Journal Club – Aberrant Dark Matter


According to Wikipedia, a journal club is a group of individuals who meet regularly to critically evaluate recent articles in the scientific literature. And of course, the first rule of Journal Club is… don’t talk about Journal Club.

So, without further ado – today’s journal article is about dark matter being in the wrong place at the wrong time.

Today’s article:
Jee et al A Study of the Dark Core in A520 with Hubble Space Telescope: The  Mystery Deepens.

This time, rather than someone suggesting what the next journal club article would be (like that happens), I thought I would pick a topical scientific paper mentioned in one of Universe Today’s fabulously thought-provoking stories and enlarge on that a bit.

This paper by Jee et al was mentioned in Ray Sanders’ excellent Hubble Spots Mysterious Dark Matter ‘Core’ article on 2 March 2012.

So, some might remember the Bullet Cluster – a seemingly clinching proof of dark matter, where two galactic clusters had collided in the past and what we see post-collision is that most of the mass of each cluster has passed straight through and out the other side. The only material remaining at the collision site is a huge jumbled clump of intergalactic gas.

This means that each galactic cluster, that has since moved on, has been stripped of much of its intergalactic gas. But lo and behold the seemingly empty intergalactic space within each of these stripped galactic clusters continues to distort the background field of view (a phenomenon known as weak gravitational lensing).

This seemed strong proof that the intergalactic spaces of each cluster must be filled with gravitationally-inducing, but otherwise invisible, stuff. In other words, dark matter. It makes sense that this dark matter would have moved straight on through the collision site because it is weakly interacting – whereas the gas caught up in the collision was not.

So, a cool finding and almost identical findings were discovered within the cluster collisions MACS J0025.4-1222, Abell 2744 and a couple of others. But now along comes Abell 520 with a completely counter example. Two or more galaxy clusters have collided, most of the visible contents have passed straight through, but back at the collision point is an apparent big clump of invisible stuff creating weak gravitational lensing – i.e. dark matter. It is the region labelled 3 on the figure at page 5 of the article.

This finding requires us to consider that we had naively concluded that the Bullet Cluster’s post-collision appearance was easily interpretable and that its outcome would surely be repeated in any equivalent collision of galaxy clusters.

But in the wake of Abell 520 we now may need to consider that the outcome of a collision between rapidly moving and utterly gargantuan collections of mass is much more complex and unpredictable than we had initially assumed. This doesn’t mean that the dark matter hypothesis has been debunked, it just means that the Bullet Cluster might not have been the clinching proof that we thought it was.

If we subsequently find fifty new Bullet cluster analogues and no more Abell 520 analogues, we might then assume that Abell 520 is just a weird outlier, which can be dismissed as an unrepresentative anomaly. But with only five or six such collision types known, one of which is Abell 520 – we can’t really call it an outlier at the moment.

So… comments? The authors offers six possible scenarios to explain this finding – got a seventh? Did we jump to conclusions with the Bullet Cluster? Could suggestions for an article for the next edition of Journal Club represent a form of negative energy?

10 Replies to “Journal Club – Aberrant Dark Matter”

  1. The best explanation are the actual findings written by the authors Mahdavi and Jee who wrote: “Galaxy clusters form where filaments intersect. The dark matter core in Abell 520 is coincident with the location of the x-ray luminosity peak but is largely devoid of bright cluster galaxies. A thin narrow filament coincident with P3 is where 11 cluster galaxies could be embedded. The most apparent filament in Abell 520 might be the NE-SW structure traced by galaxies, x-rays, and mass. Data from 11 spectroscopic redshifts supports the hypothesis that a fortuitous superposition of a line of sight filament is located near the dark matter core. A thin filament coincident with the dark matter core might explain the gravitational lensing of such a peculiar structure.”

    Mahdavi and Jee’s conclusion: “Current supporting data from only 11 spectroscopic redshifts does not provide sufficient STATISTICS to convince us of the presence of such a thin long filament near the core.”

    More redshifts of galaxies in Abell 520 should be studied to provide enough data to see if this explanation is correct. If a rare fortuitous filament LOS exists, then some might assume that all dark matter cores would have the same appearance if viewed aligned observing through a vast cosmic filaments.

    1. I read that as ‘does not…convince us’. I think that they think the statistics are key.

      The Jee et al article states: ” Hence, it is difficult to rationalize that the projected density profile of the hypothesized diffuse LOS filament possesses such a cuspiness at its center.” and concludes: ” Despite our solid confirmation on the presence of the dark core, we conclude that it is yet premature to single out the most probable cause of the dark core from the above scenarios.”

  2. When you think of the strange shapes you get to the visible component of merged galaxies, I find it surprising that we have found two Bullet-like collisions where two clusters with similar sizes have smacked into each other head-on. If dark matter only interacts via gravity, then maybe a collision will always cause the ordinary matter to slow down, while the dark matter keeps going. However, I would have expected more oblique collisions where we might get the dark equivalent of the antennae nebula, but on a larger scale.

    I would like to know what the dark matter map of a bit of visibly empty space looks like. Or, if this is too boring, maybe looking at a larger field with something interesting such as the Abell 520 in the middle. All the different calculations seem to have the same general blobs in the middle, but how much is the detail towards the edge an artefact of our boundary condition guesses?

    We shall see something when we have more data. The penultimate paragraph in SN’s article nails it.

      1. Thanks, Steve.

        Yes, there is some black stuff, but not enough of it for me to be happy. They are working out the mass along the line of sight from the distortion of the distant galaxy field, which is probably sensitive to local blobs but not to slow gradients. There is probably an assumption that the line of sight mass is tending towards zero, which will tend to give you a central blob surrounded by black.

        What happens to the dark matter blobs? Will be Bullet cluster re-combine or has the dark matter cloud detatched for good? Will it slowly evaporate if there is no normal mass to sink the kinetic energy, or will it hold together under its own mass. Is an empty bit of sky actually empty?

  3. There may be a halo of dark matter around the core that was in place before the collision. I am not sure how this might fit in with the mass reconstructions in this paper. However, suppose the two galaxies collided with each other in a region coincident with another galaxy. This may have been a direct 3-way collision, or the two galaxies collided close to and in front of the third relatively stationary galaxy. In this case the luminous matter of the two moving galaxies would merge and remain with or be within the line of sight of the matter of the stationary matter. So from our perspective or line of sight the luminous matter core would remain within the DM halo, or appear be within the DM halo, of this relatively stationary galaxy. The DM from the two moving galaxies moves outwards as with the bullet cluster.


  4. The term dark “matter” is being used, but does it really need to be “matter”?

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