My current projects

Investigation of the redshift dependence of galactic magnetic fields using Faraday Rotation Measure Synthesis of intervening sources projected on the polarized radio lobes/jets of quasars.
Implementation of an accurate bandpass correction for the Arecibo GALFACTS polarimetric survey.
Spectropolarimetry of the bilateral supernova remnant G296.5+10.0.
Studies of the switching-on conditions of methanol masers in the earliest massive protostellar-outflow phase in Cygnus X using MERLIN.
Monitoring of maser variability in the flaring source Mon R2.
Studying the interior velocity structure of individual maser `spots'.
Studying maser polarimetry and magnetic fields in W3(OH) using MERLIN.
A survey of nearby galaxies for class II methanol masers using the Parkes Methanol Multibeam.

Cosmic Magnetic Fields

Introduction

There are still wide gaps in our knowledge of the universe, such as how the first galaxies formed and evolved and in particular the role of magnetic fields in these processes. Big questions remain regarding the origins of cosmic magnetism and the way in which magnetic fields are woven into the expanding universe. The research I am conducting with the University of Sydney's Extreme Astrophysics Group attempts to unravel some of these big mysteries, using state-of-the-art telescope instrumentation.

Magnetic fields are well known at the Earth's surface, where they have been a navigational aid for centuries. They are also evident at the photosphere of the sun where they cause promininces, sunspots and a host of other solar phenomena. But how are cosmic magnetic fields generated and what effect do they have on the structure of matter in the universe?

Measuring magnetic fields in space

Cosmic magnetic fields can be measured in a variety of ways. The most common method is to measure the effect of the magnetic field on the photons that pass through the magnetised medium. A photon of radiation behaves as an electromagnetic wave, which means that the electric field vector of the wave is deviated by an external magnetic field. This can lead to polarization (electric field vector alignment) of the radiation passing through a magnetised region. Several types of astronomical object are known to generate significantly polarized radiation, including quasars, supernova remnants, masers, pulsars, and interstellar dust. Studies of these regions using radio telescopes - often at high angular resolution - allow us to determine the physical conditions in a wide range of astronomical objects.

Faraday Rotation: Unwrapping the magnetic fields of spiral galaxies

We are undertaking a project to determine the rotation measures across a sample of quasar intervening galaxy systems using radio interferometry. When polarized radiation passes through a magneto-ionic medium (e.g. ionised gas in our galaxy) the orientation of the linear polarization vectors is rotated. This effect is called Faraday Rotation and the degree of rotation depends on the electron number density, the line-of-sight magnetic field strength and the square of the frequency of the radiation. Given a good number of frequency points and using the rotation measure synthesis method, the Faraday rotation of the radiation from an astronomical object can be measured accurately, leading to a very good estimate of the magnetic field strength in that region. This is excellent news for the study of distant objects such as galaxies, as it enables us to measure the magnetic field strengths of a wide range of galaxies against the brighter background of very luminous radio sources such as quasars. If the intervenor galaxy in question is sufficiently resolved using radio interferometry, then it is possible to map out the magnetic field direction across the galaxy in great detail, including the magnetic field reversals between the spiral arms. This technique is has great potential for assisting our undertanding of the role of magnetohydrodynamic waves in creating or maintaining spiral structure in galaxies. By measuring a range of galaxies at varying redshifts, we can also study how the magnetic fields in galaxies have evolved over the last few billion years of cosmic history. This will shed light on the intrinsic nature of magnetism in the universe, and whether galaxies generate magnetic fields or whether they amplify existing `seed' fields. Image: Magnetic bubble in M82. Credit: Jane Greaves, JAC Hawaii.

Astrophysical masers

Introduction

I co-ordinate the Astrophysical Maser Group at the University of Sydney. My research involves studying interstellar masers at high resolution using radio telescopes working together as interferometric arrays. These interstellar masers are emanating from clouds of atomic and molecular gas surrounding very hot, young stars in the spiral arms of our Galaxy. The positions, sizes, motions and polarization of masers can tell us a lot about the regions in which new stars are being formed from clouds of gas and dust.

What are astronomical masers?

Masers are regions of gas in which radiation is amplified many thousands or millions of times by molecules. They emit very brightly, allowing us to observe them not only at great distances, but also from very dense and dusty regions through which most types of electromagnetic radiation cannot pass. Unlike lasers, masers emit microwaves (short-wave radio signals with wavelengths of millimetres to centimetres) and must be observed using radio telescopes. In order to produce a maser, a molecule must be in a state of population inversion. This means that the molecule has a lot of excited rotational energy levels filled at the expense of the lower rotational energy levels. This is in contrast to the usual scheme of things, as molecules are naturally very lazy creatures and prefer lolling around in the ground-state energy levels. The molecules are pushed into this inverted state by some excitation process, which needs to be on-going in order to keep the energy levels inverted over a period of time. This is called the maser pump. When these conditions are met (for example near a newly-formed star) the molecules in a dusty cloud of interstellar gas can be pumped into a population inversion (e.g. by infra-red photons emitted by the dust, which were in turn warmed by the star). In this case, spontaneous microwave radiation can be exponentially amplified by the gas. This is what is called a maser.

Masers in massive star-forming regions

I am carrying out research into the structure and evolution of regions of massive star-formation. Astronomical masers are often found in compact HII (ionised hydrogen) regions in the gas surrounding very young stars. Quite often in the case of methanol gas, the masers are seen around young protostars even before any HII region is formed. Their orientations, radial velocities and polarization properties (linear and/or circular) can be measured using radio telescopes. They often betray physical structures in their shapes, orientations and radial motions. Measuring the polarization properties of interstellar masers allows us to determine the strength and direction of the magnetic field in those regions. I hope that by a combination of approaches, including cosmic maser research, we can understand the mysterious process of massive star-formation.

Continuum emission in W3(OH) from Guilloteau et al. (1983) (left) and Wilner et al. (1999) (right) superimposed with our methanol maser spots measured with MERLIN (white dots), an OH maser filament (red contours) and extended methanol filaments (green contours).