more evidence for dark matter---another cluster collision

[marcus]

http://arxiv.org/abs/0806.2320

this has some similarities to the famous Bullet Cluster collision
where the ordinary matter gas had collided and gotten stopped and the galaxies with their DM had passed on thru

it's reassuring to get a repeat of that

[matt.o]

This is not the only one - there are a number of other similar clusters, perhaps not as dramatic as the bullet cluster, but similar nonetheless. The other papers I can think of are:

MS 1054-0321 (http://adsabs.harvard.edu/abs/2005ApJ...634..813J)

CL 0152-1357 (http://adsabs.harvard.edu/abs/2005ApJ...618...46J)

and a couple in here:

Okabe et al. (http://adsabs.harvard.edu/abs/2008PASJ...60..345O)

[Wallace]

It's looking increasingly less likely, given these and other results, that DM can be completely explained away by modified gravity of any sort.

[shalayka]

It's looking increasingly less likely, given these and other results, that DM can be completely explained away by modified gravity of any sort.

I find it odd that you say this, considering that Moffat and Brownstein's theoretical work on modified gravity relies on observations exactly like those seen in the Bullet Cluster collision. Not that I necessarily agree with their model, but still... I don't think the statement is entirely accurate.

[marcus]

It's looking increasingly less likely,..

I find it odd that you say this,..

"increasingly less likely" is not a flat statement, I hear it more as a subjective weighing of the probabilities.

I know some Moffatt and Brownstein work. I've read papers by them and watched one of them give a seminar talk on PIRSA (the Perimeter Institute recorded seminar archive), and I certainly give consideration to the notion that modified gravity might provide an alternative explanation. But as I watch the evidence piling up and try to weigh the odds, I find that I heartily agree with what Wallace said. Also to me it is looking increasingly less likely.

there is a certain amount of play or flexibility in modified gravity models and people like Moffatt have room to adjust and adapt and try to fit each new observation as it comes in. And that is what we expect and want them to do. On the chance they might after all be right. but if it goes as I now expect, they will eventually run out of flexibility and be unable to adjust. We will see.

[smallphi]

Let's not forget the 'cosmic train-wreck cluster' which doesn't show the expected behavior of dark matter in cluster collisions:

http://space.newscientist.com/article/dn12497-cosmic-train-wreck-defies-dark-matter-theories.html

[marcus]

thanks Smallphi,
here is the article about that, published in Astrophysical Journal
http://arxiv.org/abs/0706.3048
A Dark Core in Abell 520
A. Mahdavi (UVic), H. Hoekstra (UVic), A. Babul (UVic), D. Balam (UVic), P. Capak (Caltech)
10 pages, 5 figures
(Submitted on 21 Jun 2007)

"The rich cluster Abell 520 (z=0.201) exhibits truly extreme and puzzling multi-wavelength characteristics. It may best be described as a "cosmic train wreck." It is a major merger showing abundant evidence for ram pressure stripping, with a clear offset in the gas distribution compared to the galaxies (as in the bullet cluster 1E 0657-558). However, the most striking feature is a massive dark core (721 h_70 M_sun/L_sun) in our weak lensing mass reconstruction. The core coincides with the central X-ray emission peak, but is largely devoid of galaxies. An unusually low mass to light ratio region lies 500 kpc to the east, and coincides with a shock feature visible in radio observations of the cluster. Although a displacement between the X-ray gas and the galaxy/dark matter distributions may be expected in a merger, a mass peak without galaxies cannot be easily explained within the current collisionless dark matter paradigm. Interestingly, the integrated gas mass fraction (~0.15), mass-to-light ratio (220 h_70 M_sun/L_sun), and position on the X-ray luminosity-temperature and mass-temperature relations are unremarkable. Thus gross properties and scaling relations are not always useful indicators of the dynamical state of clusters."

My take on it is: here's another cluster collision aftermath where they have now mapped where the dark matter is, and mapped where the X-ray gas is, etc. And they are trying to reconstruct what happened during the collision.
they tried to fit MOND but said it was hopeless
I guess "trainwreck" could be considered as more evidence for the existence of dark matter, giving still more information about how it behaves. Still don't know what it is or the extent of its interaction. every weak-lensing map they make of it gives additional information

and yes, it doesnt always behave the way the simple theory of its behavior says it should, apparently----so the plot thickens

[matt.o]

The trainwreck is exactly that - a trainwreck - it's an absolute mess! The other examples provide "cleaner" mergers and are more suited to this type of dark matter/baryonic matter separation experiment.

[Chronos]

Admitting to the existence of any amount of cold dark matter is a nail in the MOND coffin. The entire concept was premised as an alternative to CDM.