Posted on Feb 10, 2016
In September 2014, a group of astronomers including R. Brent Tully of the University of Hawaii and Hélène Courtois of the University of Lyon determined that our Milky Way galaxy is part of a newly identified enormous supercluster of galaxies, which they dubbed "Laniakea," 500 million light-years in diameter and containing the mass of one hundred million billion Suns spread across 100,000 galaxies. The discovery clarifies the boundaries of our galactic neighborhood and establishes previously unrecognized linkages among various galaxy clusters in the local Universe.
The Milky Way resides in the outskirts of the supercluster, which means "immense heaven" in Hawaiian, whose extent has for the first time been carefully mapped using the new mapping techniques pioneered by Tully and colleagues.
The supercluster consists of four subparts, the Virgo Supercluster, the part where the Milky Way resides; Hydra-Centaurus Supercluster; the Great Attractor, the Laniakea central gravitational point near Norma Antlia Wall, known as Hydra Supercluster; Centaurus Supercluster; Pavo-Indus Supercluster; Southern Supercluster, including Fornax Cluster (S373), Dorado and Eridanus clouds. The most massive galaxy clusters of Laniakea are Virgo, Hydra, Centaurus, Abell 3565, Abell 3574, Abell 3521, Fornax, Eridanus and Norma.
The entire Laniakea supercluster consists of approximately 300 to 500 known galaxy clusters and groups. The real number may be far larger, because some of these are traversing the Zone of Avoidance, making them essentially undetectable. The project also clarifies the role of the Great Attractor, a gravitational focal point in intergalactic space that influences the motion of our Local Group of galaxies and other galaxy clusters.
Within the boundaries of the Laniakea Supercluster, galaxy motions are directed inward, in the same way that water streams follow descending paths toward a valley. The Great Attractor region is a large flat bottom gravitational valley with a sphere of attraction that extends across the Laniakea Supercluster.
"We have finally established the contours that define the supercluster of galaxies we can call home," said lead researcher Tully. "This is not unlike finding out for the first time that your hometown is actually part of much larger country that borders other nations."
Superclusters are among the largest structures in the known Universe. They are made up of groups, like our own Local Group, that contain dozens of galaxies, and massive clusters that contain hundreds of galaxies, all interconnected in a web of filaments. Though these structures are interconnected, they have poorly defined boundaries.
Galaxy clusters intrigue astronomers because their construction is believed to be influenced by the Universe's notoriously strange components — dark matter and dark energy. By studying their properties at different stages in the history of the Universe, galaxy clusters can shed light on the Universe's poorly understood dark side.
In 2011, a team consisting of over 100 astronomers from around the world, started a hunt for the cosmic monsters. Although the high-energy X-ray radiation that reveals their location is absorbed by the Earth's atmosphere, it can be detected by X-ray observatories in space. They combined an ESA XMM-Newton survey — the largest time allocation ever granted for this orbiting telescope — with observations from ESO and other observatories. The result is a huge and growing collection of data across the electromagnetic spectrum, collectively called the XXL survey.
"The main goal of the XXL survey is to provide a well-defined sample of some 500 galaxy clusters out to a distance when the Universe was half its current age," explains XXL principal investigator Marguerite Pierre of CEA, Saclay, France.
The XMM-Newton telescope imaged two patches of sky — each one hundred times the area of the full Moon — in an attempt to discover a huge number of previously unknown galaxy clusters. The XXL survey team have now released their findings in a series of papers using the 100 brightest clusters discovered.
This image above shows XXL-South Field (or XXL-S), one of the two fields observed by the XXL survey. XXL is one of the largest quests for galaxy clusters ever undertaken and provides by far the best view of the deep X-ray sky yet obtained.The area shown in this image was obtained with some 220 XMM-Newton pointings and, if viewed on the sky, would have a two dimensional area a hundred times larger than the full Moon (which spans one half degree), and that is without taking into account the depth that the survey explores.
The red circles in this image show the clusters of galaxies detected in the survey. Along with the other field — XXL-North Field (or XXL-N) — around 450 of these clusters were uncovered in the survey, which mapped them back to a time when the Universe was just half of its present age. The image also reveals some of the 12 000 galaxies that had very bright cores containing supermassive black holes that were detected in the field
Observations from the EFOSC2 instrument installed on the New Technology Telescope (NTT ), along with the FORS instrument attached to ESO's Very Large Telescope (VLT ), also were used to carefully analyse the light coming from galaxies within these galaxy clusters. Crucially, this allowed the team to measure the precise distances to the galaxy clusters, providing the three-dimensional view of the cosmos required to perform precise measurements of dark matter and dark energy.
The Daily Galaxy via NRAO and University of Hawaii