Hercules–Corona Borealis Great Wall

Hercules–Corona Borealis Great Wall
A giant Hubble mosaic of the massive galaxy cluster MACS J0717.5+3745 using a combination of 18 images. The significant amount of dark matter in this cluster, shown in light blue, may be also similar to the Hercules–Corona Borealis Great Wall. Credit: Hubble Space Telescope
Observation data (Epoch J2000)
Constellation(s)	Hercules, Corona Borealis, Lyra, Boötes and Draco
Right ascension	17h 0m
Declination	+27° 45′
Major axis	3 Gpc (10 Gly)
Minor axis	2.2 Gpc (7 Gly) h−1 0.6780
Redshift	1.6 to 2.1
Distance (co-moving)	9.612 to 10.538 billion light-years (light travel distance) 15.049 to 17.675 billion light-years (present comoving distance)
Binding mass	2×1019 M☉
See also: Galaxy groups, Galaxy clusters, List of superclusters

Hercules–Corona Borealis Great Wall or the Great GRB Wall is a massive galactic superstructure in a region of the sky seen in the data set mapping of gamma-ray bursts (GRBs) that has been found to have an unusually higher concentration of similarly distanced GRBs than the expected average distribution. It was discovered in early November 2013 by a team of American and Hungarian astronomers led by Istvan Horvath, Jon Hakkila and Zsolt Bagoly while analyzing data from the Swift Gamma-Ray Burst Mission, together with other data from ground-based telescopes. It is the largest known formation in the universe, exceeding the size of the prior Huge-LQG by about two times.

The overdensity lies at the Second, Third and Fourth Galactic Quadrants (NQ2, NQ3 and NQ4) of the sky. Thus, it lies in the Northern Hemisphere, centered on the border of the constellations Draco and Hercules. The entire clustering consists of around 19 GRBs with the redshift ranges between 1.6 and 2.1.

Typically, the distribution of GRBs in the universe appears in the sets of less than the 2σ distribution, or with less than two GRBs in the average data of the point-radius system. Thus, such a concentration as this appears extremely unlikely, given accepted theoretical models. Possible explanations of the concentration exist, including the so-called Hercules–Corona Borealis Great Wall. This would be an extremely huge structure of the universe, with a mean size in excess of 2 billion to 3 billion parsecs (6 to 10 billion light-years). Such a supercluster can explain the significant distribution of GRBs because of its tie to star formation. If such a structure did exist, it would be the largest known structure in the observable universe.

The Hercules–Corona Borealis Great Wall contains many billions of galaxies, depending on how they are counted.

The overdensity was discovered by several astronomers from United States and Hungary led by I. Horvath, J. Hakkila and Zs. Bagoly. They used data from different space telescopes operating at gamma-ray and X-ray wavelengths, plus some data from ground-based telescopes. By the end of 2012 they successfully recorded 283 GRBs and measured their redshifts spectroscopically. They subdivided them to different group subsamples of different redshifts, initially with five groups, six groups, seven groups and eight groups, but each group division in the tests suggest a weak anisotropy and concentration, but this is not the case when it is subdivided to nine groups, each containing 31 GRBs; they noticed a significant clustering of GRBs of the fourth subsample (z = 1.6 to 2.1) with 19 of the 31 GRBs of the subsample are concentrated within the vicinity of the Second, Third and Fourth Northern Galactic Quadrants (NQ2, NQ3 and NQ4) spanning no less than 120 degrees of the sky. Under current stellar evolutionary models GRBs are only caused by neutron star collision and collapse of massive stars, and as such, stars causing these events are only found in regions with more matter in general. Using the two-point Kolmogorov-Smirnov test, a nearest-neighbor test, and a Bootstrap point radius method, they found the statistical significance of this observation to be less than 0.05%. The possible binomial probability to find a clustering was p=0.0000055. It is later reported in the paper that the clustering may be associated with a previously unknown supermassive structure.