A fundamental quest of modern astronomy is to locate the earliest galaxies and study how they influenced the intergalactic medium a few hundred million years after the Big Bang. The abundance of star-forming galaxies is known to decline from redshifts of about 6 to 10, but a key question is the extent of star formation at even earlier times, corresponding to the period when the first galaxies might have emerged. Here we report spectroscopic observations of MACS1149-JD1 , a gravitationally lensed galaxy observed when the Universe was less than four per cent of its present age. We detect an emission line of doubly ionized oxygen at a redshift of 9.1096 ± 0.0006, with an uncertainty of one standard deviation. This precisely determined redshift indicates that the red rest-frame optical colour arises from a dominant stellar component that formed about 250 million years after the Big Bang, corresponding to a redshift of about 15. Our results indicate that it may be possible to detect such early episodes of star formation in similar galaxies with future telescopes.

The physical properties and elemental abundances of the interstellar medium in galaxies during cosmic reionization are important for understanding the role of galaxies in this process. We report the Atacama Large Millimeter/submillimeter Array detection of an oxygen emission line at a wavelength of 88 micrometers from a galaxy at an epoch about 700 million years after the Big Bang. The oxygen abundance of this galaxy is estimated at about one-tenth that of the Sun. The nondetection of far-infrared continuum emission indicates a deficiency of interstellar dust in the galaxy. A carbon emission line at a wavelength of 158 micrometers is also not detected, implying an unusually small amount of neutral gas. These properties might allow ionizing photons to escape into the intergalactic medium.

We present new ALMA observations and physical properties of a Lyman Break Galaxy at z = 7.15. Our target, B14-65666, has a bright ultra-violet (UV) absolute magnitude, M UV ≈ −22.4, and has been spectroscopically identified in Lyα with a small rest-frame equivalent width of ≈ 4Å. Previous HST image has shown that the target is comprised of two spatially separated clumps in the rest-frame UV. With ALMA, we have newly detected spatially resolved [Oiii] 88 µm, [Cii] 158 µm, and their underlying dust continuum emission. In the whole system of B14-65666, the [Oiii] and [Cii] lines have consistent redshifts of 7.1520 ± 0.0003, and the [Oiii] luminosity, (34.4 ± 4.1) × 10 8 L ⊙ , is about three times higher than the [Cii] luminosity, (11.0 ± 1.4) × 10 8 L ⊙ . With our two continuum flux densities, the dust temperature is constrained to be T d ≈ 50 − 60 K under the assumption of the dust emissivity index of β d = 2.0 − 1.5, leading to a large total infrared luminosity of L TIR ≈ 1 × 10 12 L ⊙ . Owing to our high spatial resolution data, we show that the [Oiii] and [Cii] emission can be spatially decomposed into two clumps associated with the two rest-frame UV clumps whose spectra are kinematically separated by ≈ 200 km s −1 . We also find these two clumps have comparable UV, infrared, [Oiii], and [Cii] luminosities. Based on these results, we argue that B14-65666 is a starburst galaxy induced by a major-merger. The merger interpretation is also supported by the large specific star-formation rate (defined as the star-formation rate per unit stellar mass), sSFR = 260 +119 −57 Gyr −1 , inferred from our SED fitting. Probably, a strong UV radiation field caused by intense star formation contributes to its high dust temperature and the [Oiii]-to-[Cii] luminosity ratio.

We present the Atacama Large Millimeter/submillimeter Array (ALMA) detection of the [O iii] 88 µm line and rest-frame 90 µm dust continuum emission in a Y -dropout Lyman break galaxy (LBG), MACS0416 Y1, lying behind the Frontier Field cluster MACS J0416.1−2403. This [O iii] detection confirms the LBG with a spectroscopic redshift of z = 8.3118 ± 0.0003, making this object one of the furthest galaxies ever identified spectroscopically. The observed 850 µm flux density of 137 ± 26 µJy corresponds to a de-lensed total infrared (IR) luminosity of L IR = (1.7±0.3)×10 11 L if assuming a dust temperature of T dust = 50 K and an emissivity index of β = 1.5, yielding a large dust mass of 4×10 6 M . The ultraviolet-to-far IR spectral energy distribution modeling where the [O iii] emissivity model is incorporated suggests the presence of a young (τ age ≈ 4 Myr), star-forming (SFR ≈ 60 M yr −1 ), moderately metal-polluted (Z ≈ 0.2Z ) stellar component with a mass of M star = 3 × 10 8 M . An analytic dust mass evolution model with a single episode of star-formation does not reproduce the metallicity and dust mass in τ age ≈ 4 Myr, suggesting a pre-existing evolved stellar component with M star ∼ 3 × 10 9 M and τ age ∼ 0.3 Gyr as the origin of the dust mass.