Black holes eat their way through galaxies by employing a complex repertoire of buffet manners, according to an international team of astronomers led by the University of Pretoria’s Dr Jack Radcliffe.  

Dramatically diverse eating preferences drive this cosmic dining decorum: some black holes chew the stellar cud and savour their meals; others stage prolonged hunger strikes that leave them starving. When shredding objects into threads of atoms, smaller black holes just a few times the mass of our Sun spaghettify everything unlucky to get close enough.

Supermassive black holes behave as though they are the very centre of the universe, beaming high-energy radio jets across spacetime near the speed of light.

These radio jets are powerful forces that astronomers are trying to understand better and are widely considered a common feature of “quasars” — the collective name for the active galactic nucleus holding a supermassive black hole, its gassy radiation disk and radio jets. 

Together, these features dictate how galaxies evolve. By heating and dispersing gas, radio jets spanning multiple light years may prevent the birth of stars. By stirring up turbulence, they can cause gas to collapse, ultimately forming new stars.

“Star formation is like a dandelion — all the stuff around it is the star’s mass, but a black hole may blow away much of that gas, preventing it from getting bigger, explains Radcliffe, a postdoctoral fellow at the South African Radio Astronomy Observatory. 

“This is why black holes are a crucial area of research. And that is what this study was trying to probe — exactly how black holes affect the evolution of a galaxy to the point of its present growth.”

How to whip up a quasar

If ever on an idle Sunday you felt like whipping one up, the Radcliffe study proposes evidence to support the thinking that radio jets are not critical to the ideal quasar recipe. 

As much as the absorption (“accretion”) of matter on to a supermassive black hole appears to be “a standard ingredient in the life of a galaxy”, Radcliffe and co-authors argue, this “may or may not generate radio jets”. 

“We have challenged the paradigm that some active black holes will always produce the jets,” Radcliffe told Daily Maverick. We’re saying this is not necessarily the case: there is a large proportion of these active objects that simply don’t have radio jets at all. We can’t even see them with deep, sensitive radio telescopes. 

“We’ve also found that radio emissions coming from these objects are, in fact, going to be dominated by radio emissions from stars rather than the central [supermassive] black hole. The black hole is active, it’s just not making jets; it must therefore belong to a population of black holes getting trickle-fed gas or dust.” 

Other studies have hinted at this, he says, “but ours is the first to show this more clearly. This has some more profound implications for the way in which galaxies must be formed.”

Violent meals drive galactic evolution

Spawned by a potent stellar explosion, a standard black hole is born when the remaining mass of this spectacular event collapses under the force of its gravity. This creates a region so dense not even light gets to jump in the galactic getaway car. 

Yet, this is just an ordinary or “stellar-mass” black hole, weighing less than 100 times the mass of our Sun. 

However, when black holes at the business end of the buffet gorge themselves on a smorgasbord of gas and dust torn off other stars, they grow. 

They might also merge with other black holes doing the same thing — and that’s when a supermassive space Godzilla millions or billions of times the Sun’s mass is born at the centre of a galaxy.

If galaxies are factories that forge stars from diffuse gas, supermassive, or central, black holes may be engine rooms driving that evolution — depending on whether they are going through an “active”, hungry phase; or “inactive”, peaceful phase. 

Milkdromeda with your coffee? 

In the universal sense, “our Milky Way is quite serene, and our black hole Sagittarius-A* [pronounced ‘Sagittarius-A Star’] is doing absolutely nothing,” Radcliffe notes. “We do not see this very bright emission we see in some other galaxies.” 

In other words, Sagittarius-A* is a great example of an inactive or starving black hole, getting fed almost nothing.

“This enables Earth to orbit the black hole at the Milky Way’s centre peacefully, in the same way we have happily orbited our Sun for 4.5-billion years without much incident.”

(Of course, Andromeda will mess all of this up when it merges with our galaxy in a few billion years, Radcliffe warns, eventually turning our nice, spiral galaxy into an elliptical galaxy called “Milkdromeda”. During the super-intense star formation and other activity caused by this merger, our black hole will turn on, and become active.) 

‘A spectacular pair of galaxies engaged in a celestial dance of cat and mouse or, in this case, mouse and mouse,’ according to NASA. ‘Located 300 million light-years away in the constellation Coma Berenices, the colliding galaxies have been nicknamed ‘The Mice’ because of the long tails of stars and gas emanating from each galaxy. Otherwise known as NGC 4676, the pair will eventually merge into a single giant galaxy.’ (Credit: NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M. Clampin (STScI), G. Hartig (STScI), the ACS Science Team and ESA — APOD 2004-06-12)

See:  APOD 2004-06-12

We might think of active black holes as Audrey II, that bluesy Venus flytrap from the American cult film Little Shop of Horrors, which devours everything in its reach.

Indeed, the preposterous Audrey II could be the black hole’s anointed emissary on Earth. Eventually confessing to being a “mean green mother from outer space”, the plant’s flytrap slurps up human bodies as though they were mere stars, gas and dust vanishing over a black hole’s event horizon. Its tentacles — gravity. Mushnik’s Flower Shop, Audrey II’s earthly stopover in “Skid Row”, New York? A proxy for the cosmos itself.  

The more ham-fisted shop florist Seymour feeds Audrey II, the more it wants.

“A black hole can eat a few times the mass of the Sun in terms of gas and dust per year. With all this stuff dropping down towards the black hole, it accelerates — in the same way an object speeds towards Earth when you drop it from a roof,” Radcliffe says. 

“As the black hole gets brighter and brighter, and faster and faster, it produces the radiation that you see.” 

Look at the stars, and how they shine for you

There must be some red flag to keep fanning these intimately interactive processes — that is, the galactic Wars of the Roses between black holes and stars.

Indeed, that red flag, or “feedback mechanism”, comes in the form of electromagnetic radiation amassing or “accreting” on the black hole’s gassy disk, which triggers the savage forces, such as radio jets, crucial to the universe’s evolution. Researchers are still not sure how radio jets are formed, yet the symbiotic feedback mechanism is a well-studied phenomenon that makes everything you know possible: the stars and how they shine for you; pop band Coldplay’s greatest hits; as well as the skin-cased parcel of meat, cells and bones representing your body. 

“If you do these very large cosmological simulations, say, using a supercomputer to model the universe from its conception to modern day, you don’t get a universe that looks like ours unless you have a black hole spewing out all this matter,” Radcliffe says. 

Ferocious behaviour at the buffet table can occur at the core of many types of galaxies, the team has found. 

“Supermassive black holes at the centre of galaxies grow during relatively short phases, when they swallow matter from their immediate environment. These growth episodes manifest themselves as violent phenomena: emitting extremely strong radiation we can then detect with our telescopes,” he says. 

“We’ve helped clarify the eating habits of black holes by showing there is massive variability in the way they eat objects,” he suggests. “Some galaxies will accrete, or eat, a lot of mass at once. Some won’t. 

“We haven’t solved it completely: we’re standing on the shoulders of giants. Scientists have been trying to answer this question for decades. All we’ve done is open up this question, as well as highlight the significance in terms of the telescopes that are going to come online.” 

‘Obscenely more sensitive’

Astronomers have trained their gaze on active galaxies — a galaxy with a supermassive black hole at its core — since the mid-20th century. Using ground as well as space telescopes, this picture has given us solid insights, yet it is incomplete.

“Observations with space telescopes have taught us a lot about active galaxies, but such telescopes are expensive,” Radcliffe points out. 

In this latest study, the constraints of space telescopes inspired him and his colleagues to trawl through data from a transcontinental network of ultra-sensitive radio telescopes. Analysing the GOODS-North field, a celestial patch about a fifth of the full moon’s size, the team targeted highly energetic objects producing radio emissions — these are objects that are extreme in their physics, such as a supernova explosion, which will decimate everything in its path; radio jets; or an accretion disk.

“Because these objects are so bright, we can use radio to travel to the edge of the observable universe — not many other types of telescopes can do that,” he explains. “Radio can see straight to anything that’s bright or active through all those black dust bands going across the sky.” 

Representing just a sliver of sky, the GOODS-North field proved big enough to yield tens of thousands of brightly twinkling galaxies, allowing the team to pinpoint all active galaxies in that zone. 

“Ultra-sensitive radio telescopes [are] uniquely suited to detect nuclear activity in galaxies,” he suggests, “but also demonstrates the existence of a class of active galaxies seemingly invisible to radio telescopes.”

In addition to spotlighting how and what black holes eat, new-generation radio telescopes such as the Northern Cape’s MeerKAT and Square Kilometre Array (SKA) are therefore likely to be ideal instruments for finding active galaxies, he notes. 

Originally from the UK, the early-career scientist, 29, says he was attracted to working in South Africa because of the, well, astronomical computing power promised by these instruments. 

According to the SKA, partially based in Australia, the project’s vast data quantities will require “high-performance central supercomputers capable of [handling] in excess of 100 petaflops (one-hundred-thousand-million-million floating-point operations per second of raw processing power)” — the equivalent of the fastest supercomputer currently in operation on Earth. 

“If there is any young South African who may be disillusioned, or does not know what they want to do, astronomy has a very bright future here,” he says. “South Africa’s MeerKAT and SKA are going to shed more light on these processes between black holes and stars. And these world-leading new instruments, which are going to be obscenely more sensitive, will allow us to probe right to the back of the universe. They will give us an unprecedented view.” 

Dr Jack Radcliffe of the South African Radio Astronomy Observatory and the University of Pretoria. (Photo: Supplied)

Radcliffe recently started as an astronomy lecturer at the University of Pretoria, which contributed to the Event Horizon Telescope effort that in 2019 gave Earthlings their first image of a black hole’s shadow. In 2021, South African researchers helped send waves of excitement rippling through this part of the Milky Way again — this time by working on the Event Horizon project that showed in bright detail the magnetic fields of a supermassive object in the Messier 87 galaxy. These fields, Radcliffe says, are “crucial to understanding the jet-production method”. 

Radio telescopes could be the best way of homing in on active supermassive black holes — however, “we’ll miss half of the story if we don’t use other techniques to catch them”. 

Among 14 wavebands explored by the team, Radcliffe gives special credit to X-ray instruments — such as NASA’s Chandra X-ray space telescope — as another key way of observing high-energy objects.

“Using lots of telescopes of different wavelengths enabled us to see what and how much black holes are eating,” he says.

Published as two papers in the journal Astronomy & Astrophysics, the research was co-authored by Radcliffe’s former PhD supervisors, professors Michael Garrett from the University of Manchester and Peter Barthel from the University of Groningen in the Netherlands.

“The evidence for massive black holes in the nuclei of all galaxies is now very strong,” according to professors Garrett and Barthel. “These black holes grow to their current mass, and it appears that our radio studies permit unique observations to address all aspects of the accretion processes, and to ultimately understand them.” DM/OBP

Journal reference: 

Astronomy & Astrophysics

Nowhere to Hide: Radio-faint AGN in the GOODS-N field

The radio emission from active galactic nuclei 

Browse the free preprints here: 

https://arxiv.org/abs/2103.08575

https://arxiv.org/abs/2104.04519