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6 mins read 16 Jan 2024

A Radio Source in the Heart of 47 Tuc

A global team of scientists, led by ICRAR astronomers, have created the most sensitive radio image of the Globular Cluster 47 Tucanae and found an interesting, undiscovered radio source at its centre - opening up the case for exciting science.

The central region of the southern hemisphere Globular Cluster, 47 Tucanae. Credit: NASA / ESA / Hubble.

Globular Clusters (GCs) are tightly bound, spherical-shaped conglomerations of stars, some containing hundreds of thousands or even millions of stars. These are the elders of our Galaxy, with many of their stars forming about 12 billion years ago when the Universe was much younger. 

These ancient clusters orbit our Galaxy at many different inclinations, hovering above and below the main Galactic disc and plane, forming a halo of bodies around the Milky Way. However, our Galaxy is nothing special - with many other galaxies observed to have their own population of GCs.

Two of the Milky Way’s brightest GCs are visible from the southern hemisphere - so bright that you can even spot them with your naked eye from a dark enough sky. The largest of them is called Omega Centauri and contains a whopping 10 million stars. Coming in second place is 47 Tucanae (47 Tuc) which also contains a hefty million stars or so. These types of clusters are bound together by their shared mutual gravity, collectively travelling in their orbits around their host galaxy. 

Given their relatively close distance (astronomically speaking) to Earth, their brightness and their compact nature, GCs make a favourable target for astrophotographers - but they also are exciting laboratories for scientists with many questions still looming about their stellar population, their histories and what lies at their dense centres. 

Radio image of the core of 47 Tuc, showing the newly discovered radio source at its centre. Credit: Paduano et al. 2023.

Now, an international team of astronomers have created the most deepest, sensitive radio image of any GC by an Australian radio telescope, and found something rather interesting in its core. What the team has uncovered is a previously undetected radio source located at the centre of the 47 Tuc. Their research has been published in The Astrophysical Journal

The team was led by the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR) in Western Australia and accumulated more than 450 hours of observations using the Australian Telescope Compact Array (ATCA) located in Narrabri (NSW) and operated by Australia’s national science agency, CSIRO. 

“The detection of the signal was an exciting discovery and could be attributed to one of two possibilities,” said lead author Dr Alessandro Paduano from ICRAR. “The first is that 47 Tuc could contain a black hole with a mass somewhere between that the supermassive black holes found in the centres of galaxies and stellar mass black holes, created by collapsed stars. The second possible source of the signal is a pulsar - a rapidly rotating neutron star that emits radio waves."

Black Holes of Different Sizes

Observed mass ranges of different types of compact, massive objects relative to the Sun’s mass. Credit: NASA.

Astronomers classify black holes by the mass they contain, which indirectly is related to the objects that can produce them. So far, evidence has been found for black holes with masses in the stellar range - that is a few times the mass of our Sun, and even up to a few tens of this value. This evidence has included observing the electromagnetic radiation that the black hole causes its surrounding regions, usually through accretion of material from a companion (or even dragging its companion around at high velocities), but also gravitational wave data of merging binary stellar-mass black hole systems. 

On the other end of the scale are supermassive black holes (SMBH), which contain millions and even billions of times the mass of our Sun. These colossal objects reside in the hearts of galaxies, including our own. Astronomers have imaged two of these (one at the core of the M87 galaxy, the other in the heart of the Milky Way), but there has also been other observational evidence for their existence (including watching stars being flung around at high velocities). 

However, the mass range that falls between these two - the intermediate-mass black holes (IMBH) remains a subject of scientific investigation, with loose evidence so far gathered. It is suspected (and with evidence from gravitational wave astronomy) that stellar mass black holes will merge and accumulate mass over time, getting bigger as they consume each other. Eventually, this accretion model will lead to IMBHs and over deep time, SMBHs. 

However, for these mergers to occur in the time that the Universe has been in existence, this type of dynamic interaction would require a dense stellar environment, where objects are close enough together to exhibit and influence their neighbours through their own gravity. The type of environments that GCs provide. 

“While IMBHs are thought to exist in GCs, there hasn’t been a clear detection of one yet,” said Paduano. “If this signal turns out to be a black hole, it would be a highly-significant discovery and the first-ever radio detection of one inside a cluster”

There is also the other possibility that is being considered by the team, that this might not be an IMBH, but instead a pulsar. “A pulsar this close to the cluster centre is also a scientifically interesting discovery, as it could be used to search for a central black hole that is yet to be detected,” he said. 

Pulsars are effective tools for astronomers, as their regular periodic ‘pulse’ is used as a clock, and so many tests of Einstein’s General Relativity can be performed with them. In fact, the first indirect evidence of gravitational waves used a pulsar in this manner, leading to the awarding of a Nobel Prize for the discovery. 

Considered one of the holy grails of General Relativity, many astronomers are excited at the possibility of discovering the first pulsar orbiting a black hole - and the science we would gain from this. If this new radio source at the centre of 47 Tuc is indeed a pulsar, many radio astronomers are going to be looking for the tell-tale signature of it being dragged around by a nearby black hole. 

CSIRO’s Australian Telescope Compact Array, which was used to make the discovery. Credit: A. Cherney / CSIRO.

Co-author Dr Tim Galvin, a research scientist with CSIRO, said the project once again demonstrated the ongoing importance of ATCA.

“This project has stretched our software to its limits, in terms of both data management and processing, and it has been really exciting to see the wealth of science that these techniques have enabled.”

“Alessandro’s research represents a culmination of years of research and technological advancements, and ATCA’s ultra-deep image of 47 Tucanae represents just the beginning of the discoveries that are yet to come.”

We acknowledge the Gomeroi people as the traditional owners and custodians of the land in which the CSIRO ATCA instrument resides. 

The paper has been published in The Astrophysical Journal