ATCA Detects Compact Radio Source in Center of 47 Tucanae

Astronomers using CSIRO’s Australia Telescope Compact Array have captured the most detailed radio image ever seen of 47 Tucanae, the second brightest globular cluster in the night sky.

47 Tucanae, otherwise known as NGC 104, is a huge, ancient globular cluster about 15,300 light-years away in the southern constellation of Tucana.

At about 120 light-years across, the cluster is so large that, despite its distance, it looks about as big as the full Moon.

Hosting millions of stars, 47 Tucanae is one of the brightest and most massive globular clusters known and is visible to the naked eye.

“Globular clusters are very old, giant balls of stars that we see around the Milky Way. They’re incredibly dense, with tens of thousands to millions of stars packed together in a sphere,” said Dr. Arash Bahramian, an astronomer with the Curtin University node of the International Centre for Radio Astronomy Research (ICRAR).

“Our image is of 47 Tucanae, one of the most massive globular clusters in the galaxy. It has over a million stars and a very bright, very dense core.”

The ultra-sensitive radio image of 47 Tucanae was created from more than 450 hours of observations on CSIRO’s Australia Telescope Compact Array (ATCA).

“47 Tucanae can be seen with the naked eye, and was first catalogued in the 1700s,” Dr. Bahramian said.

“Imaging it in such great detail allowed us to discover an incredibly faint radio signal at the center of the cluster that had not been detected before.”

“The detection of the signal was an exciting discovery and could be attributed to one of two possibilities,” said Dr. Alessandro Paduano, also from ICRAR’s Curtin University node.

“The first is that 47 Tucanae could contain a black hole with a mass somewhere between the supermassive black holes found in the centers of galaxies and the stellar black holes created by collapsed stars.”

“While intermediate-mass black holes are thought to exist in globular clusters, there hasn’t been a clear detection of one yet.”

“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.”

The second possible source of the signal is a pulsar — a rotating neutron star that emits radio waves.

“A pulsar this close to a cluster center is also a scientifically interesting discovery, as it could be used to search for a central black hole that is yet to be detected,” Dr. Paduano said.

The results were published in the Astrophysical Journal.

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Alessandro Paduano et al. 2024. Ultradeep ATCA Imaging of 47 Tucanae Reveals a Central Compact Radio Source. ApJ 961, 54; doi: 10.3847/1538-4357/ad0e68