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nota
2011-Nov-18, 05:59 AM
DM question how is DM every where no clumps or rotation or interactions

that is just what looks to me to be avg self canceling gravity of a far away shell of DM
equal in all directions so no change in the curve of space time ?
is an real effect and how it is
different from no DM

and or how DM regulates the speed of stars inside a galaxy
from far outside while being smoothly distributed

Shaula
2011-Nov-18, 06:31 AM
It is the DM in the galaxy that causes the flat rotation curve. There is hollow shell of it around us, we are embedded in a ball of DM.

DM does interact via gravity. Because that is the only way it interacts (apart from maybe via the weak force) it tends to behave kind of like a gas in thermal equilibrium. So you get this fairly smooth ball of it.

korjik
2011-Nov-18, 07:02 AM
DM question how is DM every where no clumps or rotation or interactions

that is just what looks to me to be avg self canceling gravity of a far away shell of DM
equal in all directions so no change in the curve of space time ?
is an real effect and how it is
different from no DM

and or how DM regulates the speed of stars inside a galaxy
from far outside while being smoothly distributed

It isnt from far outside. The DM of a galaxy is all through the galaxy, and is usually a much bigger sphere than the visible galaxy. DM is required to explain the way galaxies behave. Almost all of them behave like there is alot more mass than is visible in the galaxy. This makes galaxy-galaxy interactions very different in clusters, and it makes stellar orbits act different than can be explained by matter we can see.

Hornblower
2011-Nov-18, 12:54 PM
It is the DM in the galaxy that causes the flat rotation curve. There is hollow shell of it around us, we are embedded in a ball of DM.

DM does interact via gravity. Because that is the only way it interacts (apart from maybe via the weak force) it tends to behave kind of like a gas in thermal equilibrium. So you get this fairly smooth ball of it.

My bold. I think you are mistaken. To get the typical nearly flat curve over most of the galaxy, the dark matter would need to be more dense toward the core, just not as sharply so as the distribution of the observable stars.

antoniseb
2011-Nov-18, 02:12 PM
... what looks to me to be avg self canceling gravity of a far away shell of DM...

This is why I dislike the use of the term "Halo" to describe DMs placement in and around galaxies. It misleads people new to the topic. Other posters have set nota straight, so I won't redescribe the DM density profile.

Jeff Root
2011-Nov-18, 02:23 PM
What Hornblower said. Dark matter distributed throughout
a galaxy can explain flat rotation curves of the visible stars.
The dark matter would be "virialized" by gravitational interaction
such that it condenses weakly toward the center, in the same
way that the stars of a globular cluster or elliptical galaxy do.

-- Jeff, in Minneapolis

nota
2011-Nov-18, 07:44 PM
DM in the center ? other then the big BH's they all look to have ?

look I get a mass curves space/time around it
but not how diffuse DM in all directions curves anything
logic to me looks like canceled avg flat no curve gravity in that case ?

nota
2011-Nov-18, 08:07 PM
or ok every galaxy has a big center BH
what % more DM is needed near the center [and BH]
to get the observed rotation rates

and how does the DM existing out side the stars effect rotational speeds
vs DM in the stars zone [that AFAIK we can't yet detect]
vs the center's BH+DM [now that effect does look to be correct]
as gravity from the center out I can see bending space and influencing motion

but why do they need over sized halo's at all
my instincts say no gravity from a outside sphere can curve local space to a measurable effect
like being weight less at earth's center point [never mind the T&P just gravity]
esp if the DM is also local and central in near equal density


so in quick summery
big clump of DM+big BH in the center orbits are effected
hollow sphere of DM outside the stars minimum effect on anything
combo of DM everywhere with even density and no clumps also minimum effect on anything

but I keep reading DM does not clump !!!!!!!!!

korjik
2011-Nov-18, 09:16 PM
or ok every galaxy has a big center BH
what % more DM is needed near the center [and BH]
to get the observed rotation rates

and how does the DM existing out side the stars effect rotational speeds
vs DM in the stars zone [that AFAIK we can't yet detect]
vs the center's BH+DM [now that effect does look to be correct]
as gravity from the center out I can see bending space and influencing motion

but why do they need over sized halo's at all
my instincts say no gravity from a outside sphere can curve local space to a measurable effect
like being weight less at earth's center point [never mind the T&P just gravity]
esp if the DM is also local and central in near equal density


so in quick summery
big clump of DM+big BH in the center orbits are effected
hollow sphere of DM outside the stars minimum effect on anything
combo of DM everywhere with even density and no clumps also minimum effect on anything

but I keep reading DM does not clump !!!!!!!!!

?

Everyone here has told you that the DM in a galaxy is roughly in the shape of an evenly distributed solid sphere, not a hollow shell around the outside of the galaxy.

Strange
2011-Nov-18, 09:30 PM
but why do they need over sized halo's at all

Because the flat rotation curve continues way outside the stars in the galaxy: http://en.wikipedia.org/wiki/Dark_matter_halo


but I keep reading DM does not clump !!!!!!!!!

You read wrong. I believe it is clumpy within galaxies and on the large scale (http://en.wikipedia.org/wiki/File:COSMOS_3D_dark_matter_map.jpg).

Jeff Root
2011-Nov-19, 05:15 AM
Dark matter doesn't clump via electric or strong forces.
It *does* clump via gravity. But since it doesn't clump
via electric or nuclear forces, the gravitational "clumping"
is quite smooth, and is not very strong. It is more dense
toward the center of a galaxy and gradually becomes less
dense the farther you get from the center.

Also, the supermassive black holes at the center of most
galaxies are not all that massive compared to the total
mass of the entire galaxy. They have only a tiny fraction
of the galaxy's mass. Contrast that with the Solar System,
where the Sun contains more than 99% of the total mass.
Orbits of planets around the Sun are Keplerian -- orbits
which closely follow Keplers three laws. Orbits of stars
around the center of a galaxy do not follow Kepler's laws
closely because the attracting mass is much more spread
out around the galaxy and around the orbits. That is true
with or without dark matter, but dark matter makes the
orbits even less like Keplerian orbits.

-- Jeff, in Minneapolis

Shaula
2011-Nov-19, 06:46 AM
By DM doesn't clump what is meant is that it does not collapse and radiate energy like 'normal' matter will - so you will not get DM stars, galaxies etc. Instead you get a few blobs of it anchored on large mass distributions (which came first, DM or the galaxy is up for debate IIRC) with which it interacts gravitationally.

There is no shell around the galaxy. Halo just means the portion of the DM cloud that extends beyond the visible galaxy as I understand it.

Cougar
2011-Nov-19, 03:47 PM
...which came first, DM or the galaxy is up for debate IIRC...

I think the debate strongly favors DM coming first. Since it does not interact electromagnetically, it was able to start "gravitationally clumping" 400,000 years earlier than the baryonic matter, which was in thermal equilibrium. Thus, the DM clumps provided "seeds" for the subsequent baryonic structure formation, which happened too quickly if there were no such seeds.

nota
2011-Nov-20, 02:24 PM
Because the flat rotation curve continues way outside the stars in the galaxy: http://en.wikipedia.org/wiki/Dark_matter_halo



You read wrong. I believe it is clumpy within galaxies and on the large scale (http://en.wikipedia.org/wiki/File:COSMOS_3D_dark_matter_map.jpg).

from the quoted wiki link

''The dark matter halo is the single largest part of the Galaxy as it covers the space between 100,000 light-years to 300,000 light-years from the galactic center. It is also the most mysterious part of the Galaxy. It is now believed that about 95% of the Galaxy is composed of dark matter, a type of matter that does not seem to interact with the rest of the Galaxy's matter and energy in any way except through gravity. The dark matter halo is the location of nearly all of the Galaxy's dark matter, which is more than ten times as much mass as all of the visible stars, gas, and dust in the rest of the Galaxy. The luminous matter makes up approximately 90,000,000,000 (9 x 1010) solar masses. The dark matter halo is likely to include around 600,000,000,000 (6 x 1011) to 3,000,000,000,000 (3 x 1012) solar masses of dark matter.''

now wiki is claiming a 100k to 300k halo containing most of the DM
or the hollow shell around the visible matter that is mostly under 100k size

others here say no
there is more DM nearer the center of the milky way
and also DM mixed in the stars 100k space too

but that is not what I read from the wiki link above
they say shell with out other locations for most DM

so
back to the guy feeling weightless at earth's center point example
that effect should be gradual as he gets nearer the exact center
with little gravity in the last 1000 miles
and almost none in the last 100 miles
not just a zero G at the very exact center

now I do get a large center mass curving space and effecting orbits speed
but do not get a shell outside having the same effect or even much of any effect
as it looks to me like the mass of the earth outside the near center last 100 mile area
greatly reducing G not adding extra G

Strange
2011-Nov-20, 02:54 PM
I always assumed that was just really badly worded and what they meant was: from the center to between 100k and 300k light years. But maybe I am wrong...

Shaula
2011-Nov-20, 02:54 PM
The DM halo refers to the DM outside the galaxy. There is plenty of DM inside the galaxy which is what affects the rotation curves.

Essentially the DM has to be in a ball, the density of which drops off as we go out from the centre. The rotation curves we see imply that the density of DM does not drop off much at all across the radius of the visible galaxy (it is estimated to drop as 1/R). So we must be sat in the middle of something huge, so that we don't see that density drop off reflected in rotation curves. Hence the 'halo' around us. Wiki is badly worded there.

Cougar
2011-Nov-20, 04:31 PM
now wiki is claiming a 100k to 300k halo containing most of the DM
or the hollow shell around the visible matter that is mostly under 100k size

I appreciate your misunderstanding, since I was also misled in the past by the terminology that has unfortunately become standard.

The best solution would be to take the word "halo" and toss it into a black hole. It is misleading.

The dark matter is not described by a shell, and it is definitely not hollow. As Strange says, it extends from the center of the galaxy to well beyond the edge of the galaxy's visible disk. The disk is embedded in a much larger "glob" of dark matter, which, as Shaula says, is roughly spherical, and the density of the dark matter drops off the farther you get from the center.

The effect of dark matter's gravity on an orbiting star in the galaxy disk is determined by how much of the DM is interior to the star's orbit. Since there is DM permeating the galaxy, stars farther from the center will have more DM interior to their orbits. This additional mass means the star has to orbit more quickly if its orbit is to be at all stable. Stellar galactic orbits appear to be roughly stable, since the galaxy has been around for over 10 billion years, and the stars are still orbiting as part of the disk. These faster orbits are what prompted the conjecture of dark matter in the first place.

cjameshuff
2011-Nov-22, 05:22 AM
The best solution would be to take the word "halo" and toss it into a black hole. It is misleading.

It sounds like the perfect word for it to me, and I can't think of a better one. I'm still trying to figure out how it was taken to mean that all the dark matter occupied a hollow shell around the galaxy.

nota
2011-Nov-22, 06:15 AM
we donot yet have a handle on the true number of stars in the milky way
so this maybe a dangerous question
but what % of the total mass is in our center BH
what % does the center bulge have of total mass of normal matter
how much DM is near our BH vs total mass
what % of total mass is DM inside the bulge
and whats is in the arms gas ect outside the bulge normal and DM %
and finally what % of DM is outside the arms/visable area

Shaula
2011-Nov-22, 07:25 AM
It depends on the model you use. As far as I can tell there are two - an exponential one usually used in simulations and a more complex one that is thought to be a better fit. All the numbers you want would have to be calculated for both - all the basic facts, equations and constants are out there. It just requires someone to do a bit of integrations! I'd suggest you start by Googling Navarro–Frenk–White profile and the drop some numbers into that - the Wiki page has done some of the integration for that one for you.

nota
2011-Nov-23, 06:20 PM
ruff % numbers
matter inside our bulge vs DM inside

matter inside the arms/visible area vs DM there

and DM outside the whole visible area of our milky way ?


as I have read both claims of no detection of the effect of DM locally
on a earth orbit scale up to at least local stars orbits or interactions
has that changed recently ??

and most DM not centered inside visible areas with stars and gas
or concentrated centrally to any great degree
but outside visible areas mostly in a halo of DM

unlike the posters here who state it has to be central
I agree but do not understand
why the huge halo idea
and smooth wide distribution of DM
is still everywhere

antoniseb
2011-Nov-23, 06:33 PM
ruff % numbers
The values I'm giving here are very rough.
1.
matter inside our bulge vs DM inside
Inside the bulge/bar the overall ratio is about 6 grams DM per 1 gram normal matter, though the ratio very close to Sgr A* isn't well established at the moment.
2.
matter inside the arms/visable area vs DM there
Out by us DM is about 0.03 solar mass per cubic parsec, which is a little less that normal matter here locally, but when you look at the entire sphere at this radius from the center is about 10 grams of dark per 1 gram of normal.
3.
DM outside the whole visable area of our milky way ?
Just beyond the spiral arms the DM ratio should be higher than 10:1

Ivan Viehoff
2011-Nov-24, 01:09 PM
Dark matter distributed throughout
a galaxy can explain flat rotation curves of the visible stars.
The dark matter would be "virialized" by gravitational interaction
such that it condenses weakly toward the center, in the same
way that the stars of a globular cluster or elliptical galaxy do.
Can someone explain the term "virialised" please. I've researched but not got very far in understanding it.

I presume it relates to this, http://en.wikipedia.org/wiki/Virial_theorem.

I have also found this definition of it in wikipedia: "Gravitationally bound with velocities corresponding to predicted orbital velocities of general relativity".

But that explanation is not very revealing: it is not clear to me what it means in practice, nor how it relates to the virial theorem.

Jeff Root
2011-Nov-24, 01:28 PM
I havent studied the math of it *at all*. So I hope that I have
understood correctly what it means and have been using the
term correctly.

A globular cluster is a large number of stars in a relatively small
volume. But those stars are still *very* far apart. It is rare for
two stars to pass close to each other, and extremely rare for two
stars to collide. So their primary effect on one another is their
mutual gravitational attraction, which binds them together. They
are "gravitationally bound".

Such a cluster might form from a single, enormous cloud of gas.
The gas molecules are in random motion in the cloud. After the
gas collapses into stars, the stars are in random motion. But
each star follows an orbit-like path through the cluster in which
its velocity increases or decreases as it falls toward or rises away
from the cluster's center of mass.

When two stars do interact, net momentum transfer tends to be
from the higher-mass star to the lower-mass star. Higher-mass
stars tend to lose speed and end up in orbits closer to the center.
Lower-mass stars tend to gain speed and end up in orbits farther
from the center. That, as I understand it, is virialization.

There is still a very high degree of randomness in the orbits,
and most stars are still in strongly-elliptical orbits, but it is more
sorted out by mass, velocity, momentum, kinetic energy, and
gravitational potential energy than it was before virialization.

Dark matter is like stars in that it can virialize by gravitational
interaction, but the individual particles rarely if ever bump into
each other.

-- Jeff, in Minneapolis

Cougar
2011-Nov-25, 01:07 AM
what % of the total mass is in our center BH

Surprisingly, very little. Less than 1%.

antoniseb
2011-Nov-25, 01:18 PM
Surprisingly, very little. Less than 1%.

Lot's Less. The Milky Way (including DM) is about 400,000,000,000 times the mass of the Sun. Sgr A* is about 4,000,000 times the mass of the Sun. Our BH is a thousandth of 1% the mass of the galaxy. These SMBHs are typically about 0.1% the mass of the central bulge, which for the Milky Way is a small fraction of the galaxy as a whole.

nota
2011-Nov-30, 04:31 PM
so is my basic point correct ?
mass outside a body's [star] orbit should have less effect on that orbit
then the mass inside the body's orbit

and if there is DM and it is the reason for the speeds observed in star's orbits
it is the DM inside the orbit mostly or more then the DM in a halo outside the visible galaxy
that is regulating orbits speeds for stars ?

Grey
2011-Nov-30, 05:44 PM
so is my basic point correct ?
mass outside a body's [star] orbit should have less effect on that orbit
then the mass inside the body's orbit

and if there is DM and it is the reason for the speeds observed in star's orbits
it is the DM inside the orbit mostly or more then the DM in a halo outside the visible galaxy
that is regulating orbits speeds for stars ?Yes, that's essentially correct. And when astronomers talk about a dark matter halo, they don't mean a hollow sphere of matter outside the galaxy (which is what many people might imagine the term "halo" to mean; it's the way you're using it in this sentence). They mean a sphere of matter that pervades the visible parts of the galaxy, but also extends beyond it.

nota
2011-Dec-01, 12:38 AM
some how I think/guess the center BH is a key to this puzzle
some how if gravity can focus or distort light [lensing]
can gravity focus or distort it's self if it is also carried by a wave/particle ?

are there any studys to tie orbit speed variation to BH size
or in a BH less galaxy with only DM
or in a galaxy with greater or lessor DM content/density then ours ?

Cougar
2011-Dec-01, 02:51 AM
some how I think/guess the center BH is a key to this puzzle

Well, it's a key to some puzzles, but I don't think it's a key to the dark matter puzzle. Its mass is much too insignificant compared with the rest of the galaxy.


some how if gravity can focus or distort light [lensing]
can gravity focus or distort it's self if it is also carried by a wave/particle ?

How gravity is propagated, or by what mechanism it affects spacetime curvature, remains mysterious. On the other hand, two very massive objects in a tight orbit around each other apparently emit gravitational waves, which is something completely different from how gravity is propagated. Such gravitational waves are sent out as wavy distortions in spacetime itself. Though they haven't been directly detected yet (despite attempts), their existence is strongly supported by the observed decay in two very massive objects in a tight orbit around each other. Hulse and Taylor got the Nobel in physics in 1993 for just that - observing binary pulsar PSR B1913+16 and measuring the very slight decay in their orbits, which was found to closely match the amount calculated by Einstein's field equations as a result of the predicted generation of gravitational waves.

Even if there are more gravitational waves throughout the galaxy than expected, is the resulting effect "massive" enough to simulate dark matter? I'd guess not by a long shot, but the calculated estimate is left as an exercise. :)

cjameshuff
2011-Dec-01, 03:10 PM
some how I think/guess the center BH is a key to this puzzle

As mentioned, the central black hole is a tiny fraction of a galaxy's mass, and is only relevant to those stars in its immediate vicinity. The Milky Way's SMBH is probably a few million solar masses, but the mass of the galaxy that black hole is trapped in is almost a million times greater. And your hand-wavy "self lensing" effect doesn't even begin to explain the other observations of dark matter.

nota
2011-Dec-02, 02:38 AM
ok I admitted it was a guess but
what do we KNOW about gravity/DM effects

afaik we have speed of orbits for stars variation from predicted rates
and bending distant light thru a gravity well ie lensing in excess

but no local effects of DM
no DM detected
or real idea what it is

Hornblower
2011-Dec-02, 02:55 AM
ok I admitted it was a guess but
what do we KNOW about gravity/DM effects

afaik we have speed of orbits for stars variation from predicted rates
and bending distant light thru a gravity well ie lensing in excess

but no local effects of DM
no DM detected
or real idea what it is

The density of the hypothetical dark matter is too low to have a detectable effect locally within the solar system. The Sun's gravity vastly overpowers it at this close range. Only when we are lightyears away from individual stars can we see the DM's effect on a galactic scale.

We do not know what it is. So far it is a theoretical placeholder to account for what looks like excessive gravity in the outer parts of the galaxy. An alternative would be to revise the fundamental theory of gravitation, such as Dr. Milgrom has proposed with the MOND formula.