New trick uses velocity maps to measure weak lensing directly
Despite dark matter dominating the Universe with 84% of the mass compared to 16% of baryonic matter, it is non-interacting and cannot be measured directly. There are techniques to measure the fraction of the two types of matter but weak gravitational lensing is the only tool available to measure the spatial distribution of dark matter relative to its baryonic counterpart in galaxies, clusters and other structures. Understanding the matter distribution in galaxies is vital for constraining our models of cosmology and for forming a complete picture of galaxy formation and evolution. Current weak lensing techniques require hundreds of galaxies for a single weak lensing measurement, they are insensitive to the shape of the dark matter halo and they are useless for analyses that require individual, direct measurements of the weak lensing distortion, called ‘shear’.
Our PhD student Catherine de Burgh-Day and her three CAASTRO co-authors from the University of Melbourne and the Australian Astronomical Observatory (AAO) have now developed a technique to directly measure the shear around galaxies with individual measurements: Direct Shear Mapping (DSM). DSM uses velocity maps to measure shear. A velocity map shows the velocity of each part of a galaxy relative to the motion of the centre of the galaxy (which in the galaxy’s frame or reference is stationary). DSM is based on the assumption that a galaxy’s velocity map is symmetrical about the axis of no rotation (i.e. the axis about which the galaxy is rotating) and the axis of maximal rotation. The distortions imposed by weak lensing destroy these symmetries. DSM uses a Markov-Chain-Monte-Carlo Maximum-Likelihood method to fit for the shear in the velocity maps of spiral and elliptical galaxies by attempting to restore symmetries. The researchers find that in simulated data DSM can measure shears to with an error of +\-0.01, and have obtained observational data for the first DSM measurement.
DSM is the first weak lensing technique which measures shear directly. This opens up exciting new possibilities for studying dark matter, with the ability to make direct measurements that are not possible with traditional weak lensing methods. For example, the team is currently working on a sample of simulated DSM measurements in a galaxy population to measure the scatter in the stellar mass-halo mass relation. With multiple DSM measurements around a single galaxy, it will be possible to measure not just the mass of the galaxy’s dark matter halo, but also the shape. This has previously only been possible with stacked weak lensing measurements.
Catherine O. de Burgh-Day, Edward N. Taylor, Rachel L. Webster, Andrew M. Hopkins in MNRAS (2015) “Direct Shear Mapping – a new weak lensing tool”