View Full Version : Discussion: Clusters without a Home

2003-Jul-18, 01:37 AM
SUMMARY: Thousands of globular star clusters wander aimlessly between galaxies, in what was once thought to be 'empty space'. This is the finding of a joint US-UK project announced today at the International Astronomical Union General Assembly in Sydney. The group, lead by Dr. Michael West of the University of Hawaii, believes these clusters were 'torn' away from their parent galaxies and now drift as orphans. (contributed by Darren Osborne)

Comments or questions about this story? Feel free to share your thoughts.

2003-Jul-18, 01:39 AM
Any idea on how they were torn away? collision with another galaxy? I mean, that's quite a force! And were they torn away in the cluster group they were in in the galaxy or did the cluster form after?

2003-Jul-18, 08:05 AM

I went to the source, and talked to Dr. Michael West, one of the researchers who helped discover the rogue star clusters.

Here's what he had to say:

Greetings from Australia, where I'm attending the International Astronomical Union meeting. I'm happy to try to answer your questions about "rogue" star clusters. I gave a talk here yesterday about hundreds of "orphaned" star clusters that my collaborators and I recently discovered wandering through intergalactic space.

There are a number of ways that intergalactic star clusters might form:

1) They could have simply formed there, outside of any galactic home. Astronomers think that the first objects in the universe should have had masses similar to those of globular clusters, and we can measure the ages of the globular clusters in the Milky Way and they are found to indeed be old. But the question is whether they could form from the thin gas between the galaxies, or whether their birth must have taken place in a galaxy. That question can't be answered yet (though we believe that new Hubble Space Telescope observations that we have obtained may allow us to answer that eventually).

2) A very likely scenario is that some (or all) of the intergalactic star clusters that we've found were pulled out of galaxies. The universe is a dangerous place for galaxies, as collisions between galaxies can pull stars and star clusters into space, large galaxies cannibalize smaller ones, and in some cases violent collisions between galaxies or the collective gravitational pull from their neighbors can be strong enough to tearm the galaxy apart completely. The partial or complete destruction of galaxies (of which there is ample, and growing evidence) spills their contents into space. Globular clusters, because they are such dense (compact) systems of stars are held together tightly by gravity, and hence will survive the destruction of their parent galaxy. There is already evidence of intergalactic stars, so it'd be surprising if there weren't also some intergalactic star clusters. One of my colleagues, Dr. Michael Gregg, gave a presentation here in Sydney this week on the "Ghosts of Galaxies", showing many examples of galaxies in the process of being torn apart from a sudden gust of gravity caused by a passing galaxy.

3) It's possible, though perhaps speculative, that some intergalactic star clusters might really just be the naked nuclei of galaxies whose outer regions were pulled into space during galaxy collisions. The central regions of small galaxies can be quite dense, there is a whole class of galaxies known as nucleated dwarfs. By pulling off the outer regions of these galaxies, the remaining nucleus would be small and compact, and hence might masquerade as a star cluster in intergalactic space. One of my colleagues, Dr. Michael Drinkwater, presented results here at the IAU meeting in Sydney on a new type of "ultra compact dwarf" galaxies that they recently discovered. So it's possible that some "rogue" star clusters might simply be all that remains of once larger galaxies.

Michael West
Department of Physics & Astronomy
University of Hawaii at Hilo

Hope that clears things up!

2003-Jul-18, 08:22 AM
"There is already evidence of intergalactic stars, so it'd be surprising if there weren't also some intergalactic star clusters. "

why is that? I mean it seems logical for a single star to break away but an entire ready-made cluster? what sort of gravity holds these things together? what is the time frame for one of these collisions?

As always, more questions!

Thanks Fraser and of course Dr West!

2003-Jul-18, 03:47 PM
I would think that the cluster's combined mass would be the gravity holding it together, and on if there is a black hole forming within their center, maybe one so small, that it's undetected, but large enough that it gives 'weight' to the small cluster...
...and the time frame is probably like millions of years for this to happen... the bigger the cluster being ripped and pulled on, the longer it would take...

It also depends on at what speed is the passing galaxy being pushed at, or how fast is it travelling on its own accord, or from other gravitational pulling of other unknown sources... tugging here and there from black holes, seen or not, the radiation and x-rays expelling from other clusters or centrallized group of stars that may have a black hole at their center... and other cosmic phenomena.

There is a rush within a crowd of people... running ensues... a smaller person may try to hold on, but is ripped from the grasp that holds their hand... and the child is left where it was let go... only until another hand may come along and pull it or just pick it up entirely... the mass of the crowd will continue to move, but the areas between the people allow room for smaller beings to slip through, unmoved, because their mass in not sufficient enough to keep them going without them exerting enough energy to do so...
...they cannot, because they are young, and elements needed for such exertion is not available... but a group of smaller beings may have enough mass to make their own pull and be able to survive as one, either with the rushing crowd, or without it... it may depend on if the passing group mass is large enough to drag the smaller mass along or not... for the smaller group mass may have enough 'pull' to remain on their own...
surviving is another thing...

weird analogy... but i'm weird... so i guess its ok..

. ..-={Arramon}=-.. .

2003-Aug-01, 08:42 PM
This is an alarming revelation in some ways, considering that the Andromeda galaxy and the Milky Way galaxy are destined for a galactic collision, or should I say merger? I am not sure anybody knows or has thought of (by simulation) what the end results of that event will be.
Could our galaxy be ripped apart in a similar fashion by such an interaction with the larger Andromeda galaxy? Could we wind up being one of those intergalactic stars, lost in the lonely void of space between galaxies? At least we have alot of time to think about it, I suspect Los Angeles will be in the middle of the Pacific Ocean by the time this event takes place.

2003-Aug-01, 09:24 PM
I thought I would add a few more thoughts that are less deep and more practical on the subject. Looking at our local group of neighboring galaxies provides some useful insight, I believe. The Sagittarius dwarf galaxy is in the process of being ripped apart by our home galaxy as we speak, proving that these inter-galactic interactions can and do happen, resulting in deconstruction of a galaxy's internal structure and ejection of stars and possibly clusters of stars.
More importantly there are alot more dwarf galaxies than there are larger spiral and elliptical galaxies, by a ratio of something like 10:1. It would seem unlikely to me that all of these dwarfs were ripped from larger galaxies and remain intact. If they were, then that would mean alot of dwarfs come from the same parent galaxy that was ripped apart. If so, then that shouldnt be hard to determine by studying the relative motion of these dwarf galaxies to determine their point of origin (which should merge to the same point/time in space where they got ripped from their parent galaxy). It will be nice when we have enough data to be able to plot the movement of galaxies on a large scale in relation to one another, since it gives so much more useful data than a static picture (analgous to looking at a weather map and then putting it in motion to yield more useful information).
For the above reason I favor the theory that large galaxies such as ours evolved (at least in part) by accretion of smaller dwarf galaxies (and their dust) and that these dwarfs are largely pockets of dust and their stars that formed at the same time, but far enough away from other early clusters that they havent yet been incorporated into a larger galaxy yet, rather than the alternative that the dwarfs were all part of a larger galaxies to start with. The actual answer probably is that both theories are true.