Starship Asterisk*

Actually this post refers to all globular clusters orbiting the Milky Way galaxy. My question:
If the Milky Way galaxy is embedded is a much larger cloud of Dark Matter, should not the orbits of all globular clusters be affected? That is, shouldn't the orbits be different if there was no Dark Matter?

Hi AZJames

The orbits of the globular clusters are affected by the Dark Matter. We routinely use the orbital motion of GCs in other galaxies (mostly ellipticals) to infer the properties of DM. You can infer from the amount of light in the galaxies how much mass their is, and when you look how fast the clusters are moving you find that their must be some mass you can't see.


These two papers go into how this is done, in a fair amount of detail.

http://arxiv.org/pdf/0704.1189
http://arxiv.org/pdf/astro-ph/0608661

An awesome photo to say the least. The globular cluster imagery bar has just been raised.

On another subject, related to globulars, has anyone postulated or theorized why these clusters are more or less without macro structure?

Many galaxies have spiral/rotating structure (yes, I know there are some galaxies that look like giant globular clusters), yet the globs near us all seem to be these relatively uniform balls of stars.

Is the lack of overall rotational structure in globulars a result of how they formed? Their age? The fact that they're bound to a larger structure (e.g., the Milky Way)?

I've never been able to shake the feeling that globular clusters are somehow not made from the same "stuff" (e.g., from the big bang, if you believe in that theory) that makes up the rest of the universe.

-Noel

NoelC wrote:On another subject, related to globulars, has anyone postulated or theorized why these clusters are more or less without macro structure?

I imagine it has partly to do with scale. Kind of like how you get 20m swells in an ocean, but not in a lake.

Another item, though I'm not quite sure of its significance, is that both elliptical galaxies and globular clusters are old populations, and lacking in meaningful amounts of gas and dust.

I have to wonder if it's the presence of large amounts of interstellar gas that leads to the structure of spiral galaxies. Stars rarely ever collide with each other, but they will plow through clouds of gas, which would introduce a sort of drag or friction to the system. Just like how a cloud of particles orbiting a planet (or new-formed star) will rapidly form an equatorial disk, couldn't the same be said for galaxies?

The reason why you don't see any discernable structure in GCs is essentially one of scale, they are simply not massive enough to be bound if they didn't form simple spheroids. This is partly because they lack any DM as far we can tell.

There are GCs which are slightly non spherical because they are rotating, however if they were to rotate much faster they would be disrupted, as there isnt enough mass in them to hold them together. Interactions with the variable gravitational field of the MW especially when passing through or near to the disc tend to also pull off any outlying stars making the clusters more spherical.

10 millions stars in a sphere with a 150 light-years diameter is a lot of stars. Is there a way to calculate the average distance between stars ?

1.4 ly

Thanks astro_uk.
Very enlightening. As I understand it, all galaxys appear to have super-massive black holes at their cores, whereas globular clusters do not. If the precursor to galaxy formation was the presence of a SMBH, what if anything was the precursor to the formation of a globular cluster? Maybe they form around a galaxy like dust bunnies under my bed.

Hi nikki,
I goofed in my original calculation for the average distance between stars in the globular cluster, NGC 5139:

With a diameter of 150 ly, the GC has a volume of 1.77 million cubic ly (a sphere) containing 10 million stars. Assuming each star occupies the center of a spherical volume of space, then that volume is 0.18 cubic ly with a diameter of 0.70 ly, which is equal to the average interstellar distance. I wonder what is the distance between stars at the core of the GC. I imagine it might look like Times Square on New Years Eve!

I took GC as cube. It is just estimate.

As I understand it, all galaxys appear to have super-massive black holes at their cores, whereas globular clusters do not.

Yes this is true, there is a very well behaved relationship between the mass of a galaxies bulge and the mass of the supermassive black hole found there. (A version of the so called Magorrian relation) If this relation is extrapolated to the mass of GCs then some of the larger GCs would be expected to have BHs of mass around 1000Msun in them, though spotting them is exceedingly difficult. There are two clusters for which reasonable evidence exists for these so called intermediate mass black holes and the GCs and BHs appear to lie on an extrapolation of the Magorrian relation.

Its generally thought that the BHs (if they exist) formed from remnants of stars from the GCs, essentially run away accretion of massive stars in the very early life of the cluster. So the BHs are a by product of the GC formation and not any sort of cause for the formation of the GC.

Qev wrote:I have to wonder if it's the presence of large amounts of interstellar gas that leads to the structure of spiral galaxies.

Sounds good to me!

If you can figure out how spiral galaxys form I imagine that astronomers would be interested. Right now I'm not sure that anyone has a clue.

Elliptical galaxys apparently form through collision/merging of two or more pre-existing galaxys.

Perhaps there is correlation between brithtness profile radial function (magnitude per arcsec^2)- measure for central BH mass and absolute magnitude of GC?