Most massive structures in the Universebegan to form shortly after the Big Bang, in regions corresponding to thelargest fluctuations in the cosmic density field. D. P.Marrone and his team from University of Arizona identified a similar structurenamed SPT0311?58 from the 2,500-deg South Pole Telescope (SPT) survey.
Observationswith the Atacama Large Millimetre/submillimetre Array (ALMA) place the galaxyat a redshift of z?=?6.900, which corresponds to a cosmic age of 780?millionyears. SPT0311?58 is the most distant known member of the population ofmassive, infrared-bright but optically dim, dusty galaxies. The far-infrared emission from SPT0311?58provides an opportunity to study its structure clearly from the foregroundgalaxy.
The ALMA observations of the emission showed that it is in factcomposed of two distinct galaxies: SPT0311-58 E and SPT0311-58 W. Lensmodelling of the emissionindicates that the two galaxiesare separated by a projected distance of 8 kiloparsecs (kpc) in the sourceplane. SPT0311?58 E has an effective radius of 1.1 kpc, whereas SPT0311?58 Whas an elongated structure that is 7.5 kpc across. Having characterized the lensing geometry, thetwo galaxies are extremely luminous.
The implied star-formation rates arecorrespondingly enormous— (540?±?175) M? yr?1 for SPT0311-58 E and(2,900?±?1,800) M? yr?1for SPT0311-58 W, where M? isthe mass of the Sun—probably owing to the instability associated with the tidalforces experienced by merging galaxies. They have star-formation rates like theother, z?>?6 galaxies. However, unlike the latter case, there isno evidence of a black hole in either source in SPT0311?58. It is also unlikelythat active galactic nuclei are the origin of the emission line in SPT0311?58E, because the emission extend across the galaxy rather than being concentratedin the nuclear region. The masses of the components of SPT0311?58are remarkable for a time only 780 Myr after the Big Bang. The dust continuumluminosity obtained from the ALMA observations revealed that SPT0311-58 E hasan order of magnitude less gas and dust than its neighbour and physicalconditions akin to those observed in lower-metallicity galaxies in the nearbyUniverse.
The gas mass of SPT0311?58 W is well above those of all the knowngalaxies at z?>?6, that is, during the first 900 Myr of cosmichistory. These objects suggest the presenceof a dark-matter halo with a mass of more than 100 billion solar masses, makingit among the rarest dark-matter haloes that should exist at this epoch. By calculating the curves that describe the raresthaloes that should exist in the Universe at any redshift, the researchers findthat SPT0311?58 is indeed closest to the exclusion curves and therefore marksan 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 anyother known galaxy at z?>?6. Although the discovery of such asystem at this high redshift and in a survey that covered less than 10% of thesky is unprecedented, its existence is not prohibited by the current cosmologicalparadigm.
