Most massive structures in the Universe began to form shortly after the Big Bang, in regions corresponding to the largest fluctuations in the cosmic density field. D. P. Marrone and his team from University of Arizona identified a similar structure named SPT0311?58 from the 2,500-deg South Pole Telescope (SPT) survey. Observations with the Atacama Large Millimetre/submillimetre Array (ALMA) place the galaxy at a redshift of z?=?6.

900, which corresponds to a cosmic age of 780?million years. SPT0311?58 is the most distant known member of the population of massive, infrared-bright but optically dim, dusty galaxies. The far-infrared emission from SPT0311?58 provides an opportunity to study its structure clearly from the foreground galaxy. The ALMA observations of the emission showed that it is in fact composed of two distinct galaxies: SPT0311-58 E and SPT0311-58 W. Lens modelling of the emission indicates that the two galaxies are separated by a projected distance of 8 kiloparsecs (kpc) in the source plane. SPT0311?58 E has an effective radius of 1.

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1 kpc, whereas SPT0311?58 W has an elongated structure that is 7.5 kpc across.Having characterized the lensing geometry, the two galaxies are extremely luminous. The implied star-formation rates are correspondingly enormous— (540?±?175) M? yr?1 and (2,900?±?1,800) M? yr?1, where M? is the mass of the Sun—probably owing to the instability associated with the tidal forces experienced by merging galaxies.

They have luminosities and star-formation rates like the other, z?>?6 galaxies. However, unlike the latter case, there is no evidence of a black hole in either source in SPT0311?58. The far-infrared continuum and line emission of SPT0311?58 E and SPT0311?58 W imply substantial differences in their physical conditions.

It is also unlikely that active galactic nuclei are the origin of the emission line in SPT0311?58 E, because the emission extend across most of the galaxy rather than being concentrated in the nuclear region. The masses of the components of SPT0311?58 are remarkable for a time only 780 Myr after the Big Bang. The dust continuum luminosity obtained from the ALMA observations revealed  the corresponding dust and gas masses for SPT0311?58 W to be Mgas?=?(2.7?±?1.7)?×?1011M? and Mdust?=?(2.5?±?1.6)?×?109M?, and for SPT0311?58 E to be Mgas?=?(0.4?±?0.

2)?×?1011M? and Mdust?=?(0.4?±?0.2)?×?109M?.The gas mass of SPT0311?58 W is well above those of all the known galaxies at z?>?6, that is, during the first 900 Myr of cosmic history. To understand the rareness of the dark-matter halo that hosts SPT0311?58, curves that describe the rarest haloes that should exist in the Universe at any redshift are calculated. The researchers find that SPT0311?58 is indeed closest to the exclusion curves and therefore marks an exceptional peak in the cosmic density field at this time in cosmic history. Even before coalescence, the larger galaxy in the pair is more massive than any other known galaxy at z?>?6. Although the discovery of such a system at this high redshift and in a survey that covered less than 10% of the sky is unprecedented, its existence is not prohibited by the current cosmological paradigm.



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