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I was under the impression the only objects relatively close to our Milky Way but not yet a part of it were the Sagittarius Dwarf and Large and Small Magellanic Clouds and then much further out, but otherwise relatively close, the rest of the Local Group.

I was reading Florin Andrei's answer to Do all the individual stars that we can see in the night sky belong to Milky Way? and was confused about the facts of Omega Centauri. Upon researching more, I became more confused:

I hope that outlines my confusion...

My question, then: What is the orbital path of Omega Centauri (illustrations or animations would be appreciated)? Is it in or a part of our galaxy or orbiting externally more like the Magellanic Clouds?

EDIT: As seen in the comments below; I see how the second part to the question may be ambiguous. So, please focus on the first part of the question (What is the orbital path).

No speculation please, only fact; especially when talking about what Omega Centauri may have been in the past.

Kyle Oman
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TryTryAgain
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  • I'm getting a Virtual Windows installation up so I can install Starry Night and fly out in Spaceship mode to see if I can understand this more. – TryTryAgain Feb 19 '12 at 03:06
  • Are you defining the globular cluster as "part of the galaxy"? Which rather leads to the more general question: how will you define "part of the galaxy"? Which I suppose may be the whole point: if there isn't an agreed definition then there isn't an agreed partition of objects into the two sets. – dmckee --- ex-moderator kitten Feb 19 '12 at 06:31
  • I'm not defining, I'm asking. – TryTryAgain Feb 19 '12 at 17:14
  • The point is that there is not some obvious, binary definition of "part of the galaxy". If you just go with ballistic linkage then you can't exclude the Magellanic clouds. If you go with some kind of clustering algorithm then there are parameters to tweak, and they would need tweaking to include the globular clusters while still excluding the Magellanic clouds. So there is an ambiguity. If you can't pin down what definition you want to use you can't say and if your two sources are using different definitions they may come to different conclusions. – dmckee --- ex-moderator kitten Feb 19 '12 at 20:25
  • Be aware that I'm not an astronomer, and don't know if the astronomy community has anything like a consensus on this. It just isn't clear to be what how one would draw a line that would make everybody happy. – dmckee --- ex-moderator kitten Feb 19 '12 at 21:02
  • Right, I've edited my question to reflect I am more curious into the orbital path...then I will make my own conclusion on whether it is "a part of" the Milky Way or not. – TryTryAgain Feb 19 '12 at 21:03

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The answer to your original question is, I think, fairly unambiguously that Omega Centauri is part of the Milky Way. The orbital path question is maybe a little bit trickier, though a recently accepted Astrophysical Journal Letters paper addresses this (and you can skip to their Figure 4 to see the approximate position of Omega Centauri). Their basic approach is to pick out stars with very sub-solar metallicities, which are older stars that are likelier to be associated with a globular cluster or a stripped dwarf galaxy, and then further select stars that have abundances of Barium that are characteristic of Omega Centauri. Barium is produced in what is called the s-process, where elements are formed via slow capture of neutrons and eventual decay in to other elements. The s-process signature of Omega Centauri is unlike typical Milky Way stars (see their Figure 2), so stars that have similar s-process element abundances to Omega Centauri were likely once members of Omega Centauri.

They wind up with ten stars from their starting sample, all of which happen to have retrograde orbits. That is, the stars go the opposite direction compared to the majority of stars in the Milky Way. As Omega Centauri happens to be on a retrograde orbit, this further supports the notion that these stars once belonged to Omega Centauri, and were at some point in the past tidally stripped.

Next, they simulate Omega Centauri orbiting the Milky Way, and experiencing tidal stripping. Given uncertainties in its exact current physical position and velocity, they make a grid of potential orbital parameters, and simulate each of those, making sure Omega Centauri winds up with properties that match its observable properties (proxies, but not exact equivalents, to its physical position and velocity). They sum these all up to create a distribution of likely tidal stripping scenarios, and find that the positions and velocities of the ten candidate tidally stripped stars match up well with what the simulation predicts (the simulation stars are small colored points in their Figure 4, and the actual stars are larger colored circles with black borders). If so, you can get a rough idea of what the past orbital motion of Omega Centauri is. This is the black line that they plot in their Figure 4. You can see that it has been whipping around on a fairly elliptical orbit, inside of the orbit of the Sun around the galactic center.

Ideally, I would put the figures in the post, but I am not sure of the legality of posting figures from papers, so I didn't include them. Unfortunately, you'll just have to pull up the paper and look at the figures separately.