A CAT-scan view of our cosmic backyard with the MWA
Our understanding of the environment within our nook of the Milky Way Galaxy is surprisingly limited. Astronomers generally look how the light from nearby background sources (stars, nebulae, etc.) is effected but there are very few nearby sources against which this can be performed. A recent study, utilising the powerful capabilities of the Murchison Widefield Array (MWA), has employed a novel way to map the local environment using polarimetry.
Polarimetry is the measurement of the orientation of light waves as seen by an observer – it allows us to probe the environment through which the light has passed. Polarised sunglasses effectively turn our eyes into polarimetric instruments, ones that are only sensitive to vertically polarised light. Sunglasses work by restricting horizontally polarised light because reflected light, the kind that causes glare, is typically horizontally polarised.
When CAASTRO researcher Dr Emil Lenc (University of Sydney) and colleagues used the MWA as an electronic pair of sunglasses to measure the orientation of the cosmic radio waves it received, they revealed a whole new view of our local neighbourhood. Despite looking at the Galactic pole, where one would not expect to see much emission from the Galaxy itself – which is why this location is used to detect a signal from the “Epoch of Reionisation”, the MWA revealed complex, vast and bright polarised emission (click on image to view 3D reconstruction).
The interstellar medium acts like a kind of cosmic fog that affects low frequency polarised signals more than higher frequency ones. Just like a radio, the MWA can tune into different frequencies, allowing it to see polarised features at different distances. Using this capability, the researchers were effectively able to perform a “CAT-scan” of the Milky Way out to approximately 160 light years in the direction of the South Galactic Pole. Mapping this structure provided a means to measure their size and also to improve our understanding of how they formed.
Interestingly, these structures were so bright that the effect of the Earth’s ionosphere could also be studied. The ionosphere causes an extra rotation in orientation of the light waves measured by the MWA – the amount of rotation depends on the Solar weather at the time of the observation. By observing how the polarised emission rotates over time, the ionosphere can be mapped – an added side-effect that allowed two very different areas of science to be studied with the same observation. The findings pave the way for novel future applications in Solar and atmospheric science.
Emil Lenc, Bryan Gaensler et al. in The Astrophysical Journal (2016): “Low frequency observations of linearly polarized structures in the interstellar medium near the south Galactic pole”