Rendition of the Local Void and surrounds
A smoothed rendition of the structure surrounding the Local Void. The Milky Way galaxy lies at the origin of the red-green-blue orientation arrows (each 200 million lightyears in length). It is at a boundary between a large, low density void, and the high density Virgo cluster.

An astronomer from the University of Hawaiʻi Institute for Astronomy (IfA) and an international team published a new study that reveals more of the vast cosmic structure surrounding the Milky Way galaxy.

The universe is a tapestry of galaxy congregations and vast voids. In a new study reported in The Astrophysical Journal, astronomer Brent Tully’s team applies the same tools from an earlier study to map the size and shape of an extensive empty region they called the Local Void that borders the Milky Way galaxy. Using the observations of galaxy motions, they infer the distribution of mass responsible for that motion, and construct three-dimensional maps of the local universe.

Galaxies not only move with the overall expansion of the universe, they also respond to the gravitational tug of their neighbors and regions with a lot of mass. As a consequence, relative to the overall expansion, they are moving toward the densest areas and away from regions with little mass—the voids.

In 1987 Tully and Richard Fisher noted the existence of the Local Void, which has become widely accepted but remains poorly studied because it lies behind the center of the galaxy and is therefore heavily obscured from view.

Now, Tully and his team have measured the motions of 18,000 galaxies in the Cosmicflows-3 compendium of galaxy distances, constructing a cosmographic map that highlights the boundary between the collection of matter and the absence of matter that defines the edge of the Local Void. They used the same technique in 2014 to identify the full extent of the home supercluster of over 100,000 galaxies, giving it the name Laniakea, meaning “immense heaven” in Hawaiian.

For 30 years, astronomers have been trying to identify why the motions of the Milky Way, the nearest large galaxy neighbor Andromeda, and their smaller neighbors deviate from the overall expansion of the Universe by over 600 km/s (1.3 million mph). The new study shows that roughly half of this motion is generated “locally” from the combination of a pull from the massive nearby Virgo Cluster and the participation in the expansion of the Local Void as it becomes ever emptier.

An 11-minute video demonstrates the shape and extent of these cosmic structures.

Interactive visualizations that allow the user to rotate, pan and zoom maps of the mass distribution:

The paper was published on July 22, 2019 in The Astrophysical Journal.

—By Roy Gal