An international team of astronomers has discovered a previously unknown major concentration of galaxies in the constellation Vela, which they have dubbed the Vela supercluster. The gravitational attraction from this large mass concentration in our cosmic neighbourhood may have an important effect on the motion of our Local Group of Galaxies including the Milky Way. It may also help to explain the direction and amplitude of the Local Group's peculiar velocity with respect to the cosmic microwave background.

Superclusters are the largest and most massive known structures in the universe. They consist of clusters of galaxies and walls that span up to 200 million light-years across the sky. The most famous supercluster is the Shapley supercluster, some 650 million light-years away containing two dozens of massive X-ray clusters for which thousands of galaxy velocities have been measured. It is believed to be the largest of its kind in our cosmic neighbourhood.

Now a team from South Africa, the Netherlands, Germany, and Australia including two scientists at the Max-Planck-Institut fur Extraterrestrische Physik in Garching, has discovered another major supercluster, slightly further away (800 million light-years distant), which covers an even larger sky area than Shapley.

The Vela supercluster had gone unnoticed due to its location behind the plane of the Milky Way, where dust and stars obscure background galaxies, resulting in a broad band void of extragalactic sources. The team's results suggest the Vela supercluster might be as massive as Shapley, which indicates that its influence on local bulk flows is comparable to that of Shapley.

The discovery was based on multi-object spectroscopic observations of thousands of partly obscured galaxies. Observations in 2012 with the refurbished spectrograph of the Southern African Large Telescope (SALT) confirmed that eight new clusters reside within the Vela area. Subsequent spectroscopic observations with the Anglo-Australian Telescope in Australia provided thousands of galaxy redshifts and revealed the vast extent of this new structure.

Prof. Renee Kraan-Korteweg from the University of Cape Town, who led this study and has been investigating this region for more than a decade, says: "I could not believe such a major structure would pop up so prominently," when she and her colleagues analysed the spectra of the new survey.

Scientists Hans Bohringer and Gayoung Chon from the Max-Planck-Institut fur Extraterrestrische Physik in Garching have surveyed the supercluster region for X-ray luminous galaxy clusters and found two massive clusters in the region covered by the redshift survey and further massive clusters in the immediate vicinity. They thus confirm: "This discovery shows that the Vela supercluster has a significantly higher matter density than average, making it a prominent large structure."

But there is still much to do - further follow-up observations are needed to unveil the full extent, mass, and influence of the Vela supercluster. So far this region of the sky is sparsely sampled, while the part closest to the Milky Way has not been probed because dense star and dust layers block our view.

Planned observations with the new radio astronomical facility MeerKAT will in particular help to map this obscured region and further optical redshifts will be obtained with the new large-field-of-view multiobject-spectrograph, Taipan, from Australia.

The ongoing survey of X-ray luminous clusters conducted by the MPE team, Hans Bohringer and Gayoung Chon, has recently been extended to cover this region in the band of the Milky Way. The area of the Vela supercluster and its environment will receive special attention.

"We already have good indications that the Vela supercluster is embedded in a large network of cosmic filaments traced by clusters, providing insight into the even larger-scale structure embedding the Vela supercluster. With the future multi-wavelength programme we hope to unveil its full influence on the cosmography and cosmology," Gayoung Chon remarks.

Research paper: "Discovery of a Prospective Supercluster in the ZOA in Vela," Renee C. Kraan-Korteweg, Michelle E. Cluver, Maciej Bilicki, Thomas H. Jarrett, Matthew Colless, Hans Bohringer and Gayoung Chon, 2016, Monthly Notices of the Royal Astronomical Society: Letters