Attachment C: BACKGROUND: RADIO ASTRONOMY, VLBI, AND SHORT WAVELENGTHS

Why do astronomy?

Astronomy, or more properly astrophysics, is a basic science that uses the universe to study fundamental physics. Its value arises because only a limited range of physical conditions can be reproduced in the laboratory on Earth, while elsewhere in the universe we see ranges of density, speed, gravitational field, and scale size that differ by many orders of magnitude from the most extreme conditions on Earth.

Radio astronomy

Radio techniques have played a vital role in astronomy ever since the late 1940s when the influx of new ideas and technology from the war years were put to use in the study of the universe, adding a rich new dimension to traditional observations at optical wavelengths. Scientists at the CSIRO Radiophysics Laboratory and at the University of Sydney were world leaders, developing and implementing new concepts for interferometric telescopes and building one of the world's largest telescopes at Parkes. Radio astronomy remains one of the great successes of Australian science. The 1989 ASTEC Report on the Future of Australian Astronomy noted that 'astronomy appears to be a high priority discipline for Australia' relative to other disciplines, and that 'Australia appears to be a world leader in many areas of astronomy research relative to other nations'.

One key difference between radio astronomy and the more traditional optical astronomy is that the relatively long wavelengths (typically several centimetres) of radio waves means that a radio telescope must be much larger physically than an optical telescope in order to obtain the same resolution, or to distinguish fine detail. The giant 64 m ATNF radio telescope at Parkes, for example, is one of the world's largest telescopes and yet has a resolution far poorer than even a small optical telescope.

To achieve a resolution comparable to a world-class optical telescope, a radio telescope several kilometres in diameter would be needed. This is impractical so, instead, we synthesise such a telescope by building an array of antennas. This was the justification for building the Australia Telescope, which was funded as a $45M bicentennial project in 1983, and opened in 1988. As well as the 6 km Compact Array, which has a resolution comparable to optical telescopes, the Australia Telescope includes the Parkes 64 m telescope and a new 22 m antenna at Mopra.

This MNRF proposal seeks to extend the capability of Australian radio astronomy in two key domains: resolution and wavelength. High resolution is necessary to study the extreme processes in the innermost parts of quasars and regions of star formation. Shorter wavelengths are necessary to probe physical conditions which differ from those probed at longer wavelengths.

Resolution

To obtain very high resolution, we use a technique known as Very Long Baseline Interferometry (VLBI) where we combine signals from the ATNF antennas together with other antennas in Australia, to form the Australian VLBI array, which can synthesise a telescope up to 3000 km in diameter. The resolution of this array exceeds even that of the Hubble Space Telescope. However, this VLBI array is severely limited by the small number (five) of antennas in it, the locations of which are critical. Over the last few years we have been proposing the construction of an additional antenna near Adelaide. A sixth antenna at this location gives a six-fold improvement in image quality. Part of this MNRF proposal is to achieve that by making use of an existing antenna at Ceduna, SA, offered to us by Telstra, and discussed further below.

To increase the resolution even more means that we have to use overseas antennas, and so an important part of our VLBI strategy involves collaborative experiments with groups in Japan, China, Russia, Europe, and the US. To get even higher resolution means obtaining baselines larger than the Earth's diameter, and so Japan and Russia both plan to launch VLBI satellites in the next two years to obtain the highest resolution yet.

Wavelength

The principal reason for wanting to operate the AT at 3 mm is that at this wavelength we can explore mechanisms that cannot be reached at longer wavelengths. For example, the CO molecule, which is a common constituent of interstellar gas, has a powerful transition at 3 mm which will enable us to probe regions of star formation that are otherwise inaccessible. Studies using this molecule as a tracer of cool gas are proving to be extremely important for studying star-formation processes, the processes that power active galaxies, and even for searching for and studying clouds of cool gas at cosmological distances, that are the progenitors of the galaxies we see today. As well as CO, many other important molecules are accessible at this wavelength, and at 3 mm we can even study the black-body radiation from warm dust in the nuclei of active galaxies, and the remnant heat left over from the primeval fireball that signalled the origin of the universe.

The motivation to build 1 cm receivers for the AT Compact Array comes partly from the molecular transitions that lie within this band (such as water, ammonia, and methanol) that are important for the study of star formation, and partly from the need to study continuum sources at several different wavelengths in order to understand their emission mechanisms.

An additional feature of shorter wavelengths is that they offer higher resolution on the same instrument. At present the Compact Array and the Australian VLBI array cannot operate at wavelengths shorter than 3 cm, and part of this proposal is to extend the VLBI array to 1 cm, and the Compact Array to 3 mm. In both cases this will significantly increase the resolution of the instrument. A principal reason to extend the VLBI array to 1 cm is that this is the shortest wavelength on which the VSOP and Radioastron Space VLBI satellites will operate, and so by upgrading to 1 cm we will be able to make full use of the power of this technology.

The 1991 proposal to ASTEC

In 1991, the Australian radio astronomical community submitted two proposals to ASTEC. One (Australia Telescope Upgrade) was to extend the operation of the AT to shorter wavelengths (higher frequency). ASTEC (1992; "Major National Research facilities - A National Program") recommended funding for this proposal with the following comment:

"The Australia Telescope Upgrade will provide a significant addition to the capability of the existing advanced radioastronomy facility at Narrabri, NSW. Provision of access to the high frequency region of the electromagnetic spectrum should maintain Australia's internationally competitive position and exploit our unique access to the southern sky. As with construction of the Australia Telescope, this upgrade will generate opportunities for marketable innovations in the electronics industries, especially for space communications."

The other proposal (Australia-Wide VLBI Array for Astronomy and Geodesy) was to build two new antennas near Adelaide and Perth, at a cost of $6.5M each. ASTEC did not recommend this for immediate funding, but noted that the proposal was "of high merit" and recommended that it should be "reassessed at the next review". Here, instead of the $6.5M to build a new antenna near Adelaide, we are requesting $1.6M to convert an existing 30 m antenna at Ceduna for radio astronomical use. Telstra is winding down its operation at Ceduna, and has offered this antenna to the radio astronomical community at no cost, as Telstra has no further use for it.