Mapping dots in the sky using radio astronomy

Every week our observatory has a science meeting, which we can attend either in person or via Zoom. After hearing about the latest admin and project updates, we discuss the latest scientific activities and discoveries made by our scientists and engineers and their collaborators.

At a recent meeting, one astronomer presented her latest results in imaging an intriguing gas and dust structure in the sky. The image was peppered with lots of little dots, each of which was a distant galaxy. The image was obtained using a radio telescope and showed what we would see if our eyes could see radio waves.

Our ability to see detail in what we are looking at depends on the size of the lens forming the image compared with the wavelength of the light coming from what we are looking at. If our eyes are in good working order, we should be able to see details as small as about 1% of the area of the lunar disc.

Radio waves are electromagnetic waves, just like light. The only difference between them is the wavelengths (the distance between two wave crests) of radio waves are enormously greater. For example, yellow light has a wavelength of about 580 nanometres (a nanometre is a billionth of a metre). Radio waves have lengths ranging from maybe about a millimetre to hundreds or thousands of metres. The wavelength 21 cm is a very important one for radio astronomy, because emissions at that wavelength are the signature of the cold, dark hydrogen clouds found in our galaxy and beyond.

Because 21 cm radio waves have wavelengths about 400,000 times longer than the light we are seeing, to get the same ability to perceive detail as the human eye, our radio telescope would need a dish or lens around two kilometres in diameter.

The CHIME radio telescope at our observatory is about 100 metres square. The size is made possible by fixing the antenna to the ground, which limits how much sky it can see.

Fortunately there is another approach. Thanks to modern electronics and digital signal processing systems we can make a big antenna out of lots of smaller ones. The Very Large Array in New Mexico is an example, and the Atacama Large Millimetre Array (ALMA) in Chile is another.

"Millimetre" refers to the wavelength of operation, not the size of the antennas. The Canadian Hydrogen Observatory and Radio transient Detector (CHORD), under development at our observatory, will be another.

Making slick acronyms is an important part of modern science. CHORD will consist of 512 six-metre dishes, and will look rather like an insect's compound eye. However, imaging distant galaxies and other cosmic radio sources needs a far greater power to resolve detail than our eyes. That means we need even bigger arrays of dishes.

That is why we are in the process of building what is likely to be the biggest radio telescope we can install on the surface of the Earth, the Square Kilometre Array (SKA). It will consist of thousands of small antennas, located mainly in South Africa and Western Australia. It is a massive international project, of which we are a part.

Pathfinder, or precursor, instruments have been built to develop and evaluate the required technology—one in South Africa called Meerkat, and the other in Australia, named, less poetically, ASKAP (the Australian Square Kilometre Array Pathfinder).

That image we saw at our recent meeting was obtained using the ASKAP radio telescope.

There is a reason we are investing in radio telescopes, such as ALMA and the SKA, located in the Southern Hemisphere. The centre of our galaxy is of major interest to astronomers. It passes overhead in the Southern Hemisphere. Here it just bobs above the southern horizon.

•••

• Jupiter lies high in the south-southeast after sunset and Saturn low in the south-southwest. • Venus rises shortly before dawn.

• The Moon will be full on the Dec. 26.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.