Understanding magnetic fields in circumgalactic gas

Abstract

Many galaxies in the Universe, including our own Milky Way, are sites of star formation. For them to continue producing stars over cosmic time, they require a steady supply of cold gas. Situated between the galactic interstellar medium and the intergalactic medium, the circumgalactic medium (CGM) is a home for gas of diverse origins. This vast gas reservoir is a potential source of fuel for star formation within a galaxy, provided that cold circumgalactic clouds effectively descend towards the galaxy. However, the safe delivery of cold gas is often interrupted by interactions between the clouds and the surrounding medium. Numerical models suggest the formation of a strongly magnetised layer across the cloud-halo interface, which suppresses the growth of hydrodynamic instabilities. The importance of understanding CGM magnetism has been widely recognized by the theoretical astrophysics community, but little has been uncovered about the observed properties of magnetic fields in CGM.

This thesis is written to lay the groundwork for the rapidly emerging era of observational magnetism study and provide a glimpse of what will be possible in the near future. To do so, I use two powerful tools in modern astronomy: radio wavelength observations and numerical simulations. From an observational standpoint, I evaluate the methodology to detect magnetised circumgalactic clouds using the Faraday rotation measure (RM) of extragalactic radio sources. From a theoretical standpoint, I investigate how the evolution of clouds is interconnected with the presence of magnetic fields.

This thesis is a compilation of three research papers of which I am the primary author. In the first research chapter, I present research work in which we revisit one of the previously suggested candidates for magnetised clouds in the Milky Way CGM: the Magellanic Leading Arm. We conduct a thorough investigation of the Galactic Faraday rotating screens in the region and demonstrate that there has been confusion due to Faraday rotation at a foreground supernova remnant. This finding indicates that it is impossible to draw any definitive conclusions about the presence of magnetic fields in the Magellanic Leading Arm from the extragalactic RM measurements alone. As we have withdrawn a previous suggestion that the Magellanic Leading Arm is potentially magnetised, only a handful of clouds in the Milky Way CGM remain as observationally confirmed candidates for magnetised clouds.

The second research chapter is a theoretical investigation of the evolution of circumgalactic clouds using a set of 3D magnetohydrodynamic simulations. This work is distinguished from earlier studies with similar scientific motives in that we model clouds with density structures prescribed by observations, as opposed to simplified spherical uniform-density clouds. We have identified that various physical processes governing the evolution of high-velocity circumgalactic clouds, such as ram pressure stripping, the growth of hydrodynamic instabilities, and radiative cooling, are intimately linked to the presence of magnetic fields as well as clumpy substructures of the clouds.

Finally, in the third research chapter, I continue the discussion on the detectability of magnetised circumgalactic clouds. We utilise a suite of cosmological simulations and construct the Faraday rotation sky observed by an observer placed inside galaxies in the simulations. We sample the synthetic RM grids following the specifications of current and future radio polarimetric surveys. We show that the detection rate of magnetised circumgalactic clouds will significantly improve with the enhanced polarised source density and the RM measurement accuracy of upcoming radio instruments, such as the Square Kilometre Array.

The magnetised Universe has long been an under-explored piece in the galaxy evolution framework, but this thesis points to a bright future ahead that will shed light on the unknowns. Show more