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5th of July 2016
Magnetic fields around galaxy nuclei
by Craig Anderson (CASS)
When gas spirals into supermassive black holes (SMBHs) at the heart of active galactic nuclei (AGNs), powerful, oppositely-directed, high-speed jets of gas can sometimes form. These jets then blast outwards through the galaxy into intergalactic space, depositing vast amounts of material and energy into the tenuous gas that occupies these regions. This injection of energy can have a profound effect on the evolution of galaxies, so understanding how these processes work in detail is of significant interest to astronomers. There are good reasons to believe that magnetic fields in and around AGN are important here. However, the extreme distance and small size of AGN makes it challenging to obtain a clear `image' of magnetic field structure in all but the closest examples.

Our study attempts to approach this problem in a different way. When radio waves generated by AGN jets pass through magnetised gas in the surrounding regions, they are imprinted with information about the structure of the magnetic fields in this gas, which would otherwise be too small to observe directly. Using the Australia Telescope Compact Array (ATCA) radio telescope, we have attempted to observe this imprint for about 40 AGN. The data for one example can be seen in the figure above (left panel): Were there no magnetic fields along the line of sight to this particular object, the red and blue data points (which are measurements of the strength and orientation of radio waves from the AGN at different frequencies) would fall in straight horizontal lines across the plot. Instead, it is clear that the data oscillate. We can model the detailed cause of these oscillations, and from this determine that there are three bright blobs of radio-emitting gas near this AGN, of which two are seen through complicated magnetic field structures. The right-most panel provides a real example of the sort of blobs of material in jets that might generate the behaviours that we have observed: This is a real `image' of the magnetic field structure of gas in an AGN jet emanating from the nearby galaxy M87 (Algaba et al. 2016).

Image credits: Anderson et al. (2016) [arXiv1604.01403] and Algaba et al. (2016) [arXiv1603.08437]

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