9 mins read 18 Jan 2022

MWA takes a deep, wide look at the Centaurus A Galaxy

Australian astronomers used an odd-looking telescope, located in the Western Australian outback, to take one of the most detailed images of the relatively nearby bright radio galaxy, Centaurus A.

Most people are surprised when they find out that the very knowledge of galaxies existing - beyond our own Milky Way - is actually less than 100 years old. It wasn’t until Edwin Hubble turned his telescope towards the ‘nebulae’ in Virgo in the mid-1920s that we realised that our home galaxy, is in fact, one of many galaxies. 

That’s not to say that people hadn’t spotted galaxies earlier in history. They did, but they just thought they were clouds of gas and dust, floating around the Milky Way - after all, the thinking of the time was that the Milky Way was the Universe. 

One galaxy, in particular, has a strong tie to Australia - and that’s because it resides at fairly low latitudes in the southern sky, making it harder to see from northern locations. In 1826, an astronomer named James Dunlop, working from his observatory in Parammatta in Sydney happened to find this curious ‘nebula’, what he recorded as having two, almost parallel clouds of gas. 

What Dunlop didn’t realise was he was actually looking at the bright Centaurus A galaxy, located some 12 million light-years away, well beyond the edge of the Milky Way. And even after Dunlop, it wasn’t until the emergence of radio astronomy (in Sydney) that this object was truly understood for what it really was. A powerfully bright radio galaxy, and an optical jewel sitting in the Centaurus constellation - not too far from the Southern Cross. 

In December last year, an international team of astronomers - led by members of the International Centre for Radio Astronomy Research (ICRAR) -  used a not-so-regular looking telescope located in the Western Australian outback to take a look at Centaurus A in the radio frequencies. 

And what they produced was the most comprehensive image of the relatively nearby galaxy, and its feeding supermassive black hole, as outlined in the journal, Nature Astronomy

The Centaurus A galaxy in radio wavelengths. The bright inner eight-shaped core is the familiar galaxy seen in other images. This new image showcases the extension of the plasma from the galaxy out to 8-degrees - the length of 16 full moons. Credit: B. McKinley/ICRAR/Curtin Uni./C. Matherne/Louisiana State Uni.

Lead author Dr Benjamin McKinley, from the Curtin University node of the ICRAR, said the image reveals spectacular new details of the radio emission from the galaxy.

“These radio waves come from material being sucked into the supermassive black hole in the middle of the galaxy,” he said.

“It forms a disc around the black hole, and as the matter gets ripped apart going close to the black hole, powerful jets form on either side of the disc, ejecting most of the material back out into space, to distances of probably more than a million light-years.”

“Previous radio observations could not handle the extreme brightness of the jets and details of the larger area surrounding the galaxy were distorted, but our new image overcomes these limitations.”

To obtain the deep level of detail in this latest study of Centaurus A, as well as being able to look at the galaxy across a large patch of the sky, the team utilised the Murchison Widefield Array (MWA) telescope, which is managed and operated by Curtin University on behalf of ICRAR. 

Unlike your regular looking telescopes of giant optical tubes with mirrors, or even the typical dish antennas you see with some other radio telescopes (like the CSIRO Parkes radio telescope, or ASKAP antennas), the MWA is made up of a series of spider-like dipole antennas, all arranged into a certain configuration and spread across a large area of the West Australian desert. 

The spider-like antennas of the MWA, placed onto a ‘tile’ that features a 4 x 4 configuration. Credit: Dragonfly Media.

These spider-like antennas are placed in a 4 x 4 configuration atop a 4-metre squared mesh, creating what’s called ‘tiles’. And in total there are 256 tiles that are spread over several kilometres, allowing the telescope to ‘see’ the radio sky with high angular resolution across a wide low-frequency radio range (70 - 300 MHz). 

So the MWA is practically made up of 4,096 spider-like telescopes all collecting data as the southern skies roll over the open planes of the CSIRO Murchison Radio-astronomy Observatory (MRO), located a few hundred kilometres from Perth. 

MWA director Professor Steven Tingay said this research was possible because of the telescope’s extremely wide field-of-view, superb radio-quiet location, and excellent sensitivity.

“The MWA is a precursor for the Square Kilometre Array (SKA)—a global initiative to build the world’s largest radio telescopes in Western Australia and South Africa,” he said.

“The wide field of view and, as a consequence, the extraordinary amount of data we can collect, means that the discovery potential of every MWA observation is very high. This provides a fantastic step toward the even bigger SKA.”

A telescope of this nature - one with such a wide field of view, as well as high-resolution capabilities is going to produce a lot of data. And to assist with the transfer, storage and processing of such high volumes, the MWA partners with the Pawsey Supercomputing Research Centre, located in Perth. 

Comparing Supermassive Black Holes

The Centaurus A galaxy shown across a variety of wavelengths, highlighting different features of the objects that form part of the larger structure. Central large image is all wavelengths combined. Credit: NASA/ESA/Hubble/NRAO/CXC/SAO/JPL-Caltech.

Our home address happens to be approximately two-thirds of the way out from the centre of the Milky Way Galaxy – a giant spiral-disc structure that stretches approximately 100,000 light-years in diameter.

When looking towards the centre of our Galaxy, we note that at its heart, there lies a supermassive black hole (known as Sagitarrius A-star - “Sgr A*”), weighing in at 4.3 million times the mass of our Sun. We don’t directly observe this monster in the optical range, but instead infer its presence through the highly eccentric orbits of nearby stars, some of which move at mind-boggling speeds around this invisible object.

Additionally, through the use of multiwavelength observations – stretching from radio frequencies through to gamma rays - we can start to piece together details of this omnipresent structure and its relationship and effects on the surrounding region of our Galaxy.

But Sgr A* is a runt compared to the supermassive black hole inside Centaurus A, which registers at approximately 55 million solar masses. Not only is it many times larger than Sgr A*, but it’s also rather hungry - feeding on gas that falls in towards it.

As materials fall in towards the central supermassive black hole, it is shredded and forms a rotating accretion disc around the central massive compact object. And as the material orbits within the disc, frictional forces and high velocities cause it to generate lots of energy that can be detected in a number of different wavelengths, such as radio, and x-rays. 

Matter that is unfortunate enough to be caught in this death fall eventually is ejected back into space via powerful relativistic jets, which also produce an energetic signature that can be observed. 

These jets, which move matter at near the speed of light, produce x-rays out to thousands of light-years, where the radio jets extend out to over a million light-years away from the central supermassive black hole. 

“We can learn a lot from Centaurus A in particular, just because it is so close and we can see it in such detail,” Dr McKinley said.

“Not just at radio wavelengths, but at all other wavelengths of light as well.

“In this research we’ve been able to combine the radio observations with optical and x-ray data, to help us better understand the physics of these supermassive black holes,” he said.

When observing Centaurus A in multiple wavelengths, such as optical, x-rays and radio, its true structure (and beauty) begins to take shape. A bright elliptical galaxy with a central spiral disc - the product of two galaxies merging over deep time. Two giant jets, emanating from the central black hole and blowing out perpendicular from the plane of the disc. Eventually, the jets collide with the surrounding gas and lose energy, creating puffy lobes at each end. 

But this image really focuses on the central, and more optical aspect of this galaxy. In this new research, the MWA was able to take a very widefield view to look at Centaurus A, and how its vast lobes of plasma reach well beyond the visible galaxy.

The left panel showcases the whole radio source of Centaurus A, showing the extended lobes. Two right panels provide further detail around two interesting sections of the galaxy, including the familiar inner lobe region. The sprinkled dots represent distant supermassive black holes. Credit: McKinley et al. 2021.

This new data showcases that these lobes extend out to 8-degrees across the sky, as viewed from Earth - the equivalent of laying 16 full Moons side by side. Put another way, the inner lobes that are presented in the many familiar historical multi-wavelength images of Centaurus A are thought to reach approximately 16,300 light-years from the supermassive black hole at its centre. This new data from the MWA shows the larger lobes reaching as far as 1.5 million light-years from the centre, with astronomers believing it may even be bigger since the northern jet points towards us and the southern jet away from us. 

The details from this new look at this familiar southern galaxy now mean that astronomers can study the closest active supermassive black hole to Earth, in a range of wavelengths, and really start to uncover how these central engines both feed on the surrounding material, as well as feedback into it. 

Recently, the supermassive black hole at the centre of Centaurus A was the focal subject of the Event Horizon Telescope - a network of radio telescopes that form an aperture about as big as Earth. During this study, astronomers delved deep as technology permits to reveal some magnificent structures in the twisted relativistic jets and detail in relatively close proximity to the supermassive black hole. 

Zooming into the heart of the Centaurus A galaxy, the EHT has revealed the structure of the relativistic jets. Credit: Radboud Uni/EHT.

We acknowledge the Wajarri Yamatji as the traditional owners of the Murchison Radio-astronomy Observatory site on which MWA is located.

Video Credit: ICRAR.

The paper is available in the journal, Nature Astronomy.