Category: galaxies

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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?

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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 and how to determine where it is and how dense it is – although still without actually seeing it.

Today’s article:
Chae et al Dark matter density profiles of the halos embedding early-type galaxies: characterizing halo contraction and dark matter annihilation strength.

We can see how the gravitational influence of invisible dark matter is affecting the general morphology of a galaxy and the motion of the stars within that galaxy. These factors can then hint at where the dark matter is and how dense it is.

Traditional thinking positions dark matter in a halo shape around a galaxy – meaning more of it is outward than inward – which helps explain why visible objects in the outer rim of a galaxy seem to orbit the galactic center at about the same periodicity as inner visible objects. This is contrary to our local Keplerian understanding of orbital mechanics where close-in Mercury orbits the Sun (containing over 99% of the solar system’s mass) in 88 days while distant Neptune takes a leisurely 165 years.

We assume galaxies’ relatively even periodicities are a result of each galaxy’s total mass (visible and dark) being distributed throughout its structure and not concentrated in its center.

The authors use the term ‘early-type’ galaxy to describe their target population for this research. ‘Early-type’ seems unnecessary jargon – being a reference to the Hubble sequence, for which Hubble explained at some length that he was just putting galaxies in a sequence for ease of classification and he did not mean to imply any temporal sequence from the arrangement.

As it happens, our modern understanding is that these ‘early’ types, the elliptical and lenticular galaxies, are actually some of the oldest galaxy forms around. Young galaxies tend to be bright spirals. Over time, these spirals either fade, so you no longer see their spiral arms (lenticulars), or they collide with other galaxies and their ageing stars get jumbled up into random orbits to form big, blobby shapes (ellipticals).

So everywhere you see ‘early-type’ in this article – you should substitute elliptical and lenticular. Jargon prevents the general reader from being able to follow the meaning of a specialist writer – you don’t have to do this to be a scientist.

Anyhow, the researchers conducted a statistical analysis of the estimated stellar mass values and velocity dispersions of star populations within different elliptical and lenticular galaxies. Their objective was to try and get a fix on the distribution of the invisible dark matter that we think all galaxies contain.

Their analysis found that dark matter was more concentrated towards the centers of elliptical and lenticular galaxies – and the authors conclude that nearby elliptical and lenticular galaxies might hence be ideal candidates for the identification of gamma ray output from dark matter annihilation.

The last suggestion seems a bit of an intellectual leap. There have been a few reported observations of radiation output of uncertain origin from the centers of galaxies. Dark matter annihilation has been one suggested cause – but you’d think there’s a lot of stuff going on in the center of a galaxy that could offer an alternate explanation.

I could not find in the paper any suggestions as to why ‘halo contraction’ (presumably jargon for ‘dark matter concentration’) occurs in these galaxy types more often than others – which seemed the more obvious point to offer speculation on.

So… comments? Why, when knowing diddly-squat about the particle nature of dark matter, should we assume it possesses the ability to self-annihilate? Is ‘early-type’ unnecessary jargon or entrenched terminology? Is the question ‘does anyone want to suggest an article for the next edition of Journal Club’ just rhetorical?