Journal Club – Shaping The Invisible

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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?