Astronomy without a telescope -- Through a lens darkly

Massive galactic clusters – which are roughly orientated in a plane that is roughly face-on to Earth – can generate strong gravitational lensing. However, several surveys of such clusters have reached the conclusion that these clusters have a tendency towards lensing too much – at least more than is predicted based on their expected mass.

Known (to some researchers working in the area) as the ‘over-concentration problem’, it does seem to be a prima facie case of missing mass. But rather than just playing the dark matter card, researchers are pursuing more detailed observations – if only to eliminate other possible causes.

The Sunyaev-Zel’dovich (SZ) effect is a novel way of scanning the sky for massive objects like galactic clusters – which distort the Cosmic Microwave Background (CMB) via inverse Compton scattering – where photons (in this case, CMB photons) interact with very energized electrons which impart energy to the photons during a collision, shifting the protons to a shorter wavelength frequency.

The SZ effect is largely independent of red-shift – since you start with the most consistently red-shifted light in the universe and are looking for a one-off event that will have the same effect on that light whether it happens close by or far away. So, with equipment sensitive to CMB wavelengths, you can scan the whole sky – detecting both close objects, which might be directly observable in optical, as well as very distant objects which may have been red-shifted into the radio spectrum.

The SZ effect causes CMB distortions in the order of one thousandth of a Kelvin and the effect does require really massive structures – a single galaxy is not sufficient to generate the SZ effect on its own. But, when it works – the SZ effect offers a method to measure the mass of a galactic cluster – and does it in a way that is quite different to gravitational lensing.

The SZ effect is thought to be mediated by electrons in the inter-cluster medium. This means that the SZ effect is solely the result of baryonic matter, since it is a consequence of the inverse Compton effect. However, gravitational lensing is the result of the warping of space-time – which is partly due to the presence of baryonic matter, but also of dark (i.e. non-baryonic) matter.

Gralla, et al. used the Sunyaev-Zel’dovich Array, an array of eight 3.5 meter radio telescopes in California, to survey 10 strongly lensing galactic clusters. They found a consistent tendency for the Einstein radius of each gravitational lens to be around twice the value expected for the mass, determined from the SZ effect, of each cluster.

The Einstein radius is a measure of the size of the Einstein ring that would be formed if a cluster was exactly orientated in a plane that was exactly face-on to Earth – and where you, the lens and the distant light source being magnified, are all in a straight line of sight. Strongly lensing galaxies are generally only in close approximation to this geometry, but their Einstein ring and radius (and hence their mass) can be inferred easily enough.

Gralla, et al. note that this is work in progress, for now just confirming the over-concentration problem found in other surveys. They suggest one possibility is that the amount of inter-cluster medium may be less than expected – meaning that the SZ effect is underestimating the real mass of the cluster.

If, alternatively, it is a dark matter effect, there would be more dark matter in these clusters than the current ‘standard model’ for cosmology (Lambda-Cold Dark Matter) predicts. The researchers seem intent on undertaking further observations before they go there.