Journal Club – Shaping The Invisible

Article written: 25 Feb , 2012
Updated: 24 Dec , 2015


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?

15 Responses

  1. ???????? ???????????? says

    I very much enjoy the articles of Journal Club. I also found very interseting, an article I read recently, about a published research conserning dark matter. This research analysed the way light was gravitationally bent from distant clusters and superclusters and concluded that indeed every single galaxy is enveloped with dark matter, and the length of this halo streches across intergalactic space, connecting every galaxy’s halo together. So practically there is no such thing as an intergalactic void and what we perceive as ’empty space’, in reality is filled with dark matter and the galaxies we see are just the visible part of it, just like icing on the cake! I find that trully fascinating!

    Could I suggest a future article about the critisism being made towards Big Bang cosmology, the things about the universe that this cosmological model has problems explaining despite its success with other things it can explain? And an analysis of what if the universe didn’t have a start and what if it’s eternal.

    Best regards

    • Member
      Steve Nerlich says

      Thanks. Not sure JC has taken the world by storm, but it’s early days 🙂

      I think that article was mentioned on SGU this morning. They say that DM might extend out beyond the edges of the galaxy into intergalactic space. Seems to be what most people were assuming anyway and seems largely an exercise in computer modelling of grav. lensing data. But, nice work all the same.

      I don’t think anyone is ruling out genuine voids between galactic clusters.

      Re article suggestion. Wow – that’s a lot of ground to cover. Maybe we can do that in wafer-thin slices.

    • Torbjörn Larsson says

      Since the ruling standard cosmology is a big bang cosmology it is less of remaining criticism than incompleteness around it. You will find more criticism of alternatives, say cyclical cosmologies have inherent problems that the standard cosmology lacks.

      On the other hand parts of the standard cosmology are weak by themselves, for example inflation is not fully tested on its own to acceptable standards, it is also poorly constrained and to top it off it has inherent problems from singularities and in mechanisms.

      One can certainly discuss incompleteness of the standard cosmology. But a larger and more interesting (opinionated!) discussion can be made on its weak parts. (Inflation, vacuum instability and its finetuning with a quasi-stable 125 GeV standard Higgs – if it is what LHC finds*).

      * Since everything else in cosmology (cosmological constant) and its particle physics (standard model parameter set) is fine tuned to similar high order, I don’t see how Higgs would not be finetuned too. It seems to be the theme.

      Environmental (“anthropic”) principle, anyone?

  2. lcrowell says

    This is a rather long and complicated paper. It may be some time before I can digest this.

    The distribution of DM might have these complex structures. I do think that this can only be deduced by looking at the gravitational lensing of more distant objects. In that way the form factor of mass distribution can be constructed.


  3. HeadAroundU says

    1. Let’s assume for a while that it repulses itself.
    2. Jargon is fine, but diddly-squat? 😀 Crikey!
    3. Shall we extensively talk about about Dr. Pamela Gay in the next edition? 😀

  4. Peristroika says

    I would think, while knowing next to nothing, that if dark matter can’t even be bothered to interact with baryonic matter, that it’s hardly likely to self-annihilate, repulse, or any other noticeable tack. The only thing it seems to do is to maintain its own space ie: not condense past being a gas. I couldn’t read the article so thanks for the synopsis.

  5. Torbjörn Larsson says

    Despite being an interesting paper it is too long for a weekend. Maybe Journal Club could set itself a page limit (which can be voided for specific cases) or suggest subset of papers?

    why ‘halo contraction’ (presumably jargon for ‘dark matter concentration’) occurs in these galaxy types more often than others

    Not having read the paper I can only guess that the Eris simulation sets up a proper spiral galaxy DM profile while the used Bolshoi simulation will lack the needed mechanisms.

    Why … should we assume it possesses the ability to self-annihilate?

    To have a hopefully useful working hypothesis to constrain DM properties. But it is not altogether “search where the streetlight shines” but is also a a likely supersymmetry Standard Model extension predicting DM as WIMPs.

  6. super_earth says

    It will be fine to have a gamma ray map annihilating dark matter.

    The conclusion “Halo contraction boosts significantly dark matter annihilation strength in the halos
    embedding early-type galaxies so that nearby early-type galaxies may be promising
    targets for indirect dark matter search” implies that making that job is a bit less hard than thought.

    It will remain almost certainly still very difficult, but maybe in a decade or two we will have an array of gamma ray telescopes to have a decent map of not-so-dark DM.

    Meanwhile, it seems that looking with FERMI-LAT to galaxy clusters is beginning to bear fruits:

    “Evidence for extended gamma-ray emission from galaxy clusters” (jan. 2012)

    From the abstract: “For all three clusters, excess emission is observed within three degrees of the center, peaking at the GeV scale. This emission cannot be accounted for by known Fermi sources or by the galactic and extragalactic backgrounds. If interpreted as annihilation emission from supersymmetric dark matter (DM) particles, the data prefer models with a particle mass in the range 20-60 GeV annihilating into the b-bbar channel, or 2-10 GeV and >1 TeV annihilating into mu-mu final states”


    Well, this is consistent with findings in our galactic centre:

    “On The Origin Of The Gamma Rays From The Galactic Center” (sept. 2011)

    From the abstract: “If interpreted as dark matter annihilation products, the emission spectrum favors dark matter particles with a mass in the range of 7-12 GeV (if annihilating dominantly to leptons) or 25-45 GeV (if annihilating dominantly to hadronic final states)”

    “Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope” (oct. 2010)

    From the abstract: “The observed spectrum of this component, which peaks at energies between 2-4 GeV (in E2 units), is well fit by that predicted for a 7.3-9.2 GeV dark matter particle annihilating primarily to tau leptons”

    And is also consistent with the preliminary results of the direct detection experiments DAMA/LIBRA, CoGeNT and CRESST , that, quoting Hooper (one of the authors of the papers about the Milky Way galactic center),”each report signals consistent with an approximately 10 GeV dark matter particle”.

    However, the light mass results above are in tension with the non-detection of signals of light mass (around 10 GeV) WIMPS done by the XENON100 and CDMS detectors…

    And of course, no particle similar to that was found by the LHC so far, ruling out Supersymmetric WIMPS below a few hundred GeV.

    One thing more:

    The 2011 paper showed in figure 8 that for the muon channel there are two hills in the likelihood ratio statistic (Test Statistic, TS in short), one between 2 and 10 GeV and the other above 1 TeV. Could this indicate that instead of ONE dark matter particle there are TWO of them (or even more)?

    Any guess about what Nature is hiding?

    • lcrowell says

      I got here a bit late, and will try to write more on this tomorrow. The DM annihilations could be due to a number of physical processes. One is that the neutralino, a condensate of the supersymemtric pair of the photon, Z and Higgs particle, may be a Majorana particle. This means it is its own anti-particle and neutralino interactions lead to self-annihilations. Low mass WIMPS are difficult to physically justify. We might expect to have seen them in accelerator physics. However, maybe they are supersymmetric and get their masses from the MSSM Higgs charged sector at high energy. So maybe even if they are low mass they might not have been produced as yet.


      • super_earth says

        Could be TWO dark matter WIMP particles?

        Fermi-LAT and PAMELA found a gamma ray excess consistent with a TeV scale dark matter annihilating particle …

        …but Fermi-LAT found also another excess consistent with a WIMP particle between 10 GeV and 100 GeV. Also we have the direct detection experiments…

    • lcrowell says

      There should exist a supersymmetric partner to every elementary particle in the standard model spectrum, and if there is a GUT theory the same should hold. Supersymmetry (SUSY) is a very compelling idea, but it does not tell us anything about either the spectrum of elementary particles or their masses. There are though some hints of SUSY in diphoton events. An overview of this can be found at

      Supersymmetry works like this. Suppose you have some group G that pertains to the symmetries of elementary particles, or in particular a gauge theory. An element of G is g = e^{i?}, where ? in A, where A is the algebra of generators for G. Then given ? and ? in A the gauge group is given by a commutator [?, ?] = ? in A. SUSY is a grading of the algebra with operators a and b which obey anti-commutators

      {a, b} = ab + ba = ip,

      for p an operator or the momentum. In a gauge covariant form p — > p + i?, where ? is a gauge connection in the group G. Therefore an anticommutation of two graded operators gives an element in G. This extension of the group G to G’ = G xS (S = anticommutator graded part with generators s) leads to the following rules written cryptically as

      [A, A] = A — commutators of ? in A gives elements in A

      {s, s} = A — anticommutators in s gives elements in A

      [s, A] = s. commutators between elements in s and A give s.

      This in an abstract nutshell is SUSY. This needs to be broken out a bit more with Grassmann variables ? and ?-bar (bar means hermitean conjugate time the 0 Dirac matrix). If we have ? in A then there is a superfield ? which includes elements of

      ? = ? + ?-bar b + ?b-bar + ??-bar F

      for ? in A and b in s and F a constraint. These types of superfields describe doublets, where ? and b are particle fields that are supersymmetric partners or pairs.

      This is a very general scheme, and it can be extended for families of Grassmann variables which define N = 2, 4 and 8 SUSY. However, SUSY tells us very little by itself what the group G is. Further this is a massless theory, which with partners between bosons and fermions gives a zero vacuum energy. In a low energy setting these fields have broken symmetry and masses. SUSY gives little prescription for that as well. Lots of phenomenology gets pinned onto SUSY gauge theory, and the physics literature has literally tons of papers with various ideas about this. The most direct form is the minimally supersymmetric standard model.

      Now we turn to dark matter. Dark matter could be composed of a whole range of supersymmetric particles which do not interact with the photon. There could literally be an alternate world existing right beside us. However, due to the extent of galactic halos this stuff is “cold” and weakly interacting with not just ordinary matter but with itself as well. So this world is probably “boring” when compared to what we can directly observe. The lowest energy form of a supersymmetric particle field is from the SUSY pairs of the neutral Higgs, photon and Z particle, all of which have the same quantum numbers. This is thought to form a condensate called the neutralino. This particle field is most naturally a Majorana fermion, which means it is its own antiparticle. This is the “first order” theory for dark matter, which could be right — then again maybe not.

      There is another possible source for dark matter, which could exist in conjunction with the above SUSY model. The weak interactions have at low energy parity violation, or CP asymmetry. The nuclear force or QCD does not. This distinction has been a bit of a mystery. It has been proposed that a type of dilaton or scalar field exists with QCD which carries the CP violating signature. This is called the axion. I would need to break into some string theory in order to really make light of this. It is possible that the universe produced these particles in the matter dense regions, where these form the galactic halo or some portion of it. These particles, which are scalars, could constitute the lower GeV massed particles which might compose the galactic halo. There are some problems with positing this, for the axion particle has a very small mass ~ 10^{-6} to 1ev. However, this particle has a supersymmetric partner that is in the GeV range. So axions and their superpartners could be a constituent of dark matter as well.


      • super_earth says

        In the following matematical statements that you wrote about supersymmetry:

        ? in A

        ? in A

        b in s and F

        What does the “” in mean?

      • lcrowell says

        The back slash is used in TeX (LaTeX, AMSTeX, RevTeX) macro based scripting program. This is used in the physics and math community.


      • super_earth says

        But what does mean the “” in a mathematical (not in computer programming) sense?

      • lcrowell says

        The LaTeX macro system represents the Greek letter ? by $rho$. The text that you write in that form which run through the TeX interpreter replaced $rho$ with ?. One can then write complicated mathematical equations this way. For in appearing in an equation, bracketed by two $-signs. the result is a symbol which looks like the epsilon, used in mathematics to denote some element contained in a set or an algebraic system or group. The backslash is symbol which tells the TeX interpretor to replace the written word with a particular symbol spelled out by that word.


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