Scientists observe potential signs of dark matter

[wolram]

http://phys.org/news/2015-04-potential-interacting-dark.html

An international team of scientists, led by researchers at Durham University, UK, made the discovery using the Hubble Space Telescope and the European Southern Observatory's Very Large Telescope to view the simultaneous collision of four distant galaxies at the centre of a galaxy cluster 1.3 billion light years away from Earth.
Read more at: http://phys.org/news/2015-04-potential-interacting-dark.html#jCp

 

[marcus]

http://arxiv.org/abs/1504.03388
The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827
Richard Massey (Durham), Liliya Williams (Minnesota), Renske Smit (Durham), Mark Swinbank(Durham), Thomas Kitching (MSSL), David Harvey (EPFL), Mathilde Jauzac (Durham), Holger Israel(Durham), Douglas Clowe (Ohio), Alastair Edge (Durham), Matt Hilton (ACRU), Eric Jullo (LAM), Adrienne Leonard (UCL), Jori Liesenborgs (Hasselt), Julian Merten (JPL), Irshad Mohammed (Zurich), Daisuke Nagai (Yale), Johan Richard (Lyon), Andrew Robertson (Durham), Prasenjit Saha (Zurich), Rebecca Santana (Ohio), John Stott (Durham), Eric Tittley (Edinburgh)
(Submitted on 13 Apr 2015)
Galaxy cluster Abell 3827 hosts the stellar remnants of four almost equally bright elliptical galaxies within a core of radius 10kpc. Such corrugation of the stellar distribution is very rare, and suggests recent formation by several simultaneous mergers. We map the distribution of associated dark matter, using new Hubble Space Telescope imaging and VLT/MUSE integral field spectroscopy of a gravitationally lensed system threaded through the cluster core. We find that each of the central galaxies retains a dark matter halo, but that (at least) one of these is spatially offset from its stars. The best-constrained offset is 1.62+/-0.48kpc, where the 68% confidence limit includes both statistical error and systematic biases in mass modelling. Such offsets are not seen in field galaxies, but are predicted during the long infall to a cluster, if dark matter self-interactions generate an extra drag force. With such a small physical separation, it is difficult to definitively rule out astrophysical effects operating exclusively in dense cluster core environments - but if interpreted solely as evidence for self-interacting dark matter, this offset implies a cross-section sigma/m=(1.7+/-0.7)x10^{-4}cm^2/g x (t/10^9yrs)^{-2}, where t is the infall duration.
15 pages, 9 figures

An earlier short paper that reported no positive results from inspecting other collision data:
http://arxiv.org/abs/1503.07675
The non-gravitational interactions of dark matter in colliding galaxy clusters
David Harvey, Richard Massey, Thomas Kitching, Andy Taylor, Eric Tittley
(Submitted on 26 Mar 2015 (v1), last revised 13 Apr 2015 (this version, v2))
Collisions between galaxy clusters provide a test of the non-gravitational forces acting on dark matter. Dark matter's lack of deceleration in the `bullet cluster collision' constrained its self-interaction cross-section \sigma_DM/m < 1.25cm2/g (68% confidence limit) for long-ranged forces. Using the Chandra and Hubble Space Telescopes we have now observed 72 collisions, including both `major' and `minor' mergers. Combining these measurements statistically, we detect the existence of dark mass at 7.6\sigma significance. The position of the dark mass has remained closely aligned within 5.8+/-8.2 kpc of associated stars: implying a self-interaction cross-section \sigma_DM/m < 0.47 cm2/g (95% CL) and disfavoring some proposed extensions to the standard model.
5 Pages, 4 Figures and 18 pages supplementary information

 

[wolram]

Is this another hint that dark matter is not cold?

 

[Chalnoth]

Is this another hint that dark matter is not cold?

I believe these studies say more about the self-interaction of dark matter than that. The area most sensitive to the temperature of dark matter is structure formation in the early universe.

 

[wabbit]

http://arxiv.org/abs/1504.03388
The behaviour of dark matter associated with 4 bright cluster galaxies in the 10kpc core of Abell 3827, Richard Massey & al.

this suggests a particle cross-section σ/m ∼ (1.7 ± 0.7) × 10−4 cm2/g

They qualify this is several ways but this is much more precise than their recent upper bound in
http://arxiv.org/abs/1503.07675
The non-gravitational interactions of dark matter in colliding galaxy clusters, David Harvey, Richard Massey, Thomas Kitching, Andy Taylor, Eric Tittley

\sigma_DM/m < 0.47 cm2/g

Wht kind of candidates would that ~ 2 10^-4 cm2/g figure or something of similar magnitude suggest if confirmed ?

 

[wabbit]

Is this another hint that dark matter is not cold?

Hah, how cold is cold ? I have no idea but for some reason 2 10^-4 sounded pretty cold to me. Maybe not.

 

[Chalnoth]

Hah, how cold is cold ? I have no idea but for some reason 2 10^-4 sounded pretty cold to me. Maybe not.

It's not a temperature. It's a self-interaction cross-section. Basically this is related to the probability of two dark matter particles colliding with one another.

 

[wabbit]

It's not a temperature. It's a self-interaction cross-section. Basically this is related to the probability of two dark matter particles colliding with one another.

Thanks - I wasn't in doubt about that, but for some reason I thought the "cold" in CDM meant "weakly interacting". Nope, no reason really, just silliness on my part.