Thanks Alex,
glad you are enjoying my post about the Virgo Cluster.
I have a lot (70, maybe) of papers downloaded about this cluster of galaxies, and the number of published papers keeps snowballing, so it is
very very hard to keep up with what is going on, even for this single cluster of galaxies.
Virgo Cluster, at 54 million light years, is "baseline truth" for fairly rich clusters of galaxies, as 10 meter class telescopes can begin to resolve its galaxies, and it is rich in sub-clusters that can be eventually expected to merge, and it has a mixed population of galaxies with plenty of examples of each hubble type. Also, the contrast with the Fornax Cluster, which has
only a small amount of sub-structure and which is totally dominated by
non-star-forming and
gas-poor S0 and Elliptical galaxies, is remarkable. Fornax is considerably less massive than Virgo, but at least it is
our Galaxy cluster, as it is far south!
The mass of the Virgo Cluster has often been estimated, but it is still quite uncertain. All we have available as an observational diagnostic for its mass is the various velocities of its member galaxies in our line-of-sight (redshifts or blueshifts), but we cannot, as yet, observe the tiny (in angular terms) tangential motions of its galaxies, those motions at right angles to our sight-line;
So, the actual motion through space of an individual galaxy, and its orbit, is highly uncertain.
You can sort of crudely look at the range of observed radial (line-of-sight) galaxy velocities in the Virgo Cluster, and come up with a rough idea of how the various objects in this cluster are orbiting within it. And it is these galaxy motions, resulting from the gravitational field of the cluster and from the momentum of its member galaxies, that can tell us something about how a star or cluster could be moving so quickly that it can potentially be permanently ejected from the Virgo Cluster of galaxies.
(Other moving objects within Clusters of Galaxies, notably "freely orbiting" planetary nebulae and globular clusters that orbit
in the gravitational field of the cluster itself, are now beginning to be used as tracers of the mass and gravity of clusters of galaxies )
Here is a graph, from the paper I referred to in the initial post, of the line-of-sight velocities of the member galaxies, versus the number of galaxies at each specific velocity. This sort of graph is known, somewhat cryptically, to professional astronomers as a "Line-of-Sight Velocity Distribution" or just LOSVD:
This sort of plot gives at least an intuitive idea of how objects might be moving around in the gravitational field of the Virgo Cluster. The shaded purple histogram is the numbers of galaxies moving at various radial velocities (= speeds in the line of sight), and the graph marked GCs is the speeds of various globular clusters that belong to the Virgo Cluster rather than to any individual galaxy within the cluster.
(The graph marked "stars" is actually the velocity of the foreground stars in our own galaxy)
For instance, it is quite evident, even from cursory inspection of this graph, that the inter-galaxy globular clusters in the Virgo Cluster are moving in a different way from the galaxies!
The velocity of the probable ejected globular cluster is marked "HVGC-1", and the high velocity relative to the average velocity of the galaxies is glaringly evident in this plot.
