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Laniakea: our home supercluster.

Astronomers have mapped the cosmic watershed in which our Milky Way Galaxy is a droplet. This massive structure extends more than 500 million light-years and contains 100,000 large galaxies.

The work, published in the September 4th Nature, is the first to trace our local supercluster on such a large scale. It also provides a physical way to define what a supercluster is, by delineating it based on the motions of its member galaxies.

Researchers have been working out the gravitational structure in our local universe for decades. Based on work by Gerard de Vaucouleurs in the 1950s, astronomers have thought of our galaxy as being on the edge of the so-called Local Supercluster, a structure about 100 million light-years wide that's centered on the Virgo Cluster.

But astronomers have seen much larger structures in the universe, on the scale of several hundred million light-years. These maps have generally depended on calculating galaxies' 3-D locations based on their cosmological redshifts.

Brent Tully (University of Hawaii, Manoa) and colleagues have taken a different approach. They used galaxies' peculiar velocities, which are the galaxies' motions due to the local gravitational landscape. Galaxies fall toward or away from one another in this landscape; the Milky Way and many others seem to be moving toward the Great Attractor, a dense region in the vicinity of the Centaurus, Norma, and Hydra clusters about 160 million light-years away.

Peculiar velocities are on the order of a few hundred kilometers per second, whereas cosmic expansion velocities rise to several thousand km/s in the nearby universe, reaching 10,000 km/s roughly 130 million light-years out. (A galaxy recedes faster the farther away it is.) There's about 10-20% uncertainty in the peculiar velocity measurement for an individual galaxy. So only for nearby galaxies is the peculiar velocity high enough compared with the expansion velocity for astronomers to peg it confidently.

The team found a way around this problem by using a technique called Wiener filtering. This algorithm allowed the team to essentially take a step back and look at the big picture, revealing the large-scale flow patterns created by galaxies' motions.

Last year, the team used this technique to map the local universe's web of filaments, clusters, and voids. Now, they've taken a closer look using their Cosmicflows-2 catalog, which contains more than 8,100 galaxies. The new catalog reveals where the flows merge and diverge, unveiling a gargantuan structure on whose periphery the Milky Way sits. The Great Attractor is a central valley in this newly demarcated watershed.

The team calls this huge supercluster Laniakea, from the Hawaiian Iani (heaven) and akea (spacious, immeasurable).

The analysis also reveals other structures, including a separate supercluster called Perseus-Pisces and a distant concentration named Shapley about 650 million light-years away, toward which Laniakea is moving.

Finding out if our supercluster is only the elephant's trunk will require accurate distance measurements that reach three times farther than the current catalog.

Find videos exploring our local cosmic structure at www.skypub.com/laniakea.

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Title Annotation:COSMIC STRUCTURE
Author:Carlisle, Camille M.
Publication:Sky & Telescope
Date:Dec 1, 2014
Words:502
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