Mapping the Galactic magnetic field with OH masers

This thesis presents results and analysis of the survey “Mapping the Galactic Magnetic Field with OH masers”, which observed hydroxyl (OH) masers towards hundreds of high-mass star forming regions (HMSFRs) traced by 6.7-GHz methanol masers. The observations were performed with the Australia Compact Array Telescope in full-polarization mode at frequencies covering all four ground-state OH maser transitions, 1612-, 1665-, 1667-, and 1720-MHz. The main objective of the survey was to investigate if the magnetic field direction inferred from the small scale OH masers can be used to recover the residual imprint of the Galactic magnetic field at larger kiloparsec scales. 

This work presents the largest catalogue of ground-state OH masers associated with HMSFRs (strictly associated with 6.7-GHz methanol masers) detected in the Southern-hemisphere. The catalogue consists of 573 detected OH maser sources over the four transitions, of which 177 are newly detected sources. The catalogue provides maser positions, magnetic field strengths, maximum linear and circular polarization, peak and range of source velocity, Stokes I, Q, U, V flux densities, integrated flux densities, integrated luminosities, and association of the OH masers with other maser species. We found that the median magnetic field strengths varied across the transitions with 3.3, 0.4, 0.8, and 6.1mG for 1612-, 1665-, 1667-, and 1720-MHz respectively, and that they covered a range largely consistent with expected values in HMSFRs. Furthermore, we determined that more than 90% of OH masers have fractional circular polarization greater than 50%, and conversely, the majority of all OH maser sources have fractional linear polarization less than 50 per cent, confirming that OH masers associated with HMSFRs are preferentially more circularly polarized than linearly polarized.

From the sample of 573 ground-state OH maser sources detected towards 702 HMSFRs, we found 352 Zeeman pairs in 163 of the high-mass star forming regions, from which magnetic field strength and line-of-sight direction were deduced. Analysis of the inferred field direction in individual HMSFRs shows field coherence in at least 74% of the selected 96 HMSFRs, implying ordered fields at such scales. We also examined the fields at the intermediate scales of giant molecular clouds (GMCs), and at then at the larger scale of the spiral arms and sub-regions within them. Our investigation of two GMCs revealed a disordered distribution of field directions within them. Likewise, we do not find evidence that the field traced by OH masers retains the residual imprint of the Galactic-scale field in the spiral arms, or in sub-regions of the arms. Additionally, we do not find a strong correlation between the line-of-sight magnetic field direction inferred from OH masers and the large scale fields revealed by Faraday rotation measurements.