Our Galaxy Is a Lot Less Alone in Space Than We Thought

The Milky Way isn't as lonely a galaxy as you might have thought. In fact, our home galaxy is actually part of a gigantic supercluster of galaxies called Laniakea, according to new data gathered using the National Science Foundation's Green Bank Telescope (GBT).

The largest structures in the universe form a massive network of clusters, filaments, and superclusters of galaxies that break up the cosmic void. But for everything we know about these large cosmic objects, astronomers have been unable to create a detailed map of the structure in which our own galaxy resides, until now.

According to the new work, it turns out our galaxy resides in the outskirts of the Laniakea Supercluster. Laniakea, which means "immense heaven" in Hawaiian, is a supercluster 500 million light-years in diameter that contains the mass of one hundred million billion Suns spread across 100,000 galaxies.

In short, our galaxy is just one part of a massive whole. But the finding goes deeper than that: the discovery that has helped clarify the boundaries of our galactic neighborhood to establish previously unrecognized links between galaxy clusters in our local part of the universe.

"We have finally established the contours that define the supercluster of galaxies we can call home," said Brent Tully, lead researcher and astronomer at the University of Hawaii at Manoa, in a release. "This is not unlike finding out for the first time that your hometown is actually part of much larger country that borders other nations."

This latest discovery has also helped solidify the location of the so-called Great Attractor, a gravitational focal point in space that influences the movement of the galaxies in our local group and extends across the Laniakea Supercluster.

"Green Bank Telescope observations have played a significant role in the research leading to this new understanding of the limits and relationships among a number of superclusters," said Tully. The team's work was published this week in Nature.

Visible in spectroscopic surveys of the universe, supercluster structures are thought to be the result of gravitational instabilities. All the groups and clusters of galaxies are all linked by a web of filaments, or "bridges" of dark matter that link galaxies together.

But the existence of these connecting filaments means it is incredibly hard to figure out exactly where one galaxy ends and another begins. Cosmic mapmaking is thus a very difficult undertaking.

Why does such a map of cosmic superstructures matter in the first place? A detailed map can reveal the large cosmic structures surrounding our own Milky Way, things that are impossible to observe for systems that lie far from the Earth.

A more detailed map is also essential to precisely determine cosmological parameters, things like the density of dark energy that astronomers think drives the acceleration of the universe's expansion, as well as understand just how galaxies have formed and evolved.

There are a couple of ways astronomers can study the distribution of galaxies: they can look at the redshifted light from galaxies in a full sky survey, or they can look at how their motion is affected by external gravitational forces.

It's the latter method astronomers used in the latest survey. Just like a moon is acted on by both its parent planet and the other moons that orbit alongside it, a galaxy situated between large scale structures is acted upon by the structures' gravitational environments, which can be used to map out their boundaries.

This is what astronomers have done using the GBT and a handful of other radio telescopes: they have successfully mapped the velocities of galaxies throughout our local universe and used that information to define the regions of space where each supercluster dominates. This means they have been able to narrow down the boundaries of the galaxies in our local cosmic neighbourhood.

The team's goal is to ultimately map the velocity of the galaxies that make up our local group, confirming the radius that encompasses our corner of the universe and giving us a better understanding of our home planet's place in space.