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massive structures in the Universe began to form shortly after the Big Bang, in
regions corresponding to the largest fluctuations in the cosmic density field1. 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 1.1 kpc, whereas SPT0311?58 W
has an elongated structure that is 7.5 kpc across.

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Having characterized the lensing
geometry, the two galaxies are extremely luminous. The implied star-formation
rates are correspondingly enormous— (540?±?175) M? yr?1 for SPT0311-58 E and
(2,900?±?1,800) M? yr?1
for SPT0311-58 W, where M? is
the mass of the Sun—probably owing to the instability associated with the tidal
forces experienced by merging galaxies. They have 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. It is also unlikely
that active galactic nuclei are the origin of the emission line in SPT0311?58
E, because the emission extend across 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 that SPT0311-58
E has an order of magnitude less gas and dust than its neighbour and physical
conditions akin to those observed in lower-metallicity galaxies in the nearby
Universe. 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

These objects suggest the presence of a dark-matter halo with a mass of
more than 100 billion solar masses, making it among the rarest dark-matter
haloes that should exist at this epoch. By calculating the curves that describe the rarest haloes
that should exist in the Universe at any redshift, 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

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