The starlit galaxies are not randomly distributed throughout our vast and mysterious Universe. Instead, they dwell within groups, such as our own Milky Way Galaxy's Local Group--that harbors more than 40 galaxies, of which our Galaxy and the Andromeda Galaxy are the largest members, both being starry spirals that look like gigantic pin-wheels twirling majestically in Space. Clusters and superclusters of galaxies are the largest structures known to dwell in the Universe, and they commonly host hundreds to thousands of separate galaxies that are all "glued" together by their mutual gravitational bonds. In the September 4, 2014 issue of the journal Nature, an international team of astronomers announced that they have succeeded in defining the contours of the enormous supercluster of galaxies hosting our own Milky Way. They have named the supercluster Laniakea, meaning "immense heaven" in Hawaiian.
The groups, clusters, and superclusters of galaxies that inhabit our Cosmos are all interconnected in a web of filaments in which the galaxies are strung out like glittering dewdrops on the web of a gigantic and mysterious spider. The vast galaxy superclusters are situated where these filaments intersect, and all of these enormous structures are interconnected. However, they have poorly defined boundaries.
The team of astronomers, led by Dr. R. Brent Tully of the University of Hawaii at Manoa, propose a new method that can be used to evaluate these enormous large-scale structures by studying their impact on the motions of galaxies. A galaxy situated between two such structures will be trapped in a gravitational tug-of-war in which the balance of the gravitational forces from the surrounding large-scale structures determines the galaxy's motion. By mapping the velocities of galaxies distributed throughout our local Universe, the team was able to define the region of space where each supercluster dominates.
Our place in space has long been one of the most profound and enduring mysteries of humanity's existence. This new research now helps to unravel some of this long-held secret of Cosmic proportions. Lanaiakea, the astronomers write, is our home supercluster--where the Milky way dwells in starlit splendor, with others of its lovely galactic kind.
What kind of home is Lanaiakea? It is a vast region, extending approximately 160 million parsecs across. Even though it is not as big as some other known superclusters, it is the largest in our Milky Way's local neighborhood, which is strangely crowded considering the enormous swath of emptiness that is most of the cosmological void. Lanaiakea has several other supercluster neighbors, which include Coma, Perseus-Pisces, and Shapley--and together they may all make up a "hypercluster".
Actually, Lanaiakea has been hiding right out in the open. It is enormous and it exists all around us--and, yet, it has somehow eluded the prying eyes of earlier astronomical surveys. This is perhaps because the boundaries of superclusters are difficult to determine. Dr. Elmo Tempel, who is in the Department of Cosmology at Tartu Observatory in Estonia, writes in his News & Views commentary on this research, appearing in the September 4, 2014 Nature, that "The large-scale structure of the Universe is an intricate network of clusters, filaments and superclusters of galaxies, together with cosmic voids that are almost entirely bereft of galaxies. Superclusters are extended regions containing a large number of galaxies, but this concept is rather vague; researchers lack a robust, quantitative definition for them. Tully and colleagues have found a neat way of identifying the edges of superclusters by examining the motions of galaxies. In doing so, they have detected the boundaries of our home supercluster."
Lanaiakea was at long last discovered with the aid of what Dr. Tempel terms a "nifty algorithm" that enabled the team of astronomers to transform incomplete measurements of the motion of galaxies into a map of the distribution and dynamics of cosmic matter. Their map shows galaxy superclusters as hotspots--which are basins of attractions in fields of velocity flow--that can be separated from their surroundings.
It is a local map because the "nifty algorithm" has certain limitations. This is because it is dependent on direct measurements from our own planet of how speedily the galaxies are receding as a result of the cosmic expansion. The remainder of the Cosmos--harboring those glittering galaxies far, far away--resides beyond the reach of a map, at least, for the time being.