Looking Inside The Dark Heart of the Centaurus A Galaxy

As the cooler autumnal air settled with astronomical twilight on the night of 29 April 1826, James Dunlop began his evening of observing the southern skies from the recently established Parramatta observatory, located some 20 kilometres west of the British settlement at Sydney Cove.  

On that Saturday evening, Dunlop was about to make a remarkable discovery (though, at the time he would never have known) - one that for nearly 200 years, would be tied to the city of Sydney’s astronomical heritage and change our perception of the Galaxy, and greater Universe. 

Starting his observations just before 8 pm, Dunlop scanned the southern skies using his 9-inch aperture, 2.7m reflecting telescope from the relatively dark sky location, looking up at the twinkling, arching glow of the Milky Way band. Jupiter was only just rising in the east, whilst Orion was setting in the west on this crisp night. 

Dunlop’s night of observing was remarkably productive - discovering three open star clusters, one globular cluster and remarkably the first two galaxies south of 33-degrees (excluding the Magellanic clouds), amongst other observations previously made by French astronomer Nicolas-Louis de Lacaille in 1752.  

At the time, astronomers like Dunlop did not have fancy imaging equipment that are currently purchased by the general astrophotography enthusiast and amateurs. Instead, they used to sketch their observations by hand and record the position of the object observed in the sky as well as write a short description about it. 

One particular object, what appeared to him as being a double nebula, was recorded as item number 482 in his logs. “A very singular double nebula,” he scribbled, as he outlined the locations of a few stars.

What Dunlop didn’t realise was that he was discovering a new galaxy (later to be classified as NGC 5128). Exactly thirty minutes prior, he was also the first to observe another galaxy (NGC 4945). Both these objects were recorded as nebulae as it would roughly be a century before Edwin Hubble would revolutionise our view of the Universe, and the existence of galaxies beyond our own.

Now, nearly 200 years after Dunlop’s first discovery of his nebulae, a collection of radio telescopes from around the world have cast their united gaze at the centre of the bright, southern galaxy Centaurus A (the radio wavelength counterpart name for NGC 5128).  

This galaxy is also known as an ‘active radio galaxy’, and through serendipitous circumstances, the discovery of Centaurus A as a bright radio source is also poetically linked to the very same city in which Dunlop made his discovery - the city of Sydney. 

By utilising the very long baseline interferometry (VLBI) technique, scientists have developed an image of unprecedented resolution at the centre of this structure, revealing powerful jets emanating from a supermassive black hole, with striking bright outer edges - details which have never been seen before in all the years of observing Centaurus A.

The jets themselves are not a new feature of Centaurus A - in fact, jets emanating from supermassive black holes have been observed by many telescopes (and across multiple wavelengths) in many galaxies, including NGC 5128.  

These bright objects are known as active galactic nuclei (AGN) and are produced as a result of the monstrous black holes that reside at the centre of galaxies, accelerating particles away from the poles of the spinning accretion disc at high velocities.  

The findings, published in the journal Nature, now add to the growing number of AGNs that have exhibited this commonality of edge-brightened jets when observed through high-resolution VLBI methods, indicating an underlying similar mechanism across the sample population that is yet to be fully understood. 

Dr Phil Edwards, who is the Australia Telescope National Facility Science Program Director at CSIRO, Australia’s national science agency, worked as part of the global scientific collaboration that captured these deep images from Centaurus A, describing the findings as exhilarating and exciting. 

“The team was thrilled with the new image, and internally had several teams independently process the data to ensure a robust final result. The edge-brightened jet travelling towards us, and counter-jet directed away from us, are seen very clearly.” 

“Surprisingly, the region around the supermassive black hole is not bright, for reasons we don’t yet understand but are keen to investigate with further observations.” 

These newly released data have also set a new record - a 16-fold increase in the resolution of AGN jet structures, captured at such great distances. From these findings, scientists have determined that the jets originate from a distance of about the Sun-Jupiter radius (~5 astronomical units) from the supermassive black hole at the centre of the galaxy. 

Based on inferring the position of this supermassive black hole from these new findings, astronomers working on the project have said that we should be able to image the shadow of the event horizon (a feat only achieved once prior using the Event Horizon Telescope) but to do so, we would need to incorporate an array of telescopes orbiting the Earth as part of the VLBI. 

Several theories have been put forward as to the edge brightening within the jet structures, such as the role that helical magnetic fields that propagate outwards with the jets might play, in particular with producing non-thermal synchrotron radiation. 

“There are theoretical models which predict the jet will have a fast spine, surrounded by a slower moving sheath where the jet is interacting with the surrounding material in the galaxy, and those interactions may produce the edge-brightened synchrotron emission,” said Dr Edwards.  

“There is also evidence that the jet may be confined by a magnetic field wrapped around it, which might also favour brighter edges and a fainter interior.”

Additionally, the jets observed from Centaurus A in these latest findings appear to come from both the approaching and receding structure, with lots of asymmetrical features between both these directional cones of radiation, something that has scientists excited to run their models and theories against. 

“Evidence for the faint counter-jet in Centaurus A emerged in the 1990s, and comparison of the relative brightness and evolution with time of the jet and counter-jet provide valuable information in determining the orientation of the jets - that is, the angle of the jet to our line of sight to the galaxy,” said Dr Edwards. 

Many images that we receive from space, and often see across social media, are normally generated from the optical telescopes - cylindrical metallic tubes that feature mirrors and lenses to reflect and refract what is being observed either to the eye or an instrument, such as imaging devices.  

One key feature that improves the quality of imaging in optical telescopes is the size of the aperture - the large hole at the front of the telescope that allows light to pour in. The bigger the aperture, the more collecting power the telescope will have, and thus the more detail it can resolve.

However, this relation is directly related to the wavelength of light being observed, and whilst it can be easier to build bigger telescopes that feature bigger mirrors (these still require engineering challenges to be resolved), the technique becomes a lot harder when it comes to longer wavelengths, like radio waves. 

For radio telescopes, we need very large dish antennas, like CSIRO’s Parkes radio telescope (64-metre diameter is its aperture) to collect and focus lots of radio signals from space into the receiver. Already at this size, The Dish is a 1,000-tonne instrument that requires lots of support and powerful motors to help steer and direct it to follow sources across the sky. 

Building bigger radio dishes seems impossible, expensive and not very beneficial - though, several cases where the dish is built into a crater in the ground have been utilised very successfully (e.g., the Arecibo telescope was 300-metres in diameter, and the FAST is 500-metres in diameter). Though, these are not ‘steered’ telescopes and usually observe the sky sweeping over them instead. 

But radio astronomers have an extra tool up their sleeve - they can link several radio telescopes together to simulate a much larger aperture which in turn helps resolve radio frequency observations to a much higher degree. The simulated telescope then has an aperture as big as the longest distance between two antennas, which is known as the ‘baseline’. 

“The bigger the telescope, the finer the detail that can be seen in the images it produces.

This gain in “angular resolution” is what pushes radio astronomers to forming intercontinental networks of telescopes to probe the central regions of distant galaxies,” said Dr Edwards.

And that is exactly what the Event Horizon Telescope (EHT) is - a network of radio telescopes from different locations around the world, which observe and combine their data using the VLBI technique. The simulated size of the EHT can be stated as having a telescope the size of the Earth itself, which is needed to provide a high enough resolution in radio wavelengths that allows astrophysicists to resolve as much detail as they can. 

Radio telescopes as far north as Greenland, and as south as Antarctica, join telescopes in Europe and the Americas to observe distant objects in as high a resolution as possible. Whilst some of these are single-dish telescopes, the Atacama Large Millimeter Array instrument in Chile consists of 66 individual antennas, and the Submillimeter Array telescope located in Hawaii is made up of 8 antennas. 

The maximum baseline the EHT observed Centaurus A in was 10,000 kilometres, achieving an angular resolution of 25 microarcseconds, at an observing wavelength of 1.3mm. That’s the equivalent of being able to resolve a full stop at the end of a sentence on one of the Apollo mission manuals left on the Moon, as seen from the Earth. 

When compared to the resolution of the Hubble Space Telescope, the EHT’s VLBI technique and imaging capabilities when observing Centaurus A turned out to be 1,250-fold better. 

“These EHT observations were made at a wavelength of 1.3mm, and we don’t have radio telescopes in Australia able to observe at those short wavelengths (largely as such observations are best made at high altitudes),” said Dr Edwards. 

“But the EHT team recognised the benefit of collaborating with the group which had made the previously most detailed images of Cen A, which did use a network of radio telescopes across Australia and the southern hemisphere.”

Whilst the optical component of the Centaurus A galaxy was discovered by Dunlop and has since been observed by many, confirmation that the galaxy was a bright radio source didn’t occur until around the late 1940s, early 1950s. 

Using a radio antenna located on the cliffs above the Tasman in Sydney’s Dover Heights, John Bolton, Gordon Stanley and Bruce Slee from the precursor to CSIRO, the Council of Scientific and Industry Research (or CSIR), ran a radio survey of the sky over a four-month period, which led to the discovery of three new bright, yet discrete radio sources which they named Taurus A, Virgo A, and Centaurus A (Cygnus A was already known from prior studies). It was from these studies that Centaurus A the radio source was linked to NGC 5128 the optical object. 

Astronomers believe it is located between 10 - 16 million light-years away, having calculated this distance by using Cepheid Variables found in the galaxy. When observing the galaxy through visual bands, the large glowing oval-shaped hub of older stars is intersected with a tilted dark prominent dust lane that extends from one side of the galaxy to the other. In these bands, the galaxy is smaller in length, but bigger in width than our own, measuring roughly 90,000 light-years by 70,000 light-years. 

But the true revelation comes when studying this relatively nearby galaxy in a multitude of wavelengths - including radio, infrared, ultraviolet and high-energy. Each of these distinct bands reveals something remarkable about this giant, active structure. 

For example, a sprinkling of newborn massive blue stars have been detected in the ultraviolet range, especially around the central dust lane. In some regions well above the galaxy hub, filaments of hot gas streak across large portions of the sky. 

Prior to the EHT’s observations, radio telescopes around the world (of multiple frequencies) have observed this galaxy, and in particular, the prominent radio emission lobes that are created by the jets emanating from the supermassive black hole symmetrically away from at its centre. These lobes extend out across a fair portion of the southern sky.

The central dust lane - which is only an apparent shape from our edge-on perspective of the warped disc that circles the galactic nucleus - glows in the infrared bands, with dust and gas measured to move towards us from one side, and away from us in the other, as the disc rotates. 

Observing in x-rays, the central active core and jet become more prominent as electrons are accelerated at high speeds and in magnetic fields, producing synchrotron radiation. These emissions also come from the regions of the lobe fronts, as they collide with the surrounding intergalactic medium to produce x-rays. 

The high amount of activity and striking features of Centaurus A have been associated with a merger event, with what is expected to have been a smaller spiral galaxy, which only occurred approximately 100 million years ago - around the same time the dinosaurs were walking the Earth. 

As such, the galaxy offers the chance for scientists to study a large supermassive black hole that is located at its centre whilst it is active and whose mass has been measured to be equivalent to 55 million times that of the Sun (for comparison, the supermassive black hole at the centre of the Milky Way, Sgr A*, is only 4.3 million solar masses).  

Infrared cameras (Nicmos) aboard the Hubble Space Telescope have peeled back the layers of dust surrounding the nucleus and presented a tilted disk of hot gas encircling the galaxy’s centre, with a diameter of roughly 130 light-years. The accretion disc, surrounding the supermassive black hole is expected to be much smaller and closer to the beast at the centre, and the source of generation of the powerful jets. 

It’s been noted that in about 10 percent of all AGNs, some of the matter that is dragged inwards towards the supermassive black hole to be consumed is repelled and pushed back outwards via electromagnetic forces in the direction of the supermassive black holes poles. Materials caught in these jets have been measured, over a period of 10-20 years to have velocities close to that of the speed of light, which makes them relativistic. 

But it’s not just the scientists who have enjoyed Centaurus A. Given its relatively nearby distance (astronomically speaking) and relative brightness in the sky, Centaurus A is also a prime target for amateur astronomers to photograph, with its location not too difficult to find, given its close proximity to familiar southern targets like Crux and Omega Centauri.

Centaurus A is not the first galaxy that the EHT has looked into. In fact, there have now been several targets observed with the global-scale array. 

In April 2017, the EHT took extremely high-resolution images of the jet associated with the blazar 3C 279, which from our perspective showed the material to be moving at speeds of up to 20 times that of light. This apparent superluminal motion is an optical illusion based on our perspective angled view of the system. This quasar is located almost one-third of the way across the observable Universe, at approximately five billion light-years from us. 

The most famous (and iconic) EHT target was imaging the supermassive black hole at the centre of the M87 galaxies, some 55 million light-years away, which was published in April 2019. The world marvelled as the first-ever direct observation and evidence of a black hole was released by the collaboration, achieving the equivalent of image resolving a tennis ball on the Moon with the human eye on Earth. 

The supermassive black hole at the centre of M87 is far bigger than that of Centaurus A and our own galaxy, weighing in at approximately 6.5 billion solar masses, with the event horizon also measured to be 40 billion kilometres in diameter.

“The spectacular doughnut-shaped image of a black hole shadow was adjudged to be the scientific highlight of 2019, and Einstein’s theory of general relativity passed another strict observation test with flying colours!” 

“Black holes grow as they devour more and more material. Supermassive black holes will be regularly snacking on stars and gas and dust in their own galaxy, but it is likely that over cosmic time they have probably merged with other galaxies that have their own supermassive black hole, creating the biggest black holes we have detected,” added Dr Edwards. 

Sgr A* has also been a prime target for the EHT array, though for a number of reasons (esp. due to the ongoing pandemic in 2020-2021) these images have yet to be finalised and released.

Australian telescopes and antennas do not make up a part of the EHT collaboration that have produced these incredible, highly-resolved images of some of the most extreme and exotic objects in our Universe, located at vast distances from Earth.

However, we do have our own arrays here in which interferometric studies are conducted to observe phenomena at equally large distances. For example, CSIRO’s Australian Square Kilometre Pathfinder Array (ASKAP) is an interferometer that features 36 dishes (all of which are 12-metres in diameter) spread across a region of Mid West Western Australia with the greatest baseline equating to 6 kilometres. 

This telescope, which recently completed the fastest radio survey of the southern skies, has been used to pinpoint and localise the source location of where mysterious Fast Radio Bursts appear to be coming from, relative to their host galaxies. 

Additionally, the Curtin University operated Murchison Wide-Field Array (MWA) telescope, which features over 4,000 spider-like antennas, placed in a 4 x 4 configuration, across 256 tiles (also spread across several kilometres) has been conducting studies into the low-frequency radio range. This has helped catalogue 300,000 galaxies and map the sky at radio frequencies, as well as scan 10-million star systems for any signs of artificial radio transmissions. 

“Australia plays a major role in southern hemisphere VLBI, using CSIRO’s telescopes at Parkes, Narrabri and Mopra, and the University of Tasmania’s telescopes at Hobart and Ceduna as the core elements of the array. The longest baselines come from collaborating with partners in New Zealand and South Africa, and occasionally even further afield!” said Dr Edwards.

Future applications of VLBI methods will start to yield higher resolution imaging of more distant, yet increasingly compact objects. With the EHT now operational, minor tweaks might be able to improve resolution in small scales but to effectively leap over this limitation, bigger baselines are required. 

Scientists are currently in discussions about building antennas that can be orbited far from the Earth, or even on the Moon, to help increase the size of the baseline when used in conjunction with Earth-based telescopes. Whilst these ideas might border on timescales of a distant future, research continues to address many of the challenges associated with these concepts (such as maintenance of antennas on the Moon, or transmission of such large volumes of data). 

Until then, the world eagerly awaits the next targets and released imagery from the EHT array, and in particular, is looking forward to finally seeing Sgr A* - the supermassive black hole that resides in our very own Galaxy. 

Further data collection and sampling of other AGN with instruments like the EHT, will allow firmer conclusions (merited on observational data) about the structure, processes and evolution of these galactic cores, which will, in turn, assist in answering other questions - like the impacts of AGN feedback into star formation models for galaxies. 

When Dunlop commenced his observing run on that historical Saturday evening in late April, he would have no idea that his work - and his discoveries - would change the path of Australia’s astronomical heritage, and in particular anchor, the city of Sydney so closely to an astrophysical object of such importance. 

Centaurus A has captivated astronomers, astrophotographers and amateurs for nearly 200 years now, across all wavelengths of the spectrum. Its remarkably unique shape and structure, as well as its proximity, give the scientists of today an opportunity to study a complex, and actively young system undergoing changes observable in human lifetimes. 

With powerful instruments like the EHT, we’ve been able to peel back the layers of dust a little further and get a little closer to one of nature’s most exotic objects, the supermassive black hole, to study its behaviour, evolution and impact on the surrounding region. 

“Little could Dunlop have known that the unusual “double nebula” he sketched would turn out to be a spectacular radio galaxy hosting a supermassive black hole that is providing important clues about the formation, acceleration and collimation of relativistic jets of plasma!”

The city of Sydney has for 200 years walked hand in hand with Centaurus A, stepping from discovery to discovery. Who knows what the next 200 years will deliver, both from our city and from a galaxy, far, far away.