An international team of astronomers has mapped the size and shape of the Local Void, a large underdense region of the Universe, on the outskirts of which is located our Milky Way Galaxy.
Shaded gray contours outline the extent of the Local Void, while blue dots show major mass constituents (large galaxies, galaxy groups, and clusters). The curved blue lines show the derived motions of these massive objects, after removing the overall expansion of the Universe. The most important galaxy congregations are given special symbols, like the red ball identifying the Virgo Cluster. The dominant pattern of motions revealed by the orbits is a flow away from the Local Void. Image credit: University of Hawaii.
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 towards the densest areas and away from regions with little mass — the voids.
Although we live in a cosmic metropolis, back in 1987 University of Hawaii astronomer Brent Tully and NRAO astronomer Richard Fisher noted that the Milky Way is also at the edge of an extensive empty region that they called the Local Void.
The existence of the Local Void has been widely accepted, but it remained poorly studied because it lies behind the center of our Galaxy and is therefore heavily obscured from our view.
Now, Dr. Tully and colleagues 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 our home supercluster of over one hundred thousand galaxies, giving it the name Laniakea.
“For three decades, astronomers have been trying to identify why the motions of the Milky Way, our nearest large galaxy neighbor Andromeda, and their smaller neighbors deviate from the overall expansion of the Universe by over 1.3 million mph (600 km/s),” the researchers said.
“Our study shows that roughly half of this motion is generated ‘locally’ from the combination of a pull from the massive nearby Virgo Cluster and our participation in the expansion of the Local Void as it becomes ever emptier.”
The results appear in the Astrophysical Journal (arXiv.org preprint).
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R. Brent Tully et al. 2019. Cosmicflows-3: Cosmography of the Local Void. ApJ 880, 24; doi: 10.3847/1538-4357/ab2597
