Why dark matter distribution is so different in galaxies?

In summary, the distribution of dark matter in galaxies varies because of gravitational instabilities, which create density variations in both dark matter and baryonic matter. This can cause the ratio of baryonic matter to dark matter to differ from the expected ratio of 1/5. The reason for this could be due to the baryonic matter flowing into different dark matter potential wells, or the mobility of baryonic matter relative to dark matter. In theory, cold dark matter (WIMPs) can interact weakly with baryonic matter, causing them to move more slowly and potentially leading to ratio differences. Hot dark matter, on the other hand, does not interact with baryonic matter and therefore likely does not play a significant role
  • #1
RyanH42
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I learned that dark matter distrubition is homogeneius and isotrophic in cosmic scales.

I searched some galaxies dark matter distrubition.And I am actually suprised.
Why cause every galaxy has a different ratio of dark matter baryonic matter distrubition.
In cosmic scales baryonic matter/dark matter=1/5 but in galaxies its varies.
Whats the reason for that ? Why every galaxy didnt contein five times bigger dark matter.

My second question is we know that from Planck results baryonic matter density is 0.0456 and dark matter density 0.26.
It means we have a galaxy and the mass of the galaxy is will be 6m.Cause m from baryonic mass 5m from dark matter=6m.

I know that both questions are relavent each other.

Thank you
 
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  • #2
Why would you expect the dark matter distribution to be the same? That's not the case for luminous matter.
 
  • #3
Vanadium 50 said:
Why would you expect the dark matter distribution to be the same? That's not the case for luminous matter.
I want to give an example.A galaxy baryonic matter /dark matter=1/3.B galaxy baryonic matter /dark matter=1/5. its different galaxy to galaxy.
Why their ratio is not same.I don't know why.

Your answer is why not.But Is there any other explanation ?
Dark matter Homogenenity is not what I mean.I guess I used wrong word to describe myself

I wanted to mean Why ratio is different every galaxy
 
  • #4
If you understand my question right,I expected to be same.Thats more make sense and reasonable for the other one
 
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  • #5
Any ideas ?
 
  • #6
I wonder if the main cause is just randomness, or if there is a systematic trend. A random aspect might be, you have an excess gravity from a contracting dark matter region, and it pulls in baryonic matter. But the details of how the baryonic matter gets pulled in could have details that lead to random variations. I don't know exactly what would matter, but gravitational instability could be a messy process with many things that could vary. Or, we might have systematic trends, like a larger dark matter mass might produce a gravity that pulls in an unusually large amount of baryonic matter, such that the ratio of baryonic matter to dark matter increases for galaxies with large dark-matter mass. I don't know if there are correlations like that, or if it's all just random, but those are the kinds of things that would come to mind to explain it. But you do have a reasonable question there-- since gravity affects all the "cold" matter similarly, we might expect the baryonic matter to do more or less exactly the same things that the dark matter does, so we need to explain why that does not happen.
 
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  • #7
I know that we can assume Earth is in the center of universe.Every observer in the universe will going to think like that but know let's suppose we are in the center of universe.

Is there any source that.I can find the galaxy position for us(assuming we are in the center of universe).And the dark matter/baryonic matter ratio.

Its a mystery now I guess.
 
  • #9
We are the center of the OBSERVABLE universe, not of "the universe"
 
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  • #10
When I say universe I mean observable universe.I know the difference between them.Thanks for recall
 
  • #11
Some galaxies are known to be relatively devoid of dark matter, and they look different than DM rich galaxies. We already know the assumption of homogeneity does not apply to things as small as a galaxy, or even galactic clusters - so I agree with Vanadium's comment.
 
  • #12
I actually agree that there is a question here that needs answering. In the early universe, we expect some ratio between dark matter and baryonic matter that is extremely uniform, just like the CMB is. I don't know of any theory what would expect anything different there. So then we have gravitational instabilities, which create density variations in both dark matter and baryonic matter. But if both are "cold", it's hard to see why their ratio could be so different. Certainly the baryonic matter can condense faster, by virtue of its ability to emit light. But that comes late in the process, after the gravitational wells have already been established, and presumably, so has the ratio of dark matter to baryonic matter in the general vicinity. So what allows the dark matter and baryonic matter to part company enough to show such variations? Is it just that the baryonic matter flows into different dark matter potential wells, like how water might find different lakes after a flash flood? That would mean that the baryonic matter needs to have considerable mobility relative to the dark matter in which it was originally embedded.
 
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  • #13
In theory Cold Dark Matter Made by WIMP.So they would be interect weakly.And they will move more slowly due to baryonic matter and that will cause this ratio differences.
This is the answer I guess.

But this situation is only true for cold dark matter I guess.Hot dark matter never interects with baryonic matter.

And Is Hot dark matter can make extra mass ?

Cause maybe baryonic matter/mixed dark matter ratio is same for all galaxies maybe.

Hot dark matter moves very fast so maybe their energy makes mass effect.
 
  • #14
Current models don't give a significant role to hot dark matter. How firmly that has been established I could not say, and that it is hard to establish is perhaps why we've seen a Nobel prize for dark energy but not dark matter.
 
  • #15
I searched and it seems cold dark matter is also does not explain everything.

In my opinion baryonic matter/mixed dark matter ratio can be same for all galaxies.Why ? Cause maybe dark matter moves slowly due to baryonic matter (Which I will call later BM).But the hot dark matter moves faster due to BM(Cause its made of neutrinos and they are high velocity particles).So mixed of them maybe makes the BM/MDM(Mixed Dark Matter) ratio same for all galaxies.

Dark matter is a real mystery I guess.I don't know your degree in physics.Is there any person which I can ask these question If you don't know the answer ?
 
  • #16
The modern consensus is dark matter cannot be 'hot' [relatavistically speaking] due to the large scale structure of the universe. It is less clear how 'warm' it may be without disrupting large scale structure. This is 'hotly' debated in the cosmology community and is about as much as we know about it.
 
  • #17
  • #18
Hot and warm is the term to distinguish particles with highly relativistic velocities from those with lesser velocities.
 
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  • #19
I am really confused.Is there any good source that I can understand the subject better.

The answer of my question seems like given.But I am not sure that I understand the answer.

I understand cold matter and why the ratio is different from galaxy to galaxy.But I didnt understand the other types of dark matter

I made some reaserch and mixed dark matter is good approximation of observation

http://www.nature.com/nature/journal/v359/n6394/abs/359393a0.html

Or this article says MDM is not good I guess


http://m.mnras.oxfordjournals.org/content/268/1/L23.short

And Is this a good source to understand DM ? Which I asked above

https://www.google.com.tr/url?sa=t&...MpKGxXEaAreMi-xUg&sig2=NOCTZNxh6Y8t0yiYEMzfdQ

My english is middle level.So excuse me please.
 
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  • #21
If I imagine a galaxy as a small object bobbing around a cloud of dark matter, with gravity pulling it all together, it would collide with nearby clouds and galaxies may or may not and pull random amounts of dark matter and matter with it depending on how they collided? Wouldn't the stirring of the early universe create randomly sized clumps of dark matter and matter in random ratios like shaking a lava lamp, even if the initial distribution was nearly evenly distributed? It wouldn't really matter how hot dark matter is, only that it behave differently than matter in collisions, which we know it does.
 
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  • #22
Thank you
 
  • #23
I was actually asking everyone else, it was just a thought I had that seemed to make sense.
 
  • #24
Maybe someone replies.I don't know
 
  • #25
I found very interesting thing .Look this
http://www.astrowatch.net/2015/07/dead-galaxies-in-coma-cluster-may-be.html?m=1
 
  • #26
newjerseyrunner said:
If I imagine a galaxy as a small object bobbing around a cloud of dark matter, with gravity pulling it all together, it would collide with nearby clouds and galaxies may or may not and pull random amounts of dark matter and matter with it depending on how they collided? Wouldn't the stirring of the early universe create randomly sized clumps of dark matter and matter in random ratios like shaking a lava lamp, even if the initial distribution was nearly evenly distributed? It wouldn't really matter how hot dark matter is, only that it behave differently than matter in collisions, which we know it does.
your explanation was useful and it does make sense
 
  • #27
Gravity is a 2 way street. Once an overdence region of dark matter draws in ordinary matter, the gravity of ordinary matter will inhibit dispersion of the surrounding dark matter. This retentive effect will be more pronounced for cold DM, so it is conceivable DM deficient galaxies may have formed in a region that initially contained a larger fraction of warm and hot DM that has since dispersed.
 
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  • #28
Ken G said:
I wonder if the main cause is just randomness, or if there is a systematic trend.

There is a systematic trend.

The main kinds of galaxy scale structures can be strictly arranged in order of galaxy mass with almost no overlap between the catagories - dwarf galaxies always have less mass than spiral galaxies which always have less mass than elliptical galaxies which always have less mass than galactic clusters.

For spiral galaxies, this is the Tully-Fisher relation, https://en.wikipedia.org/wiki/Tully–Fisher_relation and for ellipical galaxies it is the Faber-Jackson relation, https://en.wikipedia.org/wiki/Faber–Jackson_relation The formal relations relate rotational velocity to luminosity. But, luminosity is an excellent proxy for aggregate baryonic matter (because the vast majority of mass in a galaxy is from stars and the amount of non-star matter is roughly proportional to star matter) and the amount of inferred dark matter in a halo can be inferred from a galaxy's rotational speed. There is also systemic deviation from the Faber-Jackson relation based upon the extent to which an ellipical galaxy is spherical or not. Less spherical elliptical galaxies have more dark matter than more spherical ones relative to their baryonic matter. There are also characteristic proportions of dark matter relative to baryonic matter in galactic clusters and in dwarf galaxies (where the proportion is particularly high).

Romanowsky notes more systemic relationship. http://arxiv.org/pdf/1207.4189.pdf as does Disney http://www.nature.com/nature/journal/v455/n7216/abs/nature07366.html

There are two ways of explaining dark matter phenomena.

One is to modify the law of gravity in a way that corresponds to the observed systemic relationships more or less directly. In this case, the modification to the laws of gravity produces a precise amount of dark matter equivalent impact on gravitational dynamics for any given ordinary matter distribution. Roughly speaking, dark matter effects are observed below a particular threshold of weakness in weak gravitational fields.

The other is to hypothesize that there is dark matter and to devise a way that a universe with the right amount of dark matter naturally tends to produce a spectrum of galaxies in which the size and the shape of the galaxy is related to the amount of dark matter in that galaxy because of the role that dark matter and gravitational interactions between dark matter and ordinary matter (which go both ways) have on the evolution of galaxies of a particular size.

One heuristic way to describe this process is that a certain amount of baryonic matter in a particular configuration can anchor a certain amount of dark matter, and the baryonic matter tends to push the dark matter towards a non-spherical distribution, while the dark matter tends to shape galaxies of a particular size into a particular form. But, there is not a full consensus on the dynamical process that causes dark matter and ordinary matter to co-evolve their structure in a way that shows such pronounced systemic trends. There are serious criticisms of the way that this in models in the computer models used to compare how the universe would look with a particular kind of dark matter in it to the observed universe.
 
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  • #29
I think the problem is that current cosmological models rely on the universe to be homogeneous. The problem with that is that the more we look..the more it isn't homogeneous. Hence supposedly impossible "structures" that span 1/9th to 1/3rd of the way across the observable universe. Maybe some theories about a homogeneous universe need to be rethought.
 

1. Why is dark matter distribution so important to understanding galaxies?

The distribution of dark matter in galaxies is crucial because it affects the overall structure and dynamics of the galaxy. It is the dominant source of gravity in galaxies and plays a significant role in the formation and evolution of galaxies.

2. How do we know that dark matter distribution is different in galaxies?

Scientists use various methods to study the distribution of dark matter in galaxies, such as gravitational lensing, galaxy rotation curves, and simulations. These techniques provide evidence that the distribution of dark matter is not uniform and varies from galaxy to galaxy.

3. What causes the differences in dark matter distribution among galaxies?

The exact cause of the variations in dark matter distribution among galaxies is still unknown. Some theories suggest that it could be due to the initial conditions of the universe, while others propose interactions between dark matter and other particles or forces.

4. Can we observe dark matter directly to understand its distribution in galaxies?

No, dark matter does not interact with light, making it impossible to observe directly. However, scientists can indirectly study its distribution through its gravitational effects on visible matter, such as stars and gas, in galaxies.

5. How does the distribution of dark matter affect galaxy formation and evolution?

The distribution of dark matter plays a crucial role in galaxy formation and evolution. It provides the gravitational pull needed to form and maintain the shape of galaxies, and it also affects the distribution and movement of visible matter in galaxies. Understanding the distribution of dark matter is essential to fully comprehend the evolution of galaxies.

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