The invisible world of magnetic fields

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.

(a) Magnetic field loops on the Sun, captured by NASA's TRACE satellite [1];  (b) An optical image of M51 from the Hubble Space Telescope, overlaid by total radio intensity contours from the VLA and Effelsberg telescope. Magnetic field vectors are also overplotted [2]; (c) The RM colour image of an observed radio source 5C4.114 in the Coma cluster, overlaid by total intensity radio contours at 4.935 GHz [3]; (d) Simulated image showing the log of magnetic field strength at current epoch, ranging from 0.1 nG (yellow) to 10 G (magenta). Obvious features include clusters and groups (magenta and blue) and filaments (green) [4].

(a) Magnetic field loops on the Sun, captured by NASA’s TRACE satellite [1]; (b) An optical image of M51 from the Hubble Space Telescope, overlaid by total radio intensity contours from the VLA and Effelsberg telescope. Magnetic field vectors are also overplotted [2]; (c) The RM colour image of an observed radio source 5C4.114 in the Coma cluster, overlaid by total intensity radio contours at 4.935 GHz [3]; (d) Simulated image showing the log of magnetic field strength at current epoch, ranging from 0.1 nG (yellow) to 10 G (magenta). Obvious features include clusters and groups (magenta and blue) and filaments (green) [4].

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.

A snap-shot image of a simulated cluster at current epoch from the GCMHD+ simulation. The left panel shows the cluster X-ray flux, with material infalling into the hot core. The middle panel shows the corresponding RM map, with the strongest magnetic fields in red and black. The right panel shows the power-law relation between X-ray flux and the standard deviation of RM [5].

A snap-shot image of a simulated cluster at current epoch from the GCMHD+ simulation. The left panel shows the cluster X-ray flux, with material infalling into the hot core. The middle panel shows the corresponding RM map, with the strongest magnetic fields in red and black. The right panel shows the power-law relation between X-ray flux and the standard deviation of RM [5].

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.


Image credits:

[1] NASA/LMSAL

http://www.newscientist.com/article/dn7858-new-technique-pinpoints-solar-flareups.html#.VHzs_1esVss

[2] MPIfR (R. Beck) and Newcastle University (A. Fletcher)

http://www.mpifr-bonn.mpg.de/research/fundamental/cosmag

[3] Bonafede A. et al., A&A, 2010, 513, A30

http://arxiv.org/pdf/1002.0594v1.pdf

[4] Ryu D. et al., Science, 2008, 320, 909-912

http://www.sciencemag.org/content/320/5878/909.short

[5] On A. et al., MNRAS, 2014, in prep.

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