Investigating the millions of missing stars from the centres – or cores – of two big galaxies, astronomers at Swinburne University of Technology say they may have solved this cosmic whodunit, and the main culprits are not the usual suspects.

While the scientists confirm that one of the depleted cores is the largest ever detected, they report that it may not have formed in the manner previously thought.

In normal sized galaxies, the density of stars increases smoothly as you move towards their centre. However, for decades astronomers have observed a star shortage in the centres of many big galaxies.

"The smaller of the two galaxies that we examined – the one with the smaller depleted core – likely formed from the collision of two similar galaxies, each seeded with a massive black hole several billion times the mass of our Sun," says lead-author Dr Paolo Bonfini, now at the Universidad Nacional Autónoma de México.

"In this well-studied process, the  migrate towards the centre of the newly-forged galaxy by ousting the stars already there, hurling them outward in a gravitational slingshot manoeuvre. Pairs of massive black holes effectively work together and gang up on individual stars in a galaxy's core."

Co-author of the study, Swinburne's Professor Alister Graham, says even less fortunate stars that venture too close to either black hole can be torn apart and swallowed.

"These cataclysmic events produce high-energy UV and X-ray flares as we briefly see into a star's hot interior while it is shredded by the immense gravitational field around each black hole. When the black holes themselves finally merge, a series of gravitational waves is also emitted," says Professor Graham.

However, simulations have shown that if a galaxy collision involves a larger galaxy consuming a smaller satellite galaxy, then things can be different.