Hi, my name is Alvina On and I am a final-year PhD student in the Theoretical Astrophysics Group in MSSL. My main research interests are on the evolution and diagnostics of large-scale magnetic fields. In this post, I will talk about some of my work on how magnetic fields evolve in galaxy clusters as the latter form.The Universe is magnetised. Magnetic fields are observed in stars, galaxies, galaxy clusters and the cosmic web (Figure 1). Our knowledge on stellar magnetism, such as the Sun, is derived primarily from evidence of Zeeman splitting, in which the amount of splitting in the spectral lines gives some information about the magnetic field strength. At the galactic and cluster scales, the existence of magnetic fields are deduced from radio synchrotron emission and measurements of Faraday rotation. Radio synchrotron is radiation emitted from electrons travelling at relativistic speeds under the influence of a magnetic field. Synchrotron radiation is inherently polarised and changes in its polarisation angle via Faraday rotation can be used to infer the magnetic field properties. Studies (e.g. Govoni et al. 2001; Dolag et al. 2001) have shown that the standard deviation of Faraday rotation measure (RM) and X-ray flux at each position in clusters appear to correlate, following a power-law relation. In this work, I investigate how the magnetic fields in clusters develop as the clusters evolve. I also look at how the time dependence imprints signatures observable in the radio and X-ray wavelengths. Using model clusters from cosmological GCMHD+ simulations by Barnes et al. (2012), I calculate the X-ray flux and RM from these clusters following their evolution, to determine their time-dependent properties.
My calculations produce snap-shot X-ray flux and RM images at different redshifts (an example in Figure 2). My calculations also confirm that the X-ray flux and RM in the clusters correlate and follow a power-law relation, indicating that the magnetic field strength scales with density locally. The power-law is however not steady and the slope co-evolve with structure formation. Given that the power-law relation holds, it is possible to derive the magnetic field strength and length scale, from the X-ray flux and standard deviation of RM. These works on time-dependent magnetism in clusters can be compared with future observational results from ATHENA+ which can see clusters in the X-ray up to z ~ 2.
 MPIfR (R. Beck) and Newcastle University (A. Fletcher)
 Bonafede A. et al., A&A, 2010, 513, A30
 Ryu D. et al., Science, 2008, 320, 909-912
 On A. et al., MNRAS, 2014, in prep.