z = 0–6 (ref. 6). The best fit to the literature points from ref. 44 is indicated with a solid line. The SFRs are derived using the UV + FIR method and a Chabrier IMF18. The dust-corrected ultraviolet estimates with a Meurer relation typically used at these redshifts would increase the estimated SFR by a factor of ∼2–10 (0.5–1 dex), leading to over-estimates of the expected [C ii] flux. The L[C ii] error bars are 1σ standard measurement error, while the SFR errors are 1σ from a combination of measurement error in LIR and LUV converted to star formation added in quadrature.
Our objects are generally consistent with the stellar mass-SFR or ‘main sequence’ of star forming galaxies at z ≈ 5–6 (ref. 18). Three objects fall below the relation, namely HZ1, HZ2 and HZ3, but the stellar mass of HZ1 may be over-estimated (‘Derivation of physical parameters’ in Methods). Mass errors reflect the 1σ range of masses allowed by the SED model fits including emission line strength as a free parameter. SFR errors are 1σ from a combination of measurement error in LIR, systematics in LIR, and measurement error in LUV converted to star formation added in quadrature.
A comparison of the dynamical masses estimated from the [C ii] line and stellar masses estimated from SED fitting is shown. The dotted line indicates equal stellar and dynamical masses. The average dynamic to stellar mass ratio is ∼3, which is higher than, but consistent with, similar measurements at z ≈ 1–3 (ref. 28). Errors on the dynamical masses reflect the 1σ measurement uncertainty in the size, and velocity dispersion of the sources, but the geometry is limited to sin(i) = 0.45–1 (‘Derivation of physical parameters’ in Methods). The errors on the stellar masses reflect the 1σ range of masses allowed by the SED model fits including emission line strength as a free parameter.
The global SFR history at z > 4 derived from UV measurements2 is shown for three different assumptions about the dust obscuration in the general galaxy population. Blue, no correction for dust (‘direct UV measurements’ in key); red, a dust correction assuming the Meurer relation29 (‘UV corrected with Meurer et al.’); green, a correction which linearly evolves in redshift between a Meurer relation at z ≈ 4, the SMC-like relation at z ≈ 5, and our measured value at z ≈ 6 (‘UV corrected with evolving dust’). Note the evolving dust correction leads to a downward revision by 30–40% at z ≈ 6. Redshift error bars reflect the binning of the data, errors in the SFR density reflect the 1σ measurement error in the ultraviolet luminosity density.
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Capak, P., Carilli, C., Jones, G. et al. Galaxies at redshifts 5 to 6 with systematically low dust content and high [C ii] emission. Nature 522, 455–458 (2015). https://doi.org/10.1038/nature14500
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