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Analysis of the DRAO 26 m telescope rotation measure synthesis survey of galactic polarised emisson


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Thomson, AJM 2015 , 'Analysis of the DRAO 26 m telescope rotation measure synthesis survey of galactic polarised emisson', Honours thesis, University of Tasmania.

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This thesis presents an analysis of the DRAO 26m rotation measure synthesis survey. This survey applies the process of rotation measure synthesis as formulated by Burn (1966) and Brentjens and de Bruyn (2005). Rotation measure synthesis allows the computation of Faraday depth, which in turn makes it possible to study the magneto-ionic medium via the Faraday effect. The survey provides data on both polarised emission and the resultant Faraday depth across the whole sky down to a declination of δ = -30°. Analysis here is restricted to the North and South Galactic Poles. This avoids the depolarisation effects present near the Galactic plane and also allows the analysis of the vertical magnetic field at the position of the sun. The Faraday depth for each line of sight in the survey was taken to be the position peak of the Faraday spectrum and the associated polarised emission was taken to be the height of the peak.

The North Galactic Pole was analysed as a square region surrounding b = 90°. This area was a stereographic projection of the pole with an angular size of `~1600` `deg^2`. A similar region was computed for the South Galactic Pole, centred on b = -90°. The maps produced for these regions were 201x201 pixels in size and the resultant angular size for each pixel was `0.04` `deg^2`.

Analysis of the Faraday depth in the North Galactic Pole has revealed that many of the high magnitude detections are spurious. This is indicated by a result of Sun et al. (2015) that errors in Faraday depth are associated with low polarised emission and are much higher than theoretically expected. This being the case, the best method of averaging Faraday depth was found to be a weighted average, where the weight used was the reciprocal of the error in Faraday depth squared for each line of sight. In the North Galactic Pole this method gave an average Faraday depth of 0.1 ± 0.6 `rad` `m^-2`. This is the same result as obtained in the extra-Galactic source survey by Mao et al. (2010), with 3 σ confidence. The implication of this result is that there is no mean vertical magnetic field detected at the position of the sun. This result is incompatible with the theoretical dipole and quadrupole models of the Galactic magnetic field. A trend in the Faraday depth was, however, found in this survey. This trend corresponds with a magnetic field magnitude of ~ 1 μG and implies a field reversal across the North Galactic Pole. Whether the reversal occurs in the horizontal or vertical field has not been determined. The magnitude of the magnetic field detected is dependent on the distance to the emission. As data on the distance to the emission was not analysed, assumptions on the distance had to be made in order to compute the magnetic field magnitudes.

Due to the declination range of the survey approximately half of the data for the South Galactic pole was not present. Additionally, strong scanning effects were identified and had to be excluded from analysis. The remaining Faraday depth data was associated with relatively high polarised emission; especially compared to the North Galactic Pole. Applying the same weighted average method as in the North Galactic Pole, an average Faraday depth of -3.1 ± 0.2 `rad` `m^2` was found for the South Galactic Pole. This is in strong disagreement with the result found in extra-Galactic surveys. The implication of this result is that the Faraday depth detected in this survey is local. For the region of the pole that was available in this survey, this corresponds with a mean magnetic field magnitude of ~ 1 μG. Any trends across the pole could not be analysed in this survey due to the lack of coverage below a declination of δ = -30°.

Finally, the angular structure of both polarised emission and Faraday depth was quantified using the two-dimensional autocorrelation function. This was done in the South Galactic Pole, North Galactic Pole and surrounding Northern Galactic intermediate latitudes. The angular structure found was used to find a more reliable number of independent samples in the North and South Galactic Pole regions. This enabled the error analysis of both of these regions to be updated. Using these updated errors the average Faraday depth in the North and South Galactic Poles was found to be 0.1 ± 0.5 `rad` `m^2` and -3.1 ±1 `rad` `m^2` respectively.

Analysis in this project was conducted primarily with MathWorks MATLAB. Additionally, ANTF Miriad was used for data manipulation. ANTF Kvis and SAOImage Ds9 were also used for image analysis.

Item Type: Thesis - Honours
Authors/Creators:Thomson, AJM
Keywords: Interstellar medium, Milky Way, Linear polarisation, Faraday rotation
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Copyright 2015 the author

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