Dark Matter Core Defies Explanation

Lensman

This article appeared March 2nd on Science Daily. Quite interesting. Any comments?

http://www.sciencedaily.com/releases...0302101413.htm

Tanelorn

I agree this is interesting. I wonder if there is another way to confirm what we think we are seeing?

I understand that DM was postulated to explain the distribution of rotation velocities of galaxies, so if these Galaxies had been stripped of some of their DM would it not also show up in the galaxy rotational distributions?

Lensman

You would think so. And confirming the observation would be great. It's a puzzler!

Chalnoth

Well, the galaxies still do interact more than the dark matter, so it isn't completely absurd. It is interesting, and does speak to an interesting past history for this particular cluster. But the vast majority of clusters fall into the expected pattern, with dark matter and galaxies well-aligned.

Bobbywhy

Here is an excerpt from NASA:

"The blend of blue and green in the center of the image reveals that a clump of dark matter resides near most of the hot gas, where very few galaxies are found. This finding confirms previous observations of a dark-matter core in the cluster. The result could present a challenge to basic theories of dark matter, which predict that galaxies should be anchored to dark matter, even during the shock of a collision."

http://www.nasa.gov/multimedia/image...ture_2189.html

TrickyDicky

The Bullet Cluster is claimed to provide the best evidence to date for the existence of dark matter (see for instance wikipedia entry). Is this new observation the best evidence to date against the existence of dark matter, then?

Chalnoth

Not really. As I said, the vast, vast majority of clusters are very easy to understand in terms of dark matter. The bullet cluster is also easy to understand largely because it's easy to see what happened to separate the cluster gas from the dark matter. That a few clusters are difficult to understand in terms of dark matter is expected.

My bet is that if a group of scientists are really interested in this, they'll find a reasonable solution within a few years. If they haven't already (I haven't looked, sorry).

TrickyDicky

Hmm, we are talking about clusters colliding here, do you know any good example besides the Bullet cluster and more recently MACSJ0025?

Chalnoth

There's this one as well, for instance:
http://www.scienceagogo.com/news/200...runc_sys.shtml

johne1618

Maybe this is evidence for the "Mirror Matter" dark matter hypothesis.

http://en.wikipedia.org/wiki/Mirror_matter

Mirror matter is hypothesized to be just like ordinary matter but is "right-handed" rather than "left-handed".

Perhaps most mirror matter is in the form of mirror Hydrogen and mirror Helium which forms clouds around galaxies whereas some mirror matter might be in the form of heavier mirror elements that can clump together.

TrickyDicky

Yes, the ring one, CI0024+17, that makes 3 to one(Abell 520).

Chalnoth

I don't quite see how, but I should mention that mirror matter is basically just one of many dark matter hypotheses. Mirror matter must be pretty different from normal matter in order to explain our observations, as dark matter interacts very weakly both with itself and with normal matter.

johne1618

If mirror matter exists and is mostly in the form of cold mirror hydrogen/helium surrounding galaxies then I suppose it would not interact strongly with itself if those galaxies collide.

Is that correct?

Chalnoth

Our best constraints on the interaction of dark matter with itself and with normal matter comes from the early universe, where all of the normal matter was an ionized plasma. At that time, the dark matter still didn't interact much at all with itself or with normal matter, so it can't simply be made of the same stuff as normal matter is.

johne1618

Ok - so if dark matter was mirror matter then in the early Universe it should have been a mirror ionized plasma in the same way that normal matter was an ionized plasma. Thus even if it didn't interact with normal matter at all it should still have interacted strongly with itself. We don't see evidence of that so dark matter is probably not mirror matter.

Thanks for the clarification.

Chalnoth

Well, at least not precisely. It basically has to interact with itself considerably more weakly than normal matter interacts with itself. So it can't be an exact mirror.

What might also help is if the physics causes recombination (when the universe first turns from a plasma into a gas) to occur sooner with mirror matter than with normal matter. Though I think it would still have to interact more weakly for us to not see the interactions.

marcus

I don't think anyone has given the link to the journal article yet
http://arxiv.org/abs/1202.6368
A Study of the Dark Core in A520 with Hubble Space Telescope: The Mystery Deepens
M. J. Jee, A. Mahdavi, H. Hoekstra, A. Babul, J. J. Dalcanton, P. Carroll, P. Capak
(Submitted on 28 Feb 2012)
We present a Hubble Space Telescope/Wide Field Planetary Camera 2 weak-lensing study of A520, where a previous analysis of ground-based data suggested the presence of a dark mass concentration. We map the complex mass structure in much greater detail leveraging more than a factor of three increase in the number density of source galaxies available for lensing analysis. The "dark core" that is coincident with the X-ray gas peak, but not with any stellar luminosity peak is now detected with more than 10 sigma significance. The ~1.5 Mpc filamentary structure elongated in the NE-SW direction is also clearly visible. Taken at face value, the comparison among the centroids of dark matter, intracluster medium, and galaxy luminosity is at odds with what has been observed in other merging clusters with a similar geometric configuration. To date, the most remarkable counter-example might be the Bullet Cluster, which shows a distinct bow-shock feature as in A520, but no significant weak-lensing mass concentration around the X-ray gas. With the most up-to-date data, we consider several possible explanations that might lead to the detection of this peculiar feature in A520. However, we conclude that none of these scenarios can be singled out yet as the definite explanation for this puzzle.
Published in Astrophysical Journal

The alternative scenarios being considered to explain the dark core are described on page 7.

Figures 4 and 5 show contour density maps and are graphically clear and detailed. Better than what appeared in the news article. It's worth visiting the professional journal article if only to get a look at these contour maps.