Is there a fundamental difference between a small galaxy, and a large star cluster?

  • For instance, If a large globular cluster were somehow ejected from its galaxy, and was observed by astronomers, would they call it a galaxy, or would there be characteristics of the star cluster that make it obviously not a galaxy by definition? And why are the Magellanic Clouds dwarf galaxies, and not large star clusters? Could we remove stars until the Magellanic clouds became star clusters, or is there something intrinsically galaxy like about these objects?

    I am only using globular clusters as an example, since, at least in photographs, globular clusters look like elliptical galaxies.

    There are also some astronomical objects which are in doubt between stellar clusters and elliptical galaxies, I will try to find the reference.

    @py-ser I already have, in my answer

    Omega Centauri is an interesting edge case between globular clusters and dwarf galaxies. It may even have a central black hole.

    Yes, Omega Centauri is was the reason why I asked this question. It seemed to have an ambiguously "in between" mass between a small galaxy, and a large cluster. Also, I have seen it stated that this is possibly a small galaxy's center, whose surrounding stars were stolen by the Milky Way. As a non-astronomer, it got me to wondering if there was an intrinsic difference between a small galaxy nucleus (terminology?), and a large globular cluster.

  • astromax

    astromax Correct answer

    8 years ago

    There are a couple of important distinctions between the two types of objects. Galaxies are objects which range in mass from about $10^{9}-10^{12} M_{\odot}$, and contain 'halos' of dark matter which represent the majority of the mass of the object. Now, there are things called dwarf galaxies (which are less massive than regular galaxies; I'd imagine that there really could be a bit of a blurred distinction between these two kinds of objects), however dwarf galaxies still seem to have dark matter components to them.

    Globular clusters are tightly bound collections of about $10^{6}$ stars, and do not have significant (if any) dark matter contained within or around them. So for these reasons in practice it is not hard to confuse the two.

    Some References:

    1) Evidence that dwarf galaxies are dominated by a dark-matter component.

    2) Dark matter present in globular clusters are either unlikely, or incredibly highly concentrated and approximately the same mass as the stellar component.

    So the main differences between a galaxy object and a star cluster object is the galaxy's association with dark matter? Is mass a requirement to make the distinction? I am curious if mass is an absolute requirement. Stars can vary in mass, but do not become different things. Also, both references were extremely interesting. Thank you!

    However, your answer makes it sound as if an assessment of dark matter is made to determine whether an object is a galaxy or a cluster. Isn't it more correct to note that when we look at things that have historically been classed as a galaxy or a cluster, and we look at how much dark matter appears to be associated with them, we see a correlation? While we might now use a measure of dark matter present to make a distinction in otherwise unclear cases, surely the amount of gas and the ages of the stars present will be more relevant in the classification of an object?

    astromax, since we don't yet fully understand the mechanics of dark matter, isn't it possible that a mechanism exists by which globular cluster ejecta attract dark matter to themselves? Do we know any instances of free-standing astronomical objects of the mass of globular clusters that don't have the dark-matter requirements of dwarf galaxies?

    @eraticus Well, whether total mass is an absolute distinction which can be made to determine an object's classification is not something I couldn't say is universally true. Stars are a different case - they are discrete packages of gas which at one point either collapsed due to an overdensity in a region or did not because it simply didn't have enough mass (I know there are other effects here that determine star fomation; from a purely Newtonian perspective, I think my statement is not unreasonable). When it comes to the formation of galaxies and galaxy-type objects, you have to remember where

    ... it all came from. Since DM doesn't interact electromagnetically, it was able to cool off and form overdensities for which baryons would later fall into - basically, the baryons do trace the dark matter. For a sufficiently massive dark matter overdensity, it is able to collect enough gas for there to be a galaxy within; for smaller overdensities, there may not be enough gas within to form a galaxy, and to that end, there may not be enough to even form any stars. This is the idea behind dark companion galaxies, and has been proposed as a solution to the 'missing halo problem'.

    @Jeremy In practice I don't think people go looking for dark matter in these star clusters/globular clusters. Lensing is not really something I've seen be used to determine total mass of these objects (I don't think the geometry works out for these objects), but one could use the velocity dispersion to infer total mass. But no, I do not think an assessment of dark matter is made in order for people to say if an object is truly a star cluster or a galaxy. You could be right about people using stellar population age and gas content to make this type of statement - I simply do not know.

    @called2voyage I'm not entirely sure what you mean by mechanism - if you mean gravity, then yes, these types of objects do attract dark matter from their host galaxies/clusters in the form a gravitational wake ( - this is the cause of some dynamical friction of, say, a star cluster orbiting around the center of a massive galaxy. The question remains though: Do these objects contain some known radial distribution of dark matter intrinsically, or is the dark matter present simply from the parent object? I think you'd be hard pressed...

    ... to find objects of this mass unassociated with galaxies or clusters. I don't know this for sure of course, but my intuition tells me that if it is able to be flung out of the galaxy entirely, chances are it has been disturbed in the process. It is hard for me to imagine a situation where a globular cluster could be flung out of a galaxy and remain the size and shape it was to begin with. If any have been found, please update this post!

    I just saw this in Scientific American. There is a satellite galaxy to the Milky Way that has only about 100 stars. Fossil Galaxy May Be One of First Ever Formed

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Content dated before 7/24/2021 11:53 AM