By Natasha Hurley-Walker
To the naked eye, the universe we can see on a clear night is dotted with thousands of stars, but what would it look like if human eyes could see radio waves?
Deep in the Western Australian outback a radio telescope is demonstrating just that by painting a picture of the cosmos in all the colours of the radio.
It’s called the Murchison Widefield Array (MWA), and over the past three years astronomers have used it to perform one of the largest sky surveys of all time, covering 90% of the southern sky.
This is the GaLactic and Extragalactic All-sky MWA survey, or GLEAM for short. If you camped out for the night in Murchison Shire, and could open your eyes to radio light, this video of GLEAM shows what you might see.
Unaided human vision and optical telescopes use only the visible part of the electromagnetic spectrum, a narrow window within a huge range. This optical view of the night sky shows the familiar stars of the Milky Way, and darkness where dust blocks our view of the galactic plane.
But GLEAM’s radio wavelengths show something completely different. With GLEAM we see that the Milky Way is glowing with synchrotron radiation, given off by high-energy electrons spiralling around magnetic fields spanning thousands of light years.
Peering into the cosmos
The colours that we see in GLEAM aren’t false. Red indicates the lowest radio frequencies (around the FM band of your car radio), blue indicates the highest radio frequencies (around the digital signals your TV receives), and green indicates the frequencies in between.
This radio colour view allows astronomers to see different kinds of physical processes going on in our universe.
For instance, in the galactic plane, regions of ionised plasma around the brightest stars are brighter at high frequencies and dimmer at low frequencies. These show up in blue, in contrast to the pervasive red synchrotron glow.
Also visible in the Milky Way are features like soap bubbles, which mark sites of ancient supernova explosions. This is where massive stars ran out of hydrogen fuel, imploded, and then exploded outward, creating a shell of radiating plasma expanding into space.
In the past, astronomers have found far fewer of these supernova remnants than are needed to account for the high-energy electrons that produce the synchrotron glow of the galaxy. Fortunately, GLEAM is perfectly suited to detecting these missing remnants, solving a cosmic puzzle.
In the image above, the inset highlights show the shell-like remains of ancient supernovae (blue box), ionised regions around bright stars (orange box), and radio jets coming from the nearby galaxy Centaurus (purple box). All of these features are undetectable in visible light.
Near the bottom right of the image is the Large Magellanic Cloud, our nearest neighbouring galaxy, which shines with synchrotron radio light, like the plane of our own Milky Way.
But it’s not just our own galaxy that this survey shines new light on. Scattered across the sky are hundreds of thousands of smaller dots. These are not stars, but distant radio galaxies.
They are super-massive black holes at the cores of galaxies millions to billions of light years away. The black holes accrete matter, destroying stars, and their strong magnetic fields turn the incoming matter into massive jets of plasma, launched into space at nearly the speed of light.
It is this plasma that GLEAM detects, and again, the radio colour tells astronomers whether a jet is young and just starting (blue) or old and dying (red).
A challenging viewpoint
It wasn’t easy getting to this point. The Murchison Widefield Array had to be built more than 300km from the nearest town, Geraldton, to ensure a radio-quiet environment.
The array consists of thousands of radio antennas, similar to TV aerials and somewhat resembling an army of mechanical spiders. These observe low-frequency radio waves, from the lowest end of the FM (72MHz) up to the highest end of the digital TV band (300MHz).
To build the survey, a team of 20 astronomers across Australia and New Zealand has painstakingly knitted together more than 45,000 images of the sky, inventing new algorithms at every turn in order to deal with the unique challenges of these data.
For instance, while the wide field-of-view of the MWA makes an all-sky survey possible, the ionosphere distorts the signals of every observation, sometimes creating giant plasma tubes that render a night unusable.
While the wide frequency coverage yields astronomers a scientific goldmine, it also makes source-finding and analysis more difficult. And of course, an all-sky survey isn’t small - nearly half a petabyte of data and several million CPU-hours on cutting-edge supercomputers went into its making.
The first data release was published this week in Monthly Notices of the Royal Astronomical Society. It comprises a catalogue of more than 300,000 radio galaxies and images spanning 25,000 square degrees, all of which is freely accessible to the world.
There are yet more astronomical wonders lurking in the images such as collisions between galaxy clusters - some of the largest structures in the universe - to mysterious transient radio sources, and serendipitous discoveries that will take many eyes on the data to find.
A great place to start looking is the GLEAM-o-scope, an easy-to-use interactive viewer that gives anyone in the world the power to see the sky with radio eyes.
Source: The Conversation