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Earth and The Milky Way Just Got a Few Trillion New Neighbors

By Logan Kugler 

Do you know where you live? You probably know your street address and the name of your town, state, and country. But what about your cosmic address — your location among the stars? Thanks to efforts by some astronomers in Hawaii, you can now tell people you live in Laniakea.

Scientists have known for decades that our solar system rests on an outer arm of the Milky Way galaxy. In turn, galaxies are not sprinkled randomly throughout the cosmos; they cluster into groups, which themselves are part of larger groups.

What has been known is our Milky Way is part of the Local Group, a collection of galaxies some 10 million light-years across. Now, a team of scientists led by University of Hawaii at Manoa astronomer R. Brent Tully have mapped the boundaries of a "supercluster" of galaxies stretching 500 *million* light-years through space. They named the supercluster "Laniakea," a Hawaiian word meaning "immense heaven." The name was suggested by Kapiolani Community College linguist Nawa'a Napoleon as a tribute to the Polynesian sailors who crossed the Pacific, navigating by the stars.

Tully's team determined Laniakea's contours using a method similar to the way geographers would map watersheds on Earth. In an article published in the September 2014 issue of Nature, Tully and his co-authors Hélène Courtois, Yehuda Hoffman, and Daniel Pomarède describe how they began by measuring the distance from Earth to more than eight thousand galaxies and observing the galaxies' movement. From those measurements, they calculated each galaxy's "peculiar velocity," or the difference between its observed velocity and the rate at which all galaxies are receding from each other (called the "cosmic expansion").

Mapping the velocities of all those galaxies revealed gaps between the directions different groups are moving. These divisions are three-dimensional equivalents of the mountainous divides between earthbound watersheds, on either side of which water drains to different destinations. In the case of galaxies, some can be observed to be moving more or less in our direction (once the cosmic expansion and long-range flows are accounted for)—the same way streams in the same watershed move towards the same basin—while others are moving away. Just as geographers can use the movement of water on the earth's surface to define the borders of a watershed, Tully's team was able to use the movement of galaxies through space to determine the contours of Earth's home supercluster. Laniakea, they predict, contains 100,000 galaxies with the mass of a hundred quadrillion of our Suns. (They were also able to determine that the Perseus-Pisces supercluster, first identified in 1977, borders Laniakea.)

As a side benefit to identifying a new structure in the universe—the largest yet known—the team's efforts shed light on what's called the Great Attractor. Scientists have known of a perturbation in the general cosmic expansion—a "pull" towards a particular part of the sky—for 40 years, and for nearly 30 have known in what direction it lies. Its nature, though, has remained a mystery. The mapping of Laniakea enables astronomers to conceptualize the Great Attractor as the basin of a gravitational "valley"—the end point of the supercluster's watershed.

Tully was recently named as one of four recipients of the 2014 Gruber Foundation Cosmology Prize for his role in understanding the structure and evolution of the nearby universe. He has been a regular observer at Keck Observatory since 1997 studying elliptical galaxies, jets around NGC1097 and obtaining spectra of ultra compact dwarf galaxies in the Coma Cluster

The W. M. Keck Observatory operates the largest, most scientifically productive telescopes on Earth. The two, 10-meter optical/infrared telescopes on the summit of Mauna Kea on the Island of Hawaii feature a suite of advanced instruments including imagers, multi-object spectrographs, high-resolution spectrographs, integral-field spectroscopy and world-leading laser guide star adaptive optics systems.

Keck Observatory is a private 501(c) 3 non-profit organization and a scientific partnership of the California Institute of Technology, the University of California and NASA.