International audienceMassive present-day early-type (elliptical and lenticular) galaxies probably gained the bulk of their stellar mass and heavy elements through intense, dust-enshrouded starbursts--that is, increased rates of star formation--in the most massive dark-matter haloes at early epochs. However, it remains unknown how soon after the Big Bang massive starburst progenitors exist. The measured redshift (z) distribution of dusty, massive starbursts has long been suspected to be biased low in z owing to selection effects, as confirmed by recent findings of systems with redshifts as high as ~5 (refs 2-4). Here we report the identification of a massive starburst galaxy at z = 6.34 through a submillimetre colour-selection technique. We unambiguously determined the redshift from a suite of molecular and atomic fine-structure cooling lines. These measurements reveal a hundred billion solar masses of highly excited, chemically evolved interstellar medium in this galaxy, which constitutes at least 40 per cent of the baryonic mass. A 'maximum starburst' converts the gas into stars at a rate more than 2,000 times that of the Milky Way, a rate among the highest observed at any epoch. Despite the overall downturn in cosmic star formation towards the highest redshifts, it seems that environments mature enough to form the most massive, intense starbursts existed at least as early as 880 million years after the Big Ban

The ALESS survey has followed-up a sample of 122 sub-millimeter sources in the Extended Chandra Deep Field South at 870µm with ALMA, allowing to pinpoint the positions of sub-millimeter galaxies (SMGs) to ∼ 0.3 arcsec and to find their precise counterparts at different wavelengths. This enabled the first compilation of the multi-wavelength spectral energy distributions (SEDs) of a statistically reliable survey of SMGs. In this paper, we present a new calibration of the MAGPHYS SED modelling code that is optimized to fit these ultraviolet-to-radio SEDs of z > 1 star-forming galaxies using an energy balance technique to connect the emission from stellar populations, dust attenuation and dust emission in a physically consistent way. We derive statistically and physically robust estimates of the photometric redshifts and physical parameters (such as stellar masses, dust attenuation, star formation rates, dust masses) for the ALESS SMGs. We find that the ALESS SMGs have a median stellar mass M * = (8.9 ± 0.1) × 10 10 M ⊙ , median star formation rate SFR = 280 ± 70 M ⊙ yr −1 , median overall V -band dust attenuation A V = 1.9 ± 0.2 mag, median dust mass M dust = (5.6 ± 1.0) × 10 8 M ⊙ , and median average dust temperature T dust ≃ 40 K. We find that the average intrinsic spectral energy distribution of the ALESS SMGs resembles that of local ultra-luminous infrared galaxies in the infrared range, but the stellar emission of our average SMG is brighter and bluer, indicating lower dust attenuation, possibly because they are more extended. We explore how the average SEDs vary with different parameters (redshift, sub-millimeter flux, dust attenuation and total infrared luminosity), and we provide a new set of SMG templates that can be used to interpret other SMG observations. To put the ALESS SMGs into context, we compare their stellar masses and star formation rates with those of less actively star-forming galaxies at the same redshifts. We find that, at z ≃ 2, about half of the SMGs lie above the star-forming main sequence (with star formation rates three times larger than normal galaxies of the same stellar mass), while half are consistent with being at the high-mass end of the main sequence. At higher redshifts (z ≃ 3.5), the SMGs tend to have higher star formation rates and stellar masses, but the fraction of SMGs that lie significantly above the main sequence decreases to less than a third.

We report the results of a pilot study with the Expanded Very Large Array (EVLA) of 12 CO J = 1-0 emission from four submillimetre-selected galaxies at z = 2.2-2.5, each with an existing detection of 12 CO J = 3-2, one of which comprises two distinct spatial components. Using the EVLA's most compact configuration, we detect strong, broad [medians: 990 km s −1 full width at zero intensity; 540 km s −1 full width at half-maximum (FWHM)] J = 1-0 line emission from all of our targets -coincident in position and velocity with their J = 3-2 emission. The median linewidth ratio, σ 1-0 /σ 3-2 = 1.15 ± 0.06, suggests that the J = 1-0 is more spatially extended than the J = 3-2 emission, a situation confirmed by our maps which reveal velocity structure in several cases and typical sizes of ∼16 kpc FWHM. The median brightness temperature (T b ) ratio is r 3−2/1−0 = 0.55 ± 0.05, consistent with local galaxies with L IR > 10 11 L , noting that our value may be biased high because of the J = 3-2 based sample selection. Naively, this suggests gas masses roughly two times higher than estimates made using higher J transitions of CO, with the discrepancy due entirely to the difference in assumed T b ratio. We also estimate molecular gas masses using the 12 CO J = 1-0 line and the observed global T b ratios, assuming standard underlying T b ratios for the non-star-forming and star-forming gas phases as well as a limiting star formation efficiency for the latter in all systems, i.e. without calling upon X CO (≡ α). Using this new method, we find a median molecular gas mass of (2.5 ± 0.8) × 10 10 M , with a plausible range stretching up to three times higher. Even larger masses cannot be ruled out, but are not favoured by dynamical constraints: the median dynamical mass within R ∼ 7 kpc for our sample is (2.3 ± 1.4) × 10 11 M or ∼6 times more massive than UV-selected galaxies at this epoch. We examine the Schmidt-Kennicutt (S-K) relation for all the distant galaxy populations for which CO J = 1-0 or J = 2-1 data are available, finding small systematic differences between galaxy populations. These have previously been interpreted as evidence for different modes of star formation, but we argue that these differences are to be expected, given the still considerable uncertainties, certainly when considering the probable excitation biases due to the molecular lines used, and the possibility of sustained S-K offsets during the evolution of individual gas-rich systems. Finally, we discuss the morass of degeneracies surrounding molecular gas E-mail: rji@roe.ac.uk C 2011 The Authors Monthly Notices of the Royal Astronomical Society C 2011 RAS R. J. Ivison et al. mass estimates, the possibilities for breaking them, and the future prospects for imaging and studying cold, quiescent molecular gas at high redshifts.

We analyse the physical properties of a large, homogeneously selected sample of ALMA-located sub-millimetre galaxies (SMGs) detected in the SCUBA-2 Cosmology Legacy Survey 850-µm map of the UKIDSS/UDS field. This survey, AS2UDS, identified 707 SMGs across the ∼ 1 deg 2 field, including ∼17 per cent which are undetected in the optical/near-infrared to K 25.7 mag. We interpret the UV-to-radio data of these systems using a physically motivated model, magphys and determine a median photometric redshift of z = 2.61±0.08, with a 68 th percentile range of z = 1.8-3.4, with just ∼ 6 per cent at z > 4. The redshift distribution is well fit by a model combining evolution of the gas fraction in halos with the growth of halo mass past a critical threshold of ∼4×10 12 M , thus SMGs may represent the highly efficient collapse of gasrich massive halos. Our survey provides a sample of the most massive, dusty galaxies at z 1, with median dust and stellar masses of M d = (6.8±0.3) × 10 8 M (thus, gas masses of ∼ 10 11 M ) and M * = (1.26±0.05) × 10 11 M . These galaxies have gas fractions of f gas = 0.41±0.02 with depletion timescales of ∼ 150 Myr. The gas mass function evolution of our sample at high masses is consistent with constraints at lower masses from blind CO-surveys, with an increase to z ∼ 2-3 and then a decline at higher redshifts. The space density and masses of SMGs suggests that almost all galaxies with M * 2 × 10 11 M have passed through an SMG-like phase. We find no evolution in dust temperature at a constant far-infrared luminosity across z ∼ 1.5-4. We exploit dust continuum sizes to show that SMGs appear to behave as simple homologous systems in the far-infrared, having properties consistent with a centrally illuminated starburst. Our study provides strong support for an evolutionary link between the active, gas-rich SMG population at z > 1 and the formation of massive, bulge-dominated galaxies across the history of the Universe.