Definition of a Supercluster

Question

A group of astronomers in September 2014 redefined what classifies a supercluster. Before this, the supercluster where the Milky Way resides was the Virgo Supercluster. Now, the Virgo Supercluster & three other previously defined superclusters are simply 4 lobes of the newly-defined Laniakea Supercluster.

To my understanding, this redefinition was prompted by the new information that outside of the Virgo Supercluster, the other 3 nearby superclusters are themselves being gravitationally pulled by & have "peculiar" velocity in the direction of the Great Attractor (in the center of the Virgo Supercluster). Therefore these 4 superclusters together form a loose structure that has been deemed a more fitting definition of a supercluster.

While I'm comfortable with that logic, I don't understand why the Virgo Supercluster & the other 3 still are known as superclusters. Laniakea essentially is a supercluster that contains superclusters, by that reasoning. It seems contradictory to make a new definition that overrides the last, yet have the last definition still apply.

So, what was the previous definition of a supercluster, & is there a verbatim updated definition for new superclusters somewhere? Is it possible for these both to apply simultaneously?

Answer 1

According to here, there was no precise definition before this group redefined what it meant for a group of galaxies to constitute a supercluster--before their redefinition, it seems it was just loosely defined as "extended regions with a high concentration of galaxies." They now define a supercluster to be a volume in which "the motions of galaxies are inward after removal of the mean cosmic expansion and long range flows"--basically, their new definition says a supercluster is a collection of galaxies that are tending to move closer together. When referencing the Virgo Supercluster (in their paper called the Local Supercluster) they refer to it as the "historical Local Supercluster," implying that it indeed should no longer be classified as a supercluster and instead just as a lobe of a supercluster. The people who wrote the paper have a nice video here!