Recognition of this apparent "halo" of Globular Clusters has long been interpreted as being a roughly spherical pattern, which is centered on the actual center of the Galaxy. This approach has long represented one of science's best guesses at where the actual center of the Galaxy really is. Of course, we know what direction it is in, but determining how far away it is from the Sun has always been problematical. Determining the apparent distances of the Globulars, (primarily due to Cepheid variable analysis) and then attempting to reconstruct the roughly spherical space they seem to be in, gives us an apparent distance to the center of the Galaxy of about 30,000 light years.
Many of the Globulars have apparent distances from us of around 40,000 light years, because they are well above or below the Galaxy's center point. Since many of them appear in the same area of sky in and around the constellation of Sagittarius, simple geometry and trigonometry shows that they are roughly 10,000 light years above or below the Plane of the Galaxy.
Herein lies the logical dilemma. These Globulars are quite close to the huge central mass of the Galaxy, and therefore they must revolve around the Galaxy's center point, in circles or more likely elliptic paths. It would appear that no one has considered all the consequences of this. It is almost like Physicists have been assuming they just hover there like a swarm of bees. But that wouldn't happen. They could NOT just follow elliptical paths above or below the Galactic Plane. Basic gravitational theory insists that their elliptical paths pass through the Galactic Plane, because the two halves of any path must be on opposite sides of that Plane.
At a distance of 10,000 light years from the center point of the Galaxy, it is generally thought that the star density in the Galaxy is so great as to be about one solar-mass per cubic light year. If a nicely organized Globular Cluster were to pass through such a densely populated area, it would certainly be greatly disrupted. Since many estimates of the thickness of the central Galactic disk is about 2,300 light years, the Globular would be in that environment for quite a while, with many chances of substantial perturbations of the individual cluster members' motions.
The likelihood that a Globular Cluster leaving the Galactic Plane being significantly disrupted is very high. At least a few of the Globular Cluster's member stars would certainly be given substantial new velocity vectors. Such stars would either have achieved escape velocity to entirely leave the cluster, or otherwise have their initially stable orbits within the cluster changed, leading to erratic patterns of stars in the cluster. In addition, the same types of perturbative effects would certainly have caused some Galaxy stars to join the Globular cluster, which would suggest a fairly integrated population of stars in the Globulars.
Since these clusters lie so close to the Galactic Core, their period of revolution about the Galaxy's center point, is quite short, a few tens of millions of years. It is generally accepted that the member stars of the Globulars are Population II red stars, which are as much as 15 billion years old. This implies that all of the Globular Clusters that we see must have experienced this massive disruption only a few million years ago, and hundreds of times before that, and yet they appear remarkably symmetric.
The constituent stars of Globular Clusters are generally considered to be the very oldest stars associated with our Galaxy. In each Globular, they also appear to be very similar to one another, leading to the belief that they have spent a VERY long time together, orbiting their common Globular center point. These apparent facts would seem in great variance with the many hundreds of difficult passages that the Globular would have to have survived during that long lifetime as an entity.
This seems to suggest that at least one of the assumed facts described above must be in error. Additional research, with an open mind, is called for.
There are some Physicists who estimate the Galaxy's central Core disk to be only 300 light years thick (but tilted at a 25 degree angle to the accepted Galactic Plane). And some believe that there is a substantial gap that exists around the central core, before the large mass of molecular clouds of free Hydrogen start up. First of all, this empty space would make our Galaxy rather unique, since the many thousands of other spiral galaxies photographed and studied do not show evidence of such a central empty ring. In contrast, they generally show great concentrations of stars and other matter in that vicinity. In addition, such a gap would seem to be dynamically unstable, except for limited periods of time. Certainly, such a gap could not have endured for the many billions of years that Globulars seem to have had stable existence.
Even if our Galaxy has such a relatively open gap area, through which the Globulars could pass through in their paths through the Galactic Plane, there would certainly still be SOME massive objects in the area that would materially change the paths of some of the Cluster's constituent stars. In addition, just passing through the Galaxy would induce huge tidal effects on the Cluster's constituent member stars. As the Globular approached the immense mass of the Galactic Disk, significant differential gravitational attractions would have to occur in the Cluster. This would cause accelerations that would seem to certainly greatly stretch the cluster in the direction of the its motion toward the Galaxy. As the cluster later left the Plane, the effect would NOT precisely cancel itself out. To some extent, the tidal elongation effect would include second-order effects that would last, one of which would certainly be a "ringing" effect where the Cluster's dimension in the direction of motion would oscillate for some time.
Once it was clear of the Galaxy, individual stars of the Globular would therefore have certain unique resonant motions (perturbations) from the encounter. These resonances would be disruptive to the Globular's symmetry, and they would certainly take quite some time to dampen out.
The above arguments would seem to imply that the Globulars could NOT actually pass through the Galactic Plane, at least at the points where they apparently MUST pass through it. But basic Physics ALSO tells us that there is no stable way for them to remain above or below the Galactic Plane without passing through it!
It would appear that either our understanding of the Laws of Physics is sadly lacking, or our understanding of the geometry of the situation is greatly in error. The commonly accepted view of a halo of ancient, stable Globular Clusters hovering around the Core of the Galaxy, like a swarm of bees, is just not compatible with our current understanding of the Laws of Physics. Clearly, further research is necessary. An obvious starting point would be to attempt to confirm our present estimates of the actual distances of the Globulars, and to confirm that they are as old as generally believed.
In addition, average Cluster velocity vectors could be beneficial to establishing the orbital elements of each Globular's center of mass, to determine whether the "halo" actually exists and is spherical.
( http://mb-soft.com/public/othersci.html )
C Johnson, Physicist, Physics Degree from Univ of Chicago