This old supercluster: Scientists measure the (massive) place we call home

Ever heard of Laniakea? Well, it's a supercluster of 100,000 galaxies including the Milky Way, and scientists just took a tape measure to it for the first time.

The next time someone asks you where you live, you can answer with a straight face: Laniakea – roughly translated as "spacious heaven."

That's the Hawaiian name a team of astronomers has given to the 521-million-light-year wide supercluster the Milky Way inhabits. The christening marks the first time astronomers have clearly defined the boundaries of a supercluster.

Superclusters are some of the largest structures in the cosmos, built from galaxies, which gather in local groups, which then form galaxy clusters. The Milky Way's local group, for instance, contains more than 54 galaxies, is about 10.1 million light-years across, and has an estimated mass about 1.3 trillion times the mass of the sun. Laniakea hosts roughly 100,000 galaxies that collectively tip the scales at 100 million billion solar masses.

The effort to precisely map this large-scale structure could help astronomers better understand how events at the dawn of the universe gave rise to the structure we see today.

Subtle quantum fluctuations in the earliest moments of the Big Bang – the enormous release of energy that gave rise to the universe – are thought to have ultimately led to visible variations in the universe's density. Astronomers have detected these variations in the early universe as ripples in the cosmic microwave background – the afterglow of the Big Bang. Now, understanding the structure of superclusters better could allow scientists essentially to start working backward from what they see today to see how closely the picture dovetails with other observations on the evolution of structure.

The work was conducted by a team of astronomers from the US, Israel, and France led by University of Hawaii astronomer R. Brent Tully and is described in a paper set to appear in Thursday's issue of the journal Nature.

The team did more than define Laniakea's boundaries. It also revealed another "important" measure, says Mauro Giavalisco, an astronomer at the University of Massachusetts at Amherst: "The gravitational pull that the supercluster has exerted on the surrounding matter during the cosmic eons."

"This allows characterization of the size, matter, and basically how the supercluster grew," says Dr. Giavalisco, who was not a member of Dr. Tully's team of cosmic surveyors.      

Originally, the team's goal was simply to improve the accuracy of distance measurements to galaxies in the local universe, notes Tully in an e-mail.

"Going in, we had no thought about bounding Laniakea," he writes. The boundary "just fell out of our analysis. We made progress because we had more and better distance data than had been available."

They also developed a highly sophisticated analysis approach for interpreting the data. It showed how galaxies were moving in relation to one another, revealing patterns and groupings. 

Observations with ground-and space-based telescopes yielded accurate distances to 8,161 galaxies in all regions of the sky, allowing the team to build a 3D map of their distribution. The distance measurements allowed the team to calculate the theoretical pace at which the galaxies should be receding with the expansion of the universe. But the data also provided the actual recession velocities, which were different from the theoretical calculations. The difference shows how a galaxy or cluster is being influenced by the gravitational tug from other relatively nearby galaxies or clusters. This is known as a galaxy's peculiar velocity.

The team used peculiar velocities to see where galaxies were headed. Based on those observations, they defined the supercluster's boundary as the region along which the motion of galaxies diverge. In principle, this is much like the Continental Divide, the boundary along the spine of the Rocky Mountains that determines whether water flowing down hill will head east or west. But the boundary between superclusters is not so sharply defined, because galaxies are far more diffuse at the supercluster's outer edges, says Giavalisco.

In Laniakea's case, the supercluster's galaxies and galaxy clusters are headed toward a region of enormous mass that has been dubbed The Great Attractor. Recent studies have shown that the attractor is a close pair of very dense clusters.

Intriguingly, within Laniakea, there are filaments and sheets of galaxies separated by voids – which mimics the structure of the universe on the largest scales. The researchers note that over time Laniakea, like other components of the cosmic web, will dissipate with the accelerated expansion of the universe from the influence of dark energy.

Meanwhile, Laniakea may be on a trip of her own.

"Laniakea appears to be moving toward an even bigger structure called the Shapley concentration. This feature is currently poorly understood. Are we part of something even bigger than Laniakea? This possibility remains to be explored," Tully writes.

As for the Milky Way's place in all this? It lies on the outskirts of Laniakea, on a galactic cul-de-sac the end of one peculiar-velocity roadway toward the Great Attractor.

While astronomers have long known about the existence of superclusters, a consistent definition has been elusive. Indeed, if the team's definition meets with general acceptance, some of the clutches of galaxies within Laniakea may wind up being demoted from superclusters to mere clusters.

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