# Dark Matter: what is it

#### Mike2

SpaceTiger said:
Galaxy collisions and dense clusters are, theoretically, an excellent testing ground for dark matter theories. Since the baryons are coupled to one another by non-gravitational forces, they may become separated from the dark matter, which can only interact gravitationally. If this happens, then we can use gravitational lensing to to compare the mass peaks to the light peaks. This is exactly what was done in Clowe et al. 2003 (linked earlier in the thread).
Unfortunately, there are very few systems in which this kind of analysis can be done, so I wouldn't say that they have yielded definitive proof. However, results are so far consistent with particle dark matter theories.
I think they need to rewrite that paper. It's so filled with qualifying clauses, I can't tell what they are trying to say.

Are they saying that one would expect the mass centroid to be coincident with the light centroid, but they find through lensing effects that the mass centroid in not coincident with the light centroid due to the invisible dark matter contribution? What paragraph did they say that exactly.

#### SpaceTiger

Staff Emeritus
Gold Member
Mike2 said:
What paragraph did they say that exactly.
Several places, but you can check the abstract if you're just looking for a statement like that:

Clowe et al. 2003 said:
The observed offsets of the lensing mass peaks from the peaks of the dominant visible mass component (the X-ray gas) directly demonstrate the presence, and dominance, of dark matter in this cluster

#### Mike2

SpaceTiger said:
Several places, but you can check the abstract if you're just looking for a statement like that:
riiiiiiiiight... So does he mean offset in position or amplitude?

So the X-ray sources are assumed to be caused from massive objects like BH's, right? That's why the X-ray portion is considered to contribute more mass than the visible matter, right? Or is there a mechanism for producing these X-rays that does not contribute so much to the mass distribution of the galaxy? Thanks.

#### matt.o

the x-ray source here is the intra-cluster medium of the merging group. it is well known that the ICM contributes most to the baryonic mass in a cluster (around 80-90%), it is also known that the galaxies and dark matter are collisionless during a cluster merger (to a good approximation), whilst the ICM is collisional. This collisional nature leads to a process called ram-pressure stripping, where the ICM of the infalling group is stripped away from the galaxies and dark matter due to its interaction with the ICM of the main cluster. Hence the ICM lags behind the galaxies and DM.
now, if there were no dark matter and all of the gravitational potential (causing the weak lensing) were caused by the baryonic matter, you would expect to see the highest mass concentration where the ram-pressure stripped ICM is, since it makes up ~90% of the baryonic mass in a cluster. the fact that you see the mass concentration coincident with the infalling group galaxies and not with the lagging ICM tells us there must be some other kind of matter, ie dark matter.
note that weak lensing analysis is sensitive to all matter concentrations along the line of sight to the cluster.

#### Garth

Gold Member
But note that if the DM is in the form of IMBH's it too would be largely collisionless. Such IMBH's might be the remnant of an earlier ubiquitous PopIII population, and therefore originally baryonic. The constraint on this is the BBN baryon density which is model dependent on the cosmic expansion factor during the nucleosynthesis epoch.

Garth

#### matt.o

yes, that's what I though you'd say, Garth!

I think IMBH's have been ruled out by lensing surveys. Also remember Helium, lithium and deuterium abundances have observed values very close to those predicted by BBN.

#### Mike2

matt.o said:
the x-ray source here is the intra-cluster medium of the merging group. it is well known that the ICM contributes most to the baryonic mass in a cluster (around 80-90%), it is also known that the galaxies and dark matter are collisionless during a cluster merger (to a good approximation), whilst the ICM is collisional. This collisional nature leads to a process called ram-pressure stripping, where the ICM of the infalling group is stripped away from the galaxies and dark matter due to its interaction with the ICM of the main cluster. Hence the ICM lags behind the galaxies and DM.
now, if there were no dark matter and all of the gravitational potential (causing the weak lensing) were caused by the baryonic matter, you would expect to see the highest mass concentration where the ram-pressure stripped ICM is, since it makes up ~90% of the baryonic mass in a cluster. the fact that you see the mass concentration coincident with the infalling group galaxies and not with the lagging ICM tells us there must be some other kind of matter, ie dark matter.
note that weak lensing analysis is sensitive to all matter concentrations along the line of sight to the cluster.
Thanks, that helps a lot. What is the source of the X-rays? And what is the ICM made of? Do you have an on-line source for your information? Thanks again.

#### matt.o

the source of the x-ray radiation comes from the thermal motions of the electrons and ions in the ICM. When a fast moving electron encounters a slow moving ion, it is accelerated due to the ions electric field. This produces Bremsstrahlung radiation (braking radiation). The temperature of the ICM is around 10 million degrees, hence the Bremsstrahlung radiation is in the x-ray band. there is also some line emission from iron, oxygen etc.
the ICM contains mainly ionised hydrogen and helium and electrons. Although the ICM is enriched with metals to about 1/3 of the solar metal abundance.
here are some links that came from "clusters of galaxies" intra-cluster medium;
http://www.astr.ua.edu/keel/galaxies/icm.html" [Broken]
http://www-xray.ast.cam.ac.uk/xray_introduction/Clusters_intro.html" [Broken]
if you need any more go to
and search for cluster of galaxies, intra-cluster medium, Craig Sarazin, Maxim Markevitch, Alexy Vikhlinin, Christine Jones, Willy Forman, Hans Bohringer Brian McNamara etc.

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#### Mike2

matt.o said:
the source of the x-ray radiation comes from the thermal motions of the electrons and ions in the ICM. When a fast moving electron encounters a slow moving ion, it is accelerated due to the ions electric field. This produces Bremsstrahlung radiation (braking radiation). The temperature of the ICM is around 10 million degrees, hence the Bremsstrahlung radiation is in the x-ray band. there is also some line emission from iron, oxygen etc.
the ICM contains mainly ionised hydrogen and helium and electrons. Although the ICM is enriched with metals to about 1/3 of the solar metal abundance.
here are some links that came from "clusters of galaxies" intra-cluster medium;
http://www.astr.ua.edu/keel/galaxies/icm.html" [Broken]
http://www-xray.ast.cam.ac.uk/xray_introduction/Clusters_intro.html" [Broken]
if you need any more go to
and search for cluster of galaxies, intra-cluster medium, Craig Sarazin, Maxim Markevitch, Alexy Vikhlinin, Christine Jones, Willy Forman, Hans Bohringer Brian McNamara etc.
Thanks again. It is delightful to get this level of help.

One question comes to mind about the X-ray ICM. I've not read your references yet, though I plan to do so. But off hand it would seem that X-ray don't necessarily have anything to do with baryonic mass density, but only to do with the velocity of particles. Perhaps the X-rays are produced at the point of collision between galaxies which may have nothing to do with the baryonic distribution. Ya think?

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#### Garth

Gold Member
matt.o said:
yes, that's what I though you'd say, Garth!
Sorry - I'll shut up and just wait patiently.
I think IMBH's have been ruled out by lensing surveys.
Across what mass ranges?
Also remember Helium, lithium and deuterium abundances have observed values very close to those predicted by BBN.
Indeed , apparently also concordant with the linear freely coasting model, apart from deuterium which then has to be produced by spallation.

Garth

#### SpaceTiger

Staff Emeritus
Gold Member
Garth said:
Across what mass ranges?
The allowed range is $M < 10^4~M_{sun}$, except for $0.1 < M < 10~M_{sun}$. The former is constrained by (the lack of) globular cluster disruption and the power spectrum of the Lyman alpha forest. The latter is from microlensing.

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#### Garth

Gold Member
SpaceTiger said:
The allowed range is $M < 10^4~M_{sun}$, except for $0.1 < M < 10~M_{sun}$. The former is constrained by (the lack of) globular cluster disruption and the power spectrum of the Lyman alpha forest. The latter is from microlensing.
Thank you ST.

Garth

#### SpaceTiger

Staff Emeritus
Gold Member
Mike2 said:
Thanks again. It is delightful to get this level of help.
I apologize for being terse and thanks, matt.o, for stepping in. This semester has been crazy and I think I've been cutting back a bit too much on explanation.

#### matt.o

Mike2 said:
Thanks again. It is delightful to get this level of help.
One question comes to mind about the X-ray ICM. I've not read your references yet, though I plan to do so. But off hand it would seem that X-ray don't necessarily have anything to do with baryonic mass density, but only to do with the velocity of particles.
Actually, the density of the ICM can be measured directly from the x-ray emission, assuming spherical symmetry. This is because the emission measure (em)
$$em \propto \int n_{e}^{2} dl$$
where $$n_{e}$$ is the electron density and dl is the length measured along the line of sight.
More importantly, the ICM is thought to respond to the same gravitational potential as the dark matter in the cluster. Hence assuming hydrostatic equilibrium and spherical symmetry of the ICM (which is a fair assumption in a dynamically relaxed cluster) the total mass profiles of the cluster can be measured from the combination of radial profiles of temperature and density of the ICM. That is, we can measure the total mass (including dark matter) within a certain radius using the proporties we measure from the ICM.
Mike2 said:
Perhaps the X-rays are produced at the point of collision between galaxies which may have nothing to do with the baryonic distribution. Ya think?
Not quite sure what you mean here. Although the main source of heating of the ICM is due to shocks occuring during the formation of the cluster, where super-sonic speeds are common.

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#### Mike2

matt.o said:
Actually, the density of the ICM can be measured directly from the x-ray emission, assuming spherical symmetry. This is because the emission measure (em)
$$em \propto \int n_{e}^{2} dl$$
where $$n_{e}$$ is the electron density and dl is the length measured along the line of sight.
More importantly, the ICM is thought to respond to the same gravitational potential as the dark matter in the cluster. Hence assuming hydrostatic equilibrium and spherical symmetry of the ICM (which is a fair assumption in a dynamically relaxed cluster) the total mass profiles of the cluster can be measured from the combination of radial profiles of temperature and density of the ICM.
But the paper cited to compare the centroid of baryonic to DM was a cluster with galaxies in the process merging. It talks about "pass through", etc. So I have to wonder if these assumptions of symmetry still hold.

matt.o said:
That is, we can measure the total mass (including dark matter) within a certain radius using the proporties we measure from the ICM.
Not quite sure what you mean here. Although the main source of heating of the ICM is due to shocks occuring during the formation of the cluster, where super-sonic speeds are common.
I don't see how you can measure DM this way since it weakly interacts (if at all) with all this radiation going on in the ICM

#### matt.o

Mike2 said:
But the paper cited to compare the centroid of baryonic to DM was a cluster with galaxies in the process merging. It talks about "pass through", etc. So I have to wonder if these assumptions of symmetry still hold.
You are entirely correct. Assumptions of spherical symmetry and hydrostatic equilibrium don't hold in major mergers. There is really no way to accurately measure the mass in a major merger.
Mike2 said:
I don't see how you can measure DM this way since it weakly interacts (if at all) with all this radiation going on in the ICM
I am about to go to bed now, so perhaps you could do a search onhttp://adsabs.harvard.edu/abstract_service.html" [Broken] for craig sarazin in order to answer this. Failing this, I could go into more detail at a later date. The basics of it is that both the dark matter and the ICM respond to the same gravitational potential so you can measure the mass of both from the ICM emission.

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#### Mike2

matt.o said:
You are entirely correct. Assumptions of spherical symmetry and hydrostatic equilibrium don't hold in major mergers. There is really no way to accurately measure the mass in a major merger.
I wonder if it would be better to examine mergers between just two galaxies. But then again, in a bath of ICM, it would probably be too hard to distinguish effects from just the two merging galaxies... unless you can find a cluster that consists of only two galaxies that have merged, not likely.

#### matt.o

firstly, 2 galaxies do not constitute a cluster, although they can be classified as a group. there are things known as fossil groups, ie a single galaxy which appears to have the mass of a group. these are likely to have formed from mergers of 2 or more galaxies.

Oh, and the ICM is not exactly a bath. it has densities of around 10^-2-10^-3 electrons per cm^3, which is less dense than the best vacuums we can create on earth! Plus the ICM is optically thin, hence it doesn't absorb light.

Also, galaxy-galaxy mergers are rare in massive clusters cores due to the high speeds at which the galaxies orbit.

#### vincentm

Gold Member
Okay, first off i wanna say thanks once again, but the math part confuses me more, mainly because i suck at mathematics at this point in my studies.

anyway this is what i have read in regards to DM. Please bare with me:

the stars originally in the halos of galaxies in clusters must currently permeate intergalactic space. Tidal forces between colliding galaxies during the first billion years of the cluster's existence stripped the outer halos of stars. Stripping was effective beyond a radius of about 100,000 light years in a typical galaxy.

From observations of the Doppler shifts of their spectra, we infer that the cluster galaxies move at rather high random velocities. Because we can measire the dimensions of a cluster, we can compute how much mass must be present within the rapidly moving galaxies to contain the expansion. (if this mass were not present, the galaxies would simply fly apart, there would be no cluster.) the result is surprising; the required amount of mass per galaxy is several times as large as that inferred by other types of measurements, usually of nearby galaxies, whose dynamics we can study in suffiecient detail to infer their masses. For example, by measuring the rate at shich a nearby galaxy is roatating, we can infer its mass. We can also measure the velocities of nearby galaxies ina number of isolated close pairs to determine the average mass of the pair.

We can make these statements rather more precise by introducing the mass-luminosity ratio. We measure luminosity directly and, for every unit of luminosity (usually expressed in units of solar luminosity), we can assign a certain number of units of mass (expressed in solar masses). Thus, the sun has a mass -luminosity ratio of 1; the visible regions of the Milky Way galaxy, which consist for the most part of stars less massive and considerably less luminous than the sun, have a mass-luminosity ratio between 200 and 400. Measurements of individial elliptical galaxies yield a mass-luminosity ratio of about 8, although this result is applicable only the central region luminous regions.

By studying radio emmision from meutral hydrogen, scientists have been able to measure the rate at which a spiral galaxy rotates. We can follow to the extreme parts of the galaxy should be more weakly bound. They should therefore experience a weaker centrifugal force and be rotating less rapidly But this is contrary to what is found. It appears from the measurements atht spirals have larger mass-luminosity ratios than we would predict from studying their luminous inner regions More mass must be present than we havepreviously realized. their net mass-luminosity ratios must be about 30 or even larger, Precisely what for this non luminous matter take in the out regions , or halos is not known.

rotation curves probe the outer regions of spiral galaxies, where there is little luminous matter. two different techniques have been used to study ellipticals, which are gas poor and therefore not amenable to rotation curve studies at large distances from the center of the galaxies. X-ray emmision has been discovered around ellipticals. the x-rays are produced by hot gas at about 10 million kelvins, gravitionally confined in the halos of the elliptical galaxies. To confine the gas reuires a considerable amount of mass: it is inferred that the ratio of total mass, including dark halo, to optical luminosity, which comes entirely from the inner regions, may be as large as 50.

Antoerh discovery also indicates a considerable amount of dark matter in the halo of the elliptical. Elliptical galaxies reveal the presence of faint shells on deep photographic plate. These shells extend out two or three times as far as the bulk of the starlight. As many as 20 shells have been discovered around one bright galaxy. the shells appear to be fossil "splashes" remaining from a merger of a smaller satellite galaxy into the core of the elliptical. the spacing of the shells are a measure of the gravitational field, and computer simulations of the merger result in a simple array of concentric shells. Modeling of the shells requires the presence of a massive dark halo.

Classical methods of mass determination, based on optical studies of the luminous inner regions, leave open the possibility of galaxies having considreable amounts of mass in their extended halos. GAlaxies could be very extended indeed, concievably filling most of space with exceedingly tenous halos, In clusters, the halos were stripped during collisions between the galaxies. However the excess mass should stil be present in the intergalactic medium. But the precise form of the dark mass poses a great astrophysical puzzle. the mass cannot be very luminous, or astonomers would be able to observe it directly. It cannot be gaseous, because gas, whether hot or cold, ionized or neutral, is difficult to hide. Many searches have been performed for intergalactic gas. Some gas has been discovered in rich clusters, but not enough to account for the mass discrepancy. Perhaps the most dramatic studies of dark matter in galaxy cluster have merged from the gravitational lensing by the cluster of background galaxies. the gravity field of the cluster bends the background light, acting as a lens, and produces images that are distorted into arcs, This effect was predicted by Albert Einstein but was first detected in the 1970's.

two hypostheses have merged to account for the mass that is inferred to be present in clusters and in galactic halos. One hypothesis argues that the dark mass is baryonic. It might consisst of stars of very low mass, which are so faint that they have escaped detection. Alternatively, many collapsed remnatns perhaps white dwarfs or even blackholes of an early generation of massive stars constitute the hidden mass. A second hypothesis argues that the dark matter is nonbaryonic. it consists of one of the exotic particle species that wearlier hypothesized could exist in sufficientl quantity to yield a substantial fraction of dark matter in halos and in clusters amounts to only 10 percent of the critical density required to reverse the expansion of the universe, if we measure it as the ratio of hypothesized mass to be observed luminosity averaged over a suitably large region of psace.

Black holes would hav formed as a result of catastrophic stellar explosions, and the ensuing radiation shouldm in principle, be detecable. the current consensus is that if blackholes account for the dark mass in clusters of galaxies they must have fmored sufficiently early in the universe for the cosmological redshift to have hidden the associated optical emission from our observations. at a redshift of, say 10, the protogalactic radiation produced when the massive stellar precursors of the black holes evolved and collapsed would now be visible only in ther infrared region of the spectrum. I the infrared, observations are extrmely difficult because of atmospheric emission (such as the terrestial airglow) and attenuation resulting from a sbosrtion by ozone, water vapor, and other molecules.

White dwarfs or neutron stars are a more conservative choice than blackhole for that matter. They are the only dark matter candidates that we can unambiguosly state must exist, although whether enough actually exist is another matter. If they are to be numerous enough to account for dark matter, white dwarfs must hav been produced by a large number of stars of moderate mass, formed early in the evolution of the galaxy. We cannot exclude such a hypothesis, but we can seek ways to test it. for example, the white dwarfs would have cooled down, but they might still be dimly visible as reddish dwarfs. the ejecta produced when compact remnants, blackholes, neutron stars, or white dwarfs formed would be chemically enriched and would show up in the composition of old stars. Studies suggest that remnants of very massive stars, either black holes or neutron stars, are implausible candidates for the dark matter unless the blackholes are much more massive than ordinary stars, but white dwarfs are a possibility.

Stars of low mass also are a possible source of a small fraction of the dark mass. Stars of very low mass populating the halo of our galaxy would occasionally pass close enough to the sun to be recognizeable. Thet would appear as very faint nearby stars with appreciable proper motions and the high velocities characteristic of their halo origin. Becasue fewsuch objects are seen, the orbits of such stars must restrict them predominantly to the outer halo. Presumably, their orbits are most circular, the dynamical characteristics of these objects would make them distinct from the ordinary stars in our galaxy, which have appreciable velocities in the direction of the center, move in highly elongated orbits. Alternatively, these out halo stars could be "jupiters" essentially invisible giant planets that were not massive enough [less than 0.08 solar mass] to become stars.

if we possed an adequate theory of star formation, we sgould be able to choose between hypotheses of massive versus low mass star formation. Even if low mass stars predominate, there must also have been a considerable number of massive stars in the halo of a newly formed protogalaxy the Processed gases ejected during supernova explosions of the massive stars would accounts for the origin of the neriched intergalactic matter that is observed in rich clusters of galaxies. However, our knowledge of star formation is likely to remain so imprecise that direct observations will be required to determine the form of the dark mass if it is baryonic.

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#### Mike2

matt.o said:
Oh, and the ICM is not exactly a bath. it has densities of around 10^-2-10^-3 electrons per cm^3, which is less dense than the best vacuums we can create on earth! Plus the ICM is optically thin, hence it doesn't absorb light.
That's interesting... As I understand it, the cosmological constant can be described in terms of the zero point energy and some have calculated how much energy this represents per unit volume. They even suggest how many particles of matter this is per volume. So my question is: how does this ICM density compare with that of the cosmological constant?

Also, I wonder if we can get the energy density from a black body radiation temperature. Thanks.

#### matt.o

I'm not sure that it is worthwhile comparing the density of the ICM to that of the quantum vacuum. My point was just that the ICM is not very dense.

#### Chronos

Gold Member
The average mass density of the universe [conventional] is around 1 hydrogen atom per square meter. The intergalactic medium is thought to be in the range of 10 - 100 H atoms per cubic meter. Intercluster mass density might be ~5 times denser, on average. It is a very difficult thing to model. The classical approach is based on newtonian gravity [most of this stuff does not move fast enough to worry about relativistic effects]. Dark matter is what plugs the gap to explain the apparent gravitational attraction observed, but not otherwise accounted for. DM is a rather unsavory explanation, but is more consistent with observation than the competing theories [MOND in particular]. Finding the DM particle in the lab is, however, a huge issue. It hasn't been done yet. It may also never be feasible. The energies necessary may not be achievable by any known technologies.

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#### Garth

Gold Member
Chronos said:
It is a very difficult thing to model. The classical approach is based on newtonian gravity [most of this stuff does not move fast enough to worry about relativistic effects].
However the non-linear GR effects of orbiting gravitating matter may be significant.
Dark matter is what plugs the gap to explain the apparent gravitational attraction observed, but not otherwise accounted for. DM is a rather unsavory explanation, but is more consistent with observation than the competing theories [MOND in particular]. Finding the DM particle in the lab is, however, a huge issue. It hasn't been done yet. It may also never be feasible. The energies necessary may not be achievable by any known technologies.
In which case, if the DM particle is never found, what would be the scientific status of such DM?

Garth

#### matt.o

However, the non-linear effects have no bearing on the ICM, which also requires DM to explain why it hasn't evaporated into inter-cluster space.