- A Word about Radio Astronomy
Radio waves cover the broad range of frequencies from around 3kiloHertz to about 300 gigaHertz. Radio astronomy uses various portions of this frequency range. The types of objects that can be explored in this regime range from planets and their immediate environments (for example, Jupiter is a source of radio emission), ordinary stars such as our Sun, peculiar stars such as pulsars, extreme locations such as the areas around black holes, interstellar gas in galaxies, and through to intergalactic space.
The radio spectrum was perhaps the second main spectral domain available to astronomers, after optical methods. Astronomical sources of radio waves were first found in the 1930's, and in the following years, many types of radio emission were discovered- solar, planetary, stellar, galactic.
With the development of instrumentation and techniques in the radio regime, the era of multi-wavelength astronomy began. It was realised that a much clearer understanding of the nature of astronomical sources could be found when our data cover a range of different wavelengths, since we then explore different aspects of these sources.
- How Do We Observe Radio Sources?
The Mopra radio telescope in NSW, used by HEAG for surveys of interstellar gas in our galaxy.
Since people were using radio for communications since the 1890's, the basic technology for detecting (and generating) radio waves had been known for some decades prior to the discovery of astronomical sources of radio waves.
Radio telescopes vary in form from simple antennae (possibly even just fixed wires), through to larger fixed arrays -such as the early Mills Cross in the early days of Australian radiophysics- to the classical steerable radio dish, exemplified by the Parkes radio telescope.
Recent developments include the Square Kilometre Array (SKA) and its predecessor instruments, which utilise arrays of fixed antennae, with sophisticated data-processing and analysis routines to allow simultaneous multi-directional and multi-frequency observations.
Antenna elements of the Murchison Wide-Field Array (MWA). Former HEAG student Melanie Johnston-Hollitt is involved at senior levels with SKA and MWA.
Radio telescopes can produce maps of radio emission on the sky, and if these maps are produced with sufficient resolution, we essentially obtain images of sources at radio wavelengths. Further, if data from widely separated telescopes are combined (known as interferometry), the resolution obtained is equivalent to that of a telescope with a diameter equal to the separation of those telescopes- and this could be thousands of kilometres. This idea can be extended to combining telescopes on Earth with some in space, or space-borne arrays for very large separations.
This method allows us to produce the highest resolution obtainable in images at any wavelength. Also, radio observations can be done with very high time resolution- for example, observations of pulsars with rotational periods measured in milliseconds.
- What We Do in HEAG
An example of positional alignment between gamma-ray intensity (colours) and radio emission (contour lines).
Staff and students utilise radio data at millimetre and sub-millimetre wavelengths to complement and support observations in gamma-rays.
Within our galaxy, gamma-ray photons can arise from the interaction of cosmic ray particles with the interstellar medium (ISM) in our galaxy, and thus these radio observations help us to determine more clearly the production of these gamma-rays by mapping and probing the structure of the ISM.
Also, these radio observations are undertaken in part to look for spatial co-incidence between the radio and the gamma-ray sources, since a considerable fraction of our galactic gamma-ray sources are of an unidentified nature. Multi-wavelength observations can help to elucidate the nature of these unidentified sources.
For more information, please contact Associate Professor Gavin Rowell.