New insights into our nearest actively feeding supermassive black hole

We present the most comprehensive radio image to date of our nearest neighbouring radio galaxy, Centaurus A, unlocking a region of the galaxy that allows us to study the effects of feeding and feedback of the central supermassive black hole across all scales.

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We present a new study of our nearest actively feeding supermassive black hole, using the most comprehensive radio image of the radio galaxy to date. Combining our low-frequency radio image with data from X-ray and optical telescopes, we investigate how the supermassive black hole at the centre of the galaxy is both feeding on in-falling gas, and ejecting material far out into space to form bubbles of radio-emitting plasma. These radio-emitting bubbles, or ‘lobes’ as they are known to radio astronomers, extend out to 48 times the optical span of the host galaxy, NGC5128. As Centaurus A is so close to us (in our cosmic backyard at a mere 12 million light years away), it affords us a unique opportunity for studying black hole feeding and the feedback of energy to the surrounding environment across the full electromagnetic spectrum and over a huge range of spatial scales.

Our nearest neighbouring radio galaxy, Centaurus A, imaged with the Murchison Widefield Array radio telescope. The many dots scattered throughout the image are not stars, but radio galaxies in their own right, too far away to be seen in any detail. Credit: Ben McKinley et al., 2021, Nature Astronomy (DOI: 10.1038/s41550-021-01553-3)

My interest in Centaurus A began in 2011 when my PhD supervisor presented me with a pile of hard drives, fresh from a trip to the Murchison Radio-astronomy Observatory in a remote part of Western Australia. Still coated in a thin layer of red dust, they contained data from a prototype of a new low-frequency radio telescope - the Murchison Widefield Array (MWA). The MWA was aiming to explore a novel concept in radio astronomy; to use many small and cheap antenna elements, joined together in an array that could be pointed to different regions of the sky without the need for any moving parts. The small antennas meant that it could cover a huge swath of the sky in a single ‘snapshot image’, and their low cost would allow a large number on the ground and hence a big area for collecting the faint radio signals from space. The development and testing of these technologies was essential for the success of the most ambitious international radio astronomy project ever planned - the Square Kilometre Array (SKA). It was this idea; to build one of the most sensitive radio telescopes ever conceived on an ancient and radio-quiet landscape, that had inspired me to quit my job as an Electrical Engineering Officer in the Royal Australian Air Force and pursue a career in radio astronomy.

A single 'tile' of the Murchison Widefield Array radio telescope, consisting of 16 crossed dipole antennas. The full array has 256 of these spread out over the radio-quiet Murchison Radio-astronomy Observatory site in Western Australia; future home of the Square Kilometre Array - Low. Credit: Ben McKinley, Curtin University

But the SKA was still a long way off, for now we had just 32 antenna tiles like the one seen in the panorama above. Under the guidance of my PhD supervisor and the many members of the MWA collaboration across the country and the globe who had become my collaborators and mentors, I managed to turn a bunch of ones and zeros on a dusty hard drive into an image, which was relatively low in resolution, but covered a whopping 700 square degrees of sky (the full moon occupies about 0.2 square degrees). I was then faced with a more serious dilemma - what is this giant blob of radio-emitting plasma and why do we even care? 

As I soon found out, the feedback of energy in radio galaxies has a strong influence on galaxy evolution and even very structure of the Universe itself. All galaxies are thought to harbour supermassive black holes at their centre; objects smaller than our Solar system that have masses millions of times larger than our Sun. In some galaxies these black holes are surrounded by a hot, swirling disk of matter that is being sucked in by the extreme gravitational pull. Only a small percentage of the in-falling mass actually makes it into the black hole, causing it to grow, while the rest is funnelled into narrow jets, which shine so brightly they can be observed tens of billions of lightyears away. The jets themselves can stretch out beyond the furthest stars in the galaxy, transporting mass and energy out into the space between galaxies. This intergalactic space is not empty, and as the jets pummel into it, they inflate bubbles, which appear in our radio telescopes as giant lobes like those we see in Centaurus A. 

Centaurus A may not be particularly special as a radio galaxy, in fact, we hope that it is typical of the greater population so that we can apply the lessons learned from it to the rest of the Universe. What makes it so unique to us is its close proximity. To give you an idea of how close it is as far as radio galaxies go; each of the thousands of background ‘dots’ in our radio image (shown above) from the upgraded 256-tile MWA telescope, are not stars, but radio galaxies in their own right, too distant to be resolved by the MWA. In Centaurus A, however, we can see fine details at all wavelengths, including the ability to resolve and study individual stars in the host galaxy (galaxies normally appear as a single extended ‘smudge’ in optical telescopes). In our new paper we take advantage of this to truly use Centaurus A as a ‘local laboratory’, using optical, radio and X-ray data to study how cold and hot gas is moving around the galaxy and how it relates to the outflows and inflows associated with the supermassive black hole. We find that the data best match a model where gas cools and rains down on the central galaxy, triggering outbursts of activity from the black hole and forming a self-regulated system of feeding and feedback analogous to the weather we see in our own atmosphere.

Multi-wavelength image of Centaurus A. The MWA radio emission is shown in blue and the optical galaxy sits in between the two brightest radio blobs. Hot, X-ray emitting gas is shown in orange and cold neutral hydrogen gas (HI) is purple. Smaller red blobs above the central galaxy are clouds emitting in the optical Halpha spectral line and the background shows a colour composite of stars in the optical band (the stars are actually part of our own Milky Way galaxy and are in the foreground). Credits: Connor Matherne, Louisiana State University (optical and Halpha) - Ben McKinley, Curtin University (radio) - Struve et al., 2010, A&A, 515, 67S, (HI) - Kraft et al., 2009, ApJ, 698, 2036 (X-ray).

The MWA has grown over the 10 years since I began working with it; from 32 to 128 and then 256 antenna tiles. Our picture of Centaurus A has improved dramatically as a result, culminating in this latest study. This work has been enabled by the growing network of collaborators that I have been fortunate to work with, who have brought to the table expertise in theory, multi-wavelength astronomy and even astrophotography. Centaurus A will no doubt continue to both answer and pose many questions relating to our understanding of the Universe, as it will remain forever the most detailed window into the physics of radio galaxies available to us. I look forward to the day when I can image it with the SKA!

The Murchison Widefield Array (MWA) is located on the Murchison Radio-astronomy Observatory (MRO) in Western Australia. The MRO is managed by CSIRO, Australia’s national science agency, and was established with the support of the Australian and Western Australian Governments.  The MWA is managed and operated by Curtin University, on behalf of an international consortium. We acknowledge the Wajarri Yamatji as the traditional owners of the observatory site.

Benjamin McKinley

Research Fellow, Curtin university

I'm a radio astronomer at theCurtin University node of the International Centre for Radio Astronomy Research in Perth, Western Australia. I mainly use the Murchison Widefield Array, to study galaxies in our local Universe, and to attempt to study the redshifted 21-cm line from Cosmic Dawn and the Epoch of Reionisation.