New Insights into the Milky Way Magnetic Fields through Radio Broadband Spectro-polarimetry

Magnetic field is an essential ingredient of the interstellar medium of galaxies. In particular, an accurate characterisation of the magnetic field strength and structure of the Milky Way is crucial for complete understanding of many Galactic astrophysical processes. The Faraday rotation effect can be exploited to reveal the strength and direction of the magnetic field component parallel to the line of sight, which are imprinted in the rotation measure (RM) or Faraday depth (FD) values obtained from radio polarisation observations. In this thesis, I utilised the broadband spectro-polarimetric capability of the Karl G. Jansky Very Large Array (VLA) in L-band (1–2 GHz) to gain new insights into the magnetic fields of the Milky Way in two ways. Firstly, I investigated in the reliability of the NRAO VLA Sky Survey (NVSS) RM catalogue, which is the largest RM catalogue to date with 37,543 RM values north of declination of -40° (more than one RM per sq deg). The wide sky coverage combined with the high RM density have made the NVSS RM catalogue a potent data set for the study of Galactic magnetism, including modelling of the global Milky Way magnetic fields. A full understanding of systematics of the NVSS RM catalogue is therefore indispensable to our knowledge of the magnetism of the Milky Way. In particular, I have quantified the effects of nπ-ambiguity and off-axis instrumental polarisation in the NVSS RM catalogue. Secondly, I performed new radio observations for a direct study of the magnetic fields in the mid-plane of the Milky Way in the first Galactic quadrant. This specific region hosts a complex magnetic field structure being referred to as the large-scale field reversal, which has its details such as the field strength and the exact location poorly constrained owing to the lack of reliable RM values of background polarised extragalactic radio sources (EGSs). My new observations have led to a drastic increase in the number of lines of sight with probed magnetic fields by a factor of five, from which I have discovered new features in the Galactic magneto-ionic medium that were previously unnoticed

Radio Polarisation Study of High Rotation Measure AGNs

As radio polarised emission from astrophysical objects traverse through foreground magnetised plasma, the physical conditions along the lines of sight are encrypted in the form of Rotation Measure (RM). We performed broadband spectro-polarimetric observations of high Rotation Measure (|RM| >~ 300 rad m-2) sources away from the Galactic plane (|b| > 10 deg) selected from the NVSS RM catalogue. The main goals are to verify the NVSS RM values, which could be susceptible to n{\pi}-ambiguity, as well as to identify the origin of the extreme RM values. We show that 40 % of our sample suffer from n{\pi}-ambiguity in the NVSS RM catalogue. There are also hints of RM variabilities over ~20 years epoch for most of our sources, as revealed by comparing the RM values of the two studies in the same frequency ranges after correcting for n{\pi}-ambiguity. At last, we demonstrate the possibility of applying QU-fitting to study the ambient media of AGNs.Comment: 6 pages, 2 figures; Accepted by MDPI Galaxies; Conference Proceedings for the Polarised Emission from Astrophysical Jets meeting on June 12-16 2017, Ierapetra, Greec

Sampling the Faraday rotation sky of TNG50: Imprint of the magnetised circumgalactic medium around Milky Way-like galaxies

Faraday rotation measure (RM) is arguably the most practical observational tracer of magnetic fields in the diffuse circumgalactic medium (CGM). We sample synthetic Faraday rotation skies of Milky Way-like galaxies in TNG50 of the IllustrisTNG project by placing an observer inside the galaxies at a solar circle-like position. Our synthetic RM grids emulate specifications of current and upcoming surveys; the NRAO VLA Sky Survey (NVSS), the Polarisation Sky Survey of the Universe's Magnetism (POSSUM), and a future Square Kilometre Array (SKA1-mid) polarisation survey. It has been suggested that magnetic fields regulate the survival of high-velocity clouds. However, there is only a small number of observational detections of magnetised clouds thus far. In the first part of the paper, we test conditions for the detection of magnetised circumgalactic clouds. Based on the synthetic RM samplings of clouds in the simulations, we predict upcoming polarimetric surveys will open opportunities for the detection of even low-mass and distant clouds. In the second part of the paper, we investigate the imprint of the CGM in the all-sky RM distribution. We test whether the RM variation produced by the CGM is correlated with global galaxy properties, such as distance to a satellite, specific star formation rate, neutral hydrogen covering fraction, and accretion rate to the supermassive black hole. We argue that the observed fluctuation in the RM measurements on scales less than 1 degree, which has been considered an indication of intergalactic magnetic fields, might in fact incorporate a significant contribution of the Milky Way CGM.Comment: 18 pages, 11 figures, Accepted to MNRA

A new method for spatially resolving the turbulence driving mixture in the ISM with application to the Small Magellanic Cloud

Turbulence plays a crucial role in shaping the structure of the interstellar medium. The ratio of the three-dimensional density contrast ($\sigma_{\rho/\rho_0}$) to the turbulent sonic Mach number ($\mathcal{M}$) of an isothermal, compressible gas describes the ratio of solenoidal to compressive modes in the turbulent acceleration field of the gas, and is parameterised by the turbulence driving parameter: $b=\sigma_{\rho/\rho_0}/\mathcal{M}$. The turbulence driving parameter ranges from $b=1/3$ (purely solenoidal) to $b=1$ (purely compressive), with $b=0.38$ characterising the natural mixture (1/3~compressive, 2/3~solenoidal) of the two driving modes. Here we present a new method for recovering $\sigma_{\rho/\rho_0}$, $\mathcal{M}$, and $b$, from observations on galactic scales, using a roving kernel to produce maps of these quantities from column density and centroid velocity maps. We apply our method to high-resolution HI emission observations of the Small Magellanic Cloud (SMC) from the GASKAP-HI survey. We find that the turbulence driving parameter varies between $b\sim 0.3$ and $b\sim 1.0$ within the main body of the SMC, but the median value converges to $b\sim0.51$, suggesting that the turbulence is overall driven more compressively ($b>0.38$). We observe no correlation between the $b$ parameter and HI or H$\alpha$ intensity, indicating that compressive driving of HI turbulence cannot be determined solely by observing HI or H$\alpha$ emission density, and that velocity information must also be considered. Further investigation is required to link our findings to potential driving mechanisms such as star-formation feedback, gravitational collapse, or cloud-cloud collisions.Comment: 20 pages, 16 figures, accepted to MNRA

The Rapid ASKAP Continuum Survey III: Spectra and Polarisation In Cutouts of Extragalactic Sources (SPICE-RACS) first data release

The Australian SKA Pathfinder (ASKAP) radio telescope has carried out a survey of the entire Southern Sky at 887.5 MHz. The wide area, high angular resolution, and broad bandwidth provided by the low-band Rapid ASKAP Continuum Survey (RACS-low) allow the production of a next-generation rotation measure (RM) grid across the entire Southern Sky. Here we introduce this project as Spectral and Polarisation in Cutouts of Extragalactic sources from RACS (SPICE-RACS). In our first data release, we image 30 RACS-low fields in Stokes I, Q, U at 25" angular resolution, across 744-1032 MHz with 1 MHz spectral resolution. Using a bespoke, highly parallelised, software pipeline we are able to rapidly process wide-area spectro-polarimetric ASKAP observations. Notably, we use `postage stamp' cutouts to assess the polarisation properties of 105912 radio components detected in total intensity. We find that our Stokes Q and U images have an rms noise of ∼ 80 μJy PSF-1, and our correction for instrumental polarisation leakage allows us to characterise components with ≳ 1% polarisation fraction over most of the field of view. We produce a broadband polarised radio component catalogue that contains 5818 RM measurements over an area of ∼ 1300 deg2 with an average error in RM of 1.6+1.1-1.0 rad m-2, and an average linear polarisation fraction 3.4+3.0-1.6%. We determine this subset of components using the conditions that the polarised signal-to-noise ratio is > 8, the polarisation fraction is above our estimated polarised leakage, and the Stokes I spectrum has a reliable model. Our catalogue provides an areal density of 4±2 RMs deg-2; an increase of ∼ 4 times over the previous state-of-the-art (Taylor, Stil, Sunstrum 2009, ApJ, 702, 1230). Meaning that, having used just 3% of the RACS-low sky area, we have produced the 3rd largest RM catalogue to date. This catalogue has broad applications for studying astrophysical magnetic fields; notably revealing remarkable structure in the Galactic RM sky. We will explore this Galactic structure in a follow-up paper. We will also apply the techniques described here to produce an all-Southern-sky RM catalogue from RACS observations. Finally, we make our catalogue, spectra, images, and processing pipeline publicly available

Radio polarisation study of the snail pulsar wind nebula in supernova remnant G327.1-1.1

Pulsar wind nebulae (PWNe) are believed to be acceleration sites of cosmic rays in the Galaxy. In the acceleration process, magnetic field plays an important role. Radio polarisation measurements offer a direct probe of the magnetic field geometry of astronomical objects, but such experiments have rarely been conducted on PWNe, especially for evolved systems. PWNe can be crushed by the supernova reverse shock at an age of ~ 10 kyr. Previous hydrodynamical simulations show that such interactions can result in a turbulent environment in the nebula interior, suggesting a tangled magnetic field. In this thesis, I present a radio study of the Snail PWN in the composite supernova remnant G327.1−1.1 using the Australia Telescope Compact Array. This PWN is believed to have already interacted with the supernova reverse shock. The study reveals a highly ordered magnetic field configuration in the Snail, which can be explained if either the shockwave could not penetrate into the PWN interior to drive the turbulence, or the characteristic turbulence scale is large. A toy model is built to estimate the turbulence scale assuming the latter scenario. It is found that a simulated PWN with a turbulence scale of one-eighth to one-sixth of the nebula radius and a pulsar wind filling factor of 50–75% can match the observation results. This suggests significant mixing between supernova ejecta and pulsar wind material in this system. In addition, the Snail exhibits a subsonic comet-like protrusion extending from the putative neutron star. Cometary PWNe were found to exhibit a variety of magnetic field configurations but the exact reason remains unclear. The polarisation observations of this system revealed a magnetic field parallel to the nebula elongation, similar to what was found in the Mouse (G359.23−0.82) and the handle of the Frying Pan (G315.78−0.23). This adds an important sample to cometary PWNe for future MHD modelling.published_or_final_versionPhysicsMasterMaster of Philosoph

A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - II. Effects of off-axis instrumental polarization

Ma YK, Mao SA, Stil J, et al. A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue - II. Effects of off-axis instrumental polarization. Monthly Notices of the Royal Astronomical Society . 2019;487(3):3454-3469.The NRAO VIA Sky Survey (NVSS) rotation measure (RM) catalogue has enabled numerous studies in cosmic magnetism, and will continue being a unique data set complementing future polarization surveys. Robust comparisons with these new surveys will however require further understandings in the systematic effects present in the NVSS KM catalogue, In this paper, we make careful comparisons between our new on-axis broad-band observations with the Karl G, Jansky Very Large Array and the NVSS KM results for 23 sources. We found that two unpolarized sources were reported as polarized at about 0,5 per cent level in the RM catalogue, and noted significant differences between our newly derived KM values and the catalogue values for the remaining 21 sources. These discrepancies are attributed to off-axis instrumental polarization in the NVSS KM catalogue, By adopting the 0,5 per cent above as the typical off-axis instrumental polarization amplitude, we quantified its effect on the reported RMs with a simulation, and found that on average the RM uncertainties in the catalogue have to be increased by approximate to 10 per cent to account for the off-axis instrumental polarization effect. This effect is more substantial for sources with lower fractional polarization, and is a function of the source's true RM. Moreover, the distribution of the resulting RM uncertainty is highly non-Gaussian. With the extra RM uncertainty incorporated, we found that the RM values from the two observations for most (18 out of 21) of our polarized targets can be reconciled. The remaining three are interpreted as showing hints of time variabilities in RM

A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue I. Breaking the n pi-ambiguity

Ma YK, Mao SA, Stil J, et al. A broad-band spectro-polarimetric view of the NVSS rotation measure catalogue I. Breaking the n pi-ambiguity. Monthly Notices of the Royal Astronomical Society . 2019;487(3):3432-3453.The NRAO VLA Sky Survey (NVSS) rotation measure (RM) catalogue is invaluable for the study of cosmic magnetism, However, the RM values reported in it can be affected by n: r ambiguity, resulting in deviations of the reported RM from the true values by multiples of 652.9 rad in 2, We therefore set off to observationally constrain the fraction of sources in the RM catalogue affected by this ambiguity. New broad-band spectro-polarimetric observations were performed with the Karl G, Jansky Very Large Array (VLA) at 1-2 GH.z, with 23 n7 ambiguity candidates selected by their peculiarly high IRMI values. We identified nine sources with erroneous RM values due to T/7-ambiguity and 11 with reliable RM values, In addition, we found two sources to be unpolarized and one source to be inconsistent with neither rrr ambiguity nor reliable RM cases. By comparing the statistical distributions of the above two main classes, we devised a measure of how much a source's RM deviates from that of its neighbours: A/o-, which we found to be a good diagnostic of 1.77-ambiguity. With this, we estimate that there are at least 50 sources affected by run,-ambiguity among the 37 543 sources in the catalogue. Finally, we explored the Faraday complexities of our sources revealed by our broad-band observations