# Some questions regarding WIMPs as a DM candidate

1. Apr 20, 2006

### frazzle

WIMPs as a DM candidate - Some questions about detection.

Hello everyone, I am new here, so sorry if this has already been covered. I ran a search on 'WIMP' for the forum, and it seems I'm in the clear here..

So, I am writing a report on dark matter at the moment, but there are a few things that have me confused. At the moment, I am trying to figure out some aspects of detection of the WIMP.

1) I have been reading about indirect detection of WIMPs via their annihilation products. It suggests that WIMPs may cluster at the centre of bodies such as galaxies (due to their gravitational pull) and annihilate, as particles such as the Neutralino are their own anti-particle. However, I was previously under the impression that the majority of the dark matter present in galaxies was supposed to be located within the outer regions? i.e. a dark matter 'halo'? I am also confused as to why the WIMPs would be located in the halo at all. Would gravity not pull them into the centre, in much the same way the luminous matter is largely concentrated at the centre of the galaxy?

2) Another detection method involves the annual modulation of the WIMP flux signal in ground based detectors. As I understand it, this is due to the addition and subtraction of the Earth velocity to that of the WIMPs as the Earth orbits the sun, which is in turn orbiting the galaxy centre. My confusion here, is why would the WIMPs all move in one direction, like a 'wind'? Is this due to the fact that they too are in orbit around the galaxy centre?

3) Finally, I have been reading about possible indirect WIMP detection in the form of high energy gamma-rays. They suggest that the WIMPs that have annihilated to cause this signal might be TeV in magnitude. I was under the impression that this sort of mass is well outside the 'allowed' mass boundary of such a particle?

Sorry if I have rambled a bit here. Hope I'm not imposing at all. If you've read far enough to have reached this point, thanks very much

Last edited: Apr 20, 2006
2. Apr 21, 2006

### SpaceTiger

Staff Emeritus
The vast majority of the mass of dark matter is located in the halo, but it's mostly at a very low density. Only in the regions of the highest density could there be enough dark matter annihilation to detect a signal.

Unlike luminous matter, dark matter doesn't interact (at least, not very much), so it has no way of dissipating energy. As such, the dark matter particles will oscillate around the galaxy relatively unhindered, while the luminous matter will interact with its surroundings and radiate energy away.

On average, WIMPs shouldn't be rotating much with respect to the galactic center, so in that frame there would be no wind. However, the sun is in orbit about the GC, so there should be a net flow of WIMPs relative to the sun.

You can see in the figure you posted in the High-Energy physics section -- the mass scale goes up to 10^4 GeV (10 TeV).

3. Apr 21, 2006

### Chaos' lil bro Order

Are WIMPs neutrinos? Or are they other stuff.

4. Apr 21, 2006

### EL

WIMP stands for Weakly Interacting Massive Particles. Although neutrinos are weakly interacting, they are certainly not very massive, and hence does not belong to the class of particles we call WIMPs.
A typical WIMP is the (hypothetical) Neutralino, which exists in supersymmetric extensions of the Standard Model.

5. Apr 21, 2006

### frazzle

Thankyou so much for your reply Spacetiger, that's really cleared some things up for me.

Something that has further confused me however, is that I have read that dark matter is located largely in halos around galaxies, but largely in the centre of galaxy clusters. I don't understand why/how this could be so? i.e. why would the particles cluster differently in these two scenarios?

6. Apr 21, 2006

### SpaceTiger

Staff Emeritus
It's hard to say what the source meant by that, but dark matter halos all have similar shapes, whether they're in galaxy clusters or dwarf galaxies. In pretty much all such scenarios, the halo is the densest at the center and falls off as ~1/r^2 in the visible outskirts. The luminous matter in galaxies is more condensed than in clusters, so if that's what you're comparing to, one might say that the dark matter is more centrally concentrated in a cluster. I think the outer dark matter profiles are pretty poorly known, however.

7. Apr 21, 2006

### frazzle

in case you are curious, here is the quote:

An important difference between the distribution of dark matter in galaxies
and clusters needs to be emphasised: whereas dark matter appears to
increase with distance in galaxies, in clusters exactly the reverse is true, the dark matter distribution actually decreases with distance. Indeed, for certain dwarfs (such as DD0154) the rotation curve has been measured to almost 15 optical length scales indicating that the dark matter surrounding this object is extremely spread out (see also figure 1). A foreground cluster, on the other hand, acts as a gravitational lens which focuses the light from background objects such as galaxies and QSO’s thereby allowing us to determine the depth of the cluster potential well. Observations of strong lensing by clusters indicate that dark matter is strongly concentrated in central regions with a projected mass of 10^13 − 10^14[solar masses] being contained within 0.2 - 0.3 Mpc

from astro-ph/0403324, page 4.

i suppose it may be the case that the text is a series of statements without explainations because nobody really knows why the matter appears to be distributed this way?

8. Apr 22, 2006

### Chronos

Inflation nicely explains why DM is unevenly distributed in the universe. The pre-inflation universe was uniformly saturated with DM. Inflation smeared it out in clumpy, fractal patterns across what is now the entire universe. Baryonic matter was naturally attracted [gravitationally] to these overdense regions. The rest is history.

9. Apr 22, 2006

### Garth

ST, what matter (DM & bayonic) is thought to lie in the Inter Galactic Medium between galactic clusters? Although of very low density there may be significant contribution to $\Omega_m$ because of the vast volume.

Garth

10. Apr 22, 2006

### frazzle

Aha! Is this to say that the large amount of matter at the centre of the galaxy cluster is was there 'first', and the individual galaxies were attracted to it (and then to each other) which is what caused the cluster to form?

11. Apr 22, 2006

### SpaceTiger

Staff Emeritus
Outside of bound objects, dark matter is very difficult to observe. Gravitational lensing surveys (most of which are still in preparation) ought to give us a relatively direct measure of the overall distribution of matter in the universe on large scales. The theoretical prediction from $\Lambda CDM$ is that it will be a gaussian random field, with a scale-invariant power spectrum at the largest scales.

Baryonic matter in the IGM is a bit more complicated because it depends on the detailed history of the universe -- that is, how much matter was expelled or stripped from galaxies and galaxy clusters. This matter is generally observed in absorption along sightlines to distant quasars.

12. Apr 22, 2006

### SpaceTiger

Staff Emeritus
Galaxy clusters often have a great deal of mass at the center in the form of a giant elliptical galaxy, but the formation of the cluster wasn't due to an individual galaxy or small-scale concentration of matter. It was due to the combined gravitational effect of all galaxies within an "overdense" region of the universe.

13. Apr 22, 2006

### Garth

Thank you ST. Are there any upper limits? We have already looked elsewhere at WHIM, but what about cold hydrogen and the material causing the Lyman $\alpha$ forest?

Garth

14. Apr 22, 2006

### SpaceTiger

Staff Emeritus
We can estimate the amount of mass in the Ly$\alpha$ forest and I think it comes well short of what's needed for $\Omega_b$ from nucleosynthesis and the CMB. I don't know the numbers, but a search of arxiv might turn something up.

15. Apr 23, 2006

### Garth

Yes thank you - I have two recent papers Rao's Damped Lyman alpha Surveys and Statistics: A Review and Wolfe et al's DAMPED Ly alpha SYSTEMS.

The latter paper gives the neutral gas $\Omega_g \sim 0.001$ and stellar $\Omega_* \sim 0.002$. However, the first paper makes the point:
but later makes the point
So there may be more out there than is thought.

Garth

Last edited: Apr 23, 2006
16. Apr 23, 2006

### SpaceTiger

Staff Emeritus
Damped systems are only a small fraction of the full Ly$\alpha$ forest. Specifically, it's only those absorbers with hydrogen column densities greater than about 1020 cm-2. Even though they may carry most of the neutral gas, the Ly$\alpha$ lines in the forest will often be from warmer, more heavily ionized gas with large hydrogen column densities.

Last edited: Apr 23, 2006
17. Apr 23, 2006

### Garth

Is it thought that this latter ionized gas is WHIM at 105 - 107oK or just warm gas?

Garth

18. Apr 23, 2006

### SpaceTiger

Staff Emeritus
The WHIM is very highly ionized, so it produces virtually no signature in the Ly$\alpha$ forest. This is a part of the reason that we have such difficulty constraining the mass of the WHIM. The Ly$\alpha$ forest is largely diffuse, ionized gas, presumably at temperatures nearer 104 K.

19. Apr 23, 2006

### SpaceTiger

Staff Emeritus
Last edited by a moderator: Apr 22, 2017
20. Apr 24, 2006

### Garth

Just to include a paper today on WHIM Warm-hot intergalactic medium contribution to baryonic matter which states
, which is even lower (by a factor of ~ 3) than the lower limit of Nicastro, Elvis, Fiore & Mathur's paper: Measured Cosmological Mass Density in the WHIM: the Solution to the 'Missing Baryons' Problem, which as I have posted before gives an upper limit of:
$\Omega_b$WHIM > 4.3 × 100.47 % = 12.6%
and a lower limit of:
$\Omega_b$WHIM > 1.3 × 100.32 % = 2.7%.

Garth

Last edited by a moderator: Apr 22, 2017