Modelling the Galactic magnetic field on the plane in two dimensions

Abstract

We present a method for parametric modelling of the physical components of\nthe Galaxy's magnetised interstellar medium, simulating the observables, and\nmapping out the likelihood space using a Markov Chain Monte-Carlo analysis. We\nthen demonstrate it using total and polarised synchrotron emission data as well\nas rotation measures of extragalactic sources. With these three datasets, we\ndefine and study three components of the magnetic field: the large-scale\ncoherent field, the small-scale isotropic random field, and the ordered field.\nIn this first paper, we use only data along the Galactic plane and test a\nsimple 2D logarithmic spiral model for the magnetic field that includes a\ncompression and a shearing of the random component giving rise to an ordered\ncomponent. We demonstrate with simulations that the method can indeed constrain\nmultiple parameters yielding measures of, for example, the ratios of the\nmagnetic field components. Though subject to uncertainties in thermal and\ncosmic ray electron densities and depending on our particular model\nparametrisation, our preliminary analysis shows that the coherent component is\na small fraction of the total magnetic field and that an ordered component\ncomparable in strength to the isotropic random component is required to explain\nthe polarisation fraction of synchrotron emission. We outline further work to\nextend this type of analysis to study the magnetic spiral arm structure, the\ndetails of the turbulence as well as the 3D structure of the magnetic field.\n