The Astrophysical Journal, 631:301–311, 2005 September 20

PHOTOMETRIC CONFIRMATION OF MACHO LARGE MAGELLANIC CLOUD MICROLENSING EVENTS

It’s also not clear from a theoretical perspective whether you’d expect there to be much dark matter left in the nucleus of a dwarf galaxy that has been so stripped. The dark matter distributions in galaxies are much more extended than the stars, so if you strip off 90% of the stars (which you’d need to have done to be left with just omega Cen), you have to strip off 99% of the dark matter. Whether that remaining 1% would be enough to cause a measurable difference to the orbits of the stars depends on the details of how the dark matter is distributed, which is still a matter of debate.

[TMB]

Re: dark matter in GCs : Stars in globular clusters don’t show any evidence of a mass discrepancy, i.e. no dark matter associated with the GCs. That’s the basic difference between the most massive GCs and the smallest dwarf galaxies – they actually both have similar numbers of stars, but the galaxies have dark matter too.

Thanks for that answer, much appreciated.

PS. I also second (#19.) andy’s question on the level of dark matter associated with Omega Centauri.

http://en.wikipedia.org/wiki/MACHOs

Which notes that :

Several groups have searched for MACHOs by searching for the microlensing amplification of light. These groups have ruled out dark matter being explained by MACHOs with mass in the range 0.00000001 solar masses to 100 solar masses. One group, the MACHO collaboration, claims to have found enough microlensing to predict the existence of many MACHOs with mass of about 0.5 solar masses, enough to make up perhaps 20% of the dark matter in the galaxy. This suggests that MACHOs could be white dwarfs or red dwarfs which have similar masses. However, red and white dwarfs are not completely dark; they do emit some light, and so can be searched for with the Hubble Telescope and with proper motion surveys. These searches have ruled out the possibility that these objects make up a significant fraction of dark matter in our galaxy.

It also looks from there as though cosmologists now don’t think that enough Massive Compact Halo Objects would’ve been created in the first place to explain more than a fraction of Dark Matter.

No wonder hard-working Americans turn their back on the “theory” of global warming.

And Phil: nice Arthur C. Clarke reference in the title.

Stars in globular clusters don’t show any evidence of a mass discrepancy, i.e. no dark matter associated with the GCs. That’s the basic difference between the most massive GCs and the smallest dwarf galaxies – they actually both have similar numbers of stars, but the galaxies have dark matter too.

But it’s not that there’s dark matter everywhere and somehow a big hole in the dark matter where the GC is! There’s no dark matter *associated* with the cluster, i.e. it concentrated where the stars are concentrated. There’s still a smooth background distribution of dark matter associated with the galaxy in which the cluster resides.

The key is that because there’s no concentration of dark matter in the cluster, the density inside and outside is the same, so there’s no net inward gravitational force on the stars due to the dark matter. It’s that net force that would make them move faster.

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Great image. Fascinating object. Last time I was up at Stockport Observatory with the local astronomical society I viewed Omega Centauri, 47 Tucane, The Great Hercules Globular (M13) and M15 near the nose of Pegasus among many others.

@6. Jack Mitcham Says:

… I thought a small number of MACHOs had been detected, but I’m having trouble finding papers that confirm it. It appears that several microlensing events have been detected in the LMC, but there is some debate as to whether the cause was a halo object, an object in the galactic disk, an object in the LMC, or some other phenomenon. Still, I think saying “no MACHOs have been found” is a little strong, I might have said “no MACHOs have been confirmed.”

I’ll second that. It’s what I thought too.

@13. andy Says:

Talking of the LMC, what’s the current view on whether it is actually a satellite galaxy? A while back there was some news that measurements had indicated that the Magellanic Clouds were in fact on their first approach to the Milky Way.

I also remember reading about that. If I recall correctly – a later study showed that both the Magellanic Clouds are actually gravitationally bound to the Milky Way after all. But I might be mistaken here &, alas, I can’t recall a source to cite for you on this. Sorry, I guess that doesn’t help all that much does it?

@11. Doug Watts :

if dark matter pervades the universe, shouldn’t it also affect the apparent rate at which stars rotate around the gravitational center of a globular cluster? And if not, why are globular clusters somehow devoid of dark matter? Or is there just not ‘enough’ of it in globular clusters for it to exert a measurable effect?

I’m not sure of that either, sorry.

I do know however that some globulars have been severly disrupted by passing through the plane of the Milky Way and had stars torn off them. For instance, Kapteyn’s Star a nearby red dwarf with the second highest proper motion (apparent speed across the sky) is thought to have been part of Omega Centauri. Kapteyn’s Star is now 12 ly away and Omega Cen is 15,800 so its travelled a *very* long way from home!

Omega Cen is actually believed to be a old dwarf galaxy since incorporated nto our own Milky Way so its not typical but other globulars it seems have also lost stars and been tidally disrupted. Other globular clusters have undergone core collapse where the stars have collapsed in towards the centre.

Not sure if these observations mean that the globulars don’t have *any* dark matter in them but it could possibly indicate that – perhaps?

“Click it”? That’s it? Just “click it”? Not “englobulate” or “hyperinstellarize” or…I dunno…something!

You’ve let me down, Phil. [<-- insert disapproving and dour expression here]

Here’s a question you smart folks can answer. We know that dark matter has been presumed, and nay, proven, to exist in and around galaxies because they rotate far too fast for their visible mass, and the stars on the outer visible edges rotate far too quickly if all that was on the outer edges was just the visible stuff. Some guy named Phil Plait mentioned this in a book. Do globular clusters also exhibit the same “too fast” rotation rates from which we infer dark matter from galaxies; and the same disconnect between rotation rates of stars near the center of the cluster and stars on the outskirts that we have seen in galaxies?

I thought a small number of MACHOs had been detected, but I’m having trouble finding papers that confirm it. It appears that several microlensing events have been detected in the LMC, but there is some debate as to whether the cause was a halo object, an object in the galactic disk, an object in the LMC, or some other phenomenon.

Still, I think saying “no MACHOs have been found” is a little strong, I might have said “no MACHOs have been confirmed.”