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transreality
2010-Jan-29, 01:10 AM
Globular Clusters form in the halo at the same time as the major galaxy. The Clusters appear not to central have black holes, but are conservative in appearance, not being tidally disrupted like dwarf galaxies. It is possible that these attributes are the case because like the arms of the major local galaxy, the clusters are embedded in the surrounding dark matter halo. It may even be possible that this embedding is responsible for keeping the clusters from collapsing to form a singularity at the core. If the clusters are embedded in the galactic halo, then can we use the clusters to map the extent of the halo?

It may even be possible that the morphology of the clusters reflects the local density of dark matter. This would be reflected in the variation to the clusters as they are caught up in galactic collisions, since the halo would be subject to local variation the clusters might be prone to collapsing, as they are deprived of dark matter, they might form bright central nodes.

Obviously this technqiue would not extend far beyond the galactic haloes, since formation of the clusters seems related to the formation of the galaxy.
It would seem that at least for the milky way, many of the globular clusters have been characterised and mapped, where would it be possible to find this data, or if this type of study has been conducted?

Hornblower
2010-Jan-29, 02:54 AM
Globular Clusters form in the halo at the same time as the major galaxy. The Clusters appear not to central have black holes, but are conservative in appearance, not being tidally disrupted like dwarf galaxies. It is possible that these attributes are the case because like the arms of the major local galaxy, the clusters are embedded in the surrounding dark matter halo. It may even be possible that this embedding is responsible for keeping the clusters from collapsing to form a singularity at the core. If the clusters are embedded in the galactic halo, then can we use the clusters to map the extent of the halo?

It may even be possible that the morphology of the clusters reflects the local density of dark matter. This would be reflected in the variation to the clusters as they are caught up in galactic collisions, since the halo would be subject to local variation the clusters might be prone to collapsing, as they are deprived of dark matter, they might form bright central nodes.

Obviously this technqiue would not extend far beyond the galactic haloes, since formation of the clusters seems related to the formation of the galaxy.
It would seem that at least for the milky way, many of the globular clusters have been characterised and mapped, where would it be possible to find this data, or if this type of study has been conducted?If a globular cluster had a tendency to collapse, I would not expect the presence of dark matter to to inhibit that tendency. If anything the additional mass within the cluster might increase such a tendency.

transreality
2010-Jan-29, 08:17 AM
If a globular cluster had a tendency to collapse, I would not expect the presence of dark matter to to inhibit that tendency. If anything the additional mass within the cluster might increase such a tendency.

Wouldn't most of the relevant dark matter be surrounding, and external, to the cluster.

thecolorofash
2010-Jan-29, 09:03 AM
I think this is an interesting question, since I have always wondered why clusters do not collapse.

Hornblower
2010-Jan-29, 11:51 AM
Why should they collapse?

noncryptic
2010-Jan-29, 12:35 PM
Globular Clusters form in the halo at the same time as the major galaxy. The Clusters appear not to central have black holes,

Black Holes in Globular Clusters
September 17, 2002
STScI-PR02-18
http://www.stsci.edu/~marel/m15release.html
"Medium-size black holes actually do exist, according to the latest findings from NASA's Hubble Space Telescope, but scientists had to look in some unexpected places to find them.
The previously undiscovered black holes provide an important link that sheds light on the way black holes grow. Even more odd, these new black holes were found in the cores of glittering, "beehive" swarms of stars -- called globular star clusters -- that orbit our Milky Way and other galaxies."...

transreality
2010-Jan-29, 12:49 PM
wikipedia: globular clusters (http://en.wikipedia.org/wiki/Globular_cluster)

"Claims of intermediate mass black holes have been met with some skepticism. The densest objects in globular clusters are expected to migrate to the cluster center due to mass segregation. These will be white dwarfs and neutron stars in an old stellar population like a globular cluster. As pointed out in two papers by Holger Baumgardt and collaborators, the mass-to-light ratio should rise sharply towards the center of the cluster, even without a black hole, in both M15[27] and Mayall II."

Cougar
2010-Jan-29, 04:16 PM
If a globular cluster had a tendency to collapse, I would not expect the presence of dark matter to to inhibit that tendency. If anything the additional mass within the cluster might increase such a tendency.Wouldn't most of the relevant dark matter be surrounding, and external, to the cluster.
Well, yeah, but as Newton showed in his Principia back in 1687, the gravitational effect of such uniform mass external to the cluster essentially 'cancels out,' leaving no gravitational effect on the cluster.

With uniform mass (dark matter) all around the cluster, what gets pulled east also gets pulled west (and up-down, etc.) If a body is out near the east edge of the cluster, it is closer to the east-side mass of the external dark matter, hence stronger gravity in that direction; HOWEVER, now there is MORE MASS back in the west direction. Newton showed the result of these two considerations equals zero. Smart guy.

Newton's assumptions were a bit different than the above scenario, but the conclusion is at least approximately correct for this case, I expect.

chornedsnorkack
2010-Jan-29, 04:44 PM
How can any system with 3 or more bodies be stable especially if the ratio of most massive to least massive body exceeds 2?

In a globular cluster, assuming no interaction with galaxy and no dark matter, some stars would, because of random collisions/close passes, be ejected. Each time it happens the cluster forever loses its mass and at least some energy.

Some stars are also sent to bound but eccentric orbits, where they shall spend most time in the outskirts of the cluster but enter the interior each orbit, and be liable to further collisions.

And some stars happen to undergo collisions in the outskirts of the cluster, that result in bound orbits which do not enter the interior of the cluster and are stable till another (rare) collision.

As some stars are driven to the outskirts of the cluster, the core shrinks - and collisions in the core will accordingly take place at an increasing rate and expel the stars at increasing speed. And this will go on.

StupendousMan
2010-Jan-29, 04:54 PM
How can any system with 3 or more bodies be stable especially if the ratio of most massive to least massive body exceeds 2?


You've described some processes which can cause the structure of a globular cluster to change. Fine. But as long as the timescale for the changes due to these processes is longer than a Hubble time, the globular clusters we observe will appear largely pristine.

As many do.

If you look in a good book on the subject, you'll find an estimate of the timescales required for these processes to affect a typical globular cluster. You could even work some of the out yourself to an order of magnitude. They are all pretty long, and most are longer than a Hubble time.

korjik
2010-Jan-29, 07:28 PM
If the halo is not isotropic, then perterbations in the orbit of a globular cluster should map the distribution of the halo. Getting enough readings to see a perterbation would take a while tho.

:)

chornedsnorkack
2010-Jan-30, 09:08 AM
You've described some processes which can cause the structure of a globular cluster to change. Fine. But as long as the timescale for the changes due to these processes is longer than a Hubble time, the globular clusters we observe will appear largely pristine.

As many do.

But not all.

Roughly 20% of clusters are said to have collapsed cores.


If you look in a good book on the subject, you'll find an estimate of the timescales required for these processes to affect a typical globular cluster. You could even work some of the out yourself to an order of magnitude. They are all pretty long, and most are longer than a Hubble time.

Well, I found a rule of thumb description.

Relaxation time is roughly crossing time * 0,1 * N/ln N.

And evaporation time should be roughly 140 times relaxation time.

Well, for a typical globular cluster, with N=100 000, half-mass radius of 10 pc and speed 10 km/s, crossing time is in the region of 1 000 000 years, relaxation time in the region of 1 000 000 000 years, and evaporation time over 100 000 000 000 years.

But stellar evolution is said to speed up evaporation.

Now, how is this affected by the masses?

Add stars here, make the number 400 000 and still within 10 pc radius. Then the speed is doubled, and crossing time halved.

But relaxation time? The change in ln N is small, so while N is quadrupled and crossing time halved, relaxation time is nearly doubled (not quite doubled because of the ln N factor).

Am I on the right track here?

transreality
2010-Jan-30, 01:39 PM
If the halo is not isotropic, then perterbations in the orbit of a globular cluster should map the distribution of the halo.


How do the clusters orbit relative to the halo? If the clusters move with the halo then their distribution could be directly related to their probability of formation, and possibly local matter density. If the clusters move through the halo, then possible the total effect of the halo would be to concentrate clusters relative to the density of the halo. In any case, wouldn't the clumpedness of clusters provide a proxy for this map.

StupendousMan
2010-Jan-30, 02:05 PM
Am I on the right track here?

Yes. As you can see, the timescale for significant evolution of the structure of globular clusters is really long, longer in most cases than the current age of the universe.

chornedsnorkack
2010-Jan-30, 02:32 PM
Yes. As you can see, the timescale for significant evolution of the structure of globular clusters is really long, longer in most cases than the current age of the universe.

Not really. It is sensitive to the size and structure of cluster. E. g. decreasing the radius of the cluster at constant total mass and object count will decrease the crossing time with 3/2 power of radius, and the relaxation and evaporation times decrease accordingly.

And if a part of the cluster gets concentrated as a core, that means decrease of relaxation time both because the crossing time decreases and because the object count decreases.

The end result is that 20 % of clusters have had core collapse in Hubble time.

StupendousMan
2010-Jan-31, 02:58 AM
Me: " ... longer than a Hubble time in most cases ..."

You: "Not really ... The end result is that 20 % of clusters have had core collapse in Hubble time."

I guess we agree.

transreality
2010-Feb-24, 11:12 PM
I hope I might just revive this thread for a few questions that have arisen in connection with the subject and the article in Universe Today about alien globular clusters in the galactic halo (http://www.universetoday.com/2010/02/23/alien-star-clusters-are-invading-the-milky-way/).

It states that 25% of clusters have come from external sources, mainly dwarf galaxies devoured by the Milky way. Is it then quite likely that the Dark Matter halo of the Milky way has grown by a similar proportion? If this process is happening in all galaxies could this be a process that would drive the development of spiral morphology in galaxies, eg; as the halo increases in mass the arms might get tighter wound? If so this would suggest that GC provenance could be a good proxy for the encounter history of the galaxy.. So that ellipticals could be expected to have a even higher proportion of alien clusters through a lot of encounters, or less through stripping of the outer halo?

antoniseb
2010-Feb-24, 11:29 PM
... Is it then quite likely that the Dark Matter halo of the Milky way has grown by a similar proportion? ...

Probably. It isn't known how consistent the reactive-matter/dark-matter ratio is across large and small galaxies, but if DM is made of WIMPs (such as neutralinos) than you can think of each WIMP as a test mass in an n-body problem, and many of them would get captured in a galaxy-galaxy collision, and some would escape.

transreality
2010-Feb-25, 02:29 AM
And if each globular cluster also acts as a test particle, then the spatial extent and density of the globular cluster population should indeed map the distribution of dark matter around a galaxy.

Practically, how far away is it feasable to take a census of globular clusters around galaxies, or determine their elemental constituents and evolutionary stage?

StupendousMan
2010-Feb-25, 01:29 PM
When you have a big question, such as


Practically, how far away is it feasable to take a census of globular clusters around galaxies, or determine their elemental constituents and evolutionary stage?

one good technique to finding the answer is to look for articles on the topic in the "Annual Reviews of Astronomy and Astrophysics". These articles are written to summarize several years worth of recent research, and are aimed at a slightly less technical level than the usual research paper.

You can go to ADS

http://adsabs.harvard.edu/abstract_service.html

then type the words describing your topic into the "Abstract Words" box. Then scroll down the form to the "Filters" section, and type into the "Select/deselect publications" box the word

ARA&A

(the abbreviation for "Annual Reviews of Astronomy and Astrophysics"). Then click on the "Send query" button.

You'll receive a list of article titles. You can click on titles to see the abstracts. In some (most?) cases, you can read the entire articles.

transreality
2010-Feb-26, 03:01 AM
When you have a big question, such as



one good technique to finding the answer is to look for articles on the topic in the "Annual Reviews of Astronomy and Astrophysics". These articles are written to summarize several years worth of recent research, and are aimed at a slightly less technical level than the usual research paper.



Thanks for that, i've been reading a few. For something that is described as a building block of our knowledge of the Universe, not so much has been done with GC. However, there are some surveys of the populations of a few galaxies, namely Fornax dwarf, M87 and some Virgo elliptics, the MW and Andromeda, so probably has a reasonable representation of galactic types.

Basically there are two populations of GC in the MW, old ones that form with the bulge and less-old ones that are acquired from component dwarfs. The latter dominate in the halo. Elliptics have large proportion of old ones. They are like a spirals bulge. So does an elliptic no longer have a halo from which it can recruit GC? I would have thought a large elliptic would form inside a large halo, so has the halo been stripped away by interaction from large elliptic galaxies?

StupendousMan
2010-Feb-26, 03:08 AM
So does an elliptic no longer have a halo from which it can recruit GC? I would have thought a large elliptic would form inside a large halo, so has the halo been stripped away by interaction from large elliptic galaxies?

The current thinking is the opposite: big ellipticals have very extended, very massive dark-matter halos.

transreality
2010-Feb-28, 09:57 PM
At the same time dwarf galaxies are rare in the neighbourhood of giant elliptical galaxies (ref (http://adsabs.harvard.edu/full/1977PASP...89....5B)). They may have large halos but that halo don't have a population of subsequently forming companion galaxies, at least implying gas is not concentrating in this halo. So the elliptical halo has very low concentration of baryonic matter, and has low angular momentum. Spirals are characterised by angular momentum, absent in ellipticals. So the rotation of the dark matter halo could be a mechanism for increasing the rate at which the baryonic matter concentrates, possibly allowing spirals to form agn at smaller halo masses that ellipticals. Are the population one stars and companion dwarf galaxies the result of 'scavenging' material from the halo due to the angular momentum of the system, alone?