Double Nucleus Galaxies: Ravenous Black Holes And Ripples In Space-Time Continuum

In a study published in the Monthly Notices of the Royal Astronomical Society, Swinburne researchers examined 50 regular galaxies to determine their composition and structure.

The researchers, Associate Professor Alister Graham and Dr Lee Spitler, found that 12 of these galaxies contained a double nucleus -- that is, they had both a super massive black hole and a dense star cluster containing up to ten million stars at their centre.

Traditionally astronomers believed that most small galaxies had a compact star cluster at their centre while giant galaxies had a super massive black hole.

Double nucleus galaxies were thought to be extremely rare; however the Swinburne researchers have shown that they are actually quite common.

According to Graham, the prevalence of double nucleus galaxies significantly increases the likelihood of several bizarre astronomical phenomena occurring.

The first of these is a phenomenon where black holes ‘eat up’ nearby stars.

“When stars get too close to massive black holes, the gravitational attraction is such that they can be devoured,” Graham said. “When you’ve got up to a million stars within the immediate vicinity of a black hole, the chance of this occurring increases significantly.”

The predominance of double nucleus galaxies also means there are likely to be many more ‘hyper velocity stars’ in existence than astronomers had thought.

“This is when a star approaches a massive black hole and gets caught in a gravitational slingshot. When this happens stars can be ejected from galaxies at speeds in excess of 500 kilometres per second,” Graham said.

Another implication of the research – and according to the researchers one of the most exciting – is the increased likelihood of the phenomenon known as gravitational radiation.

“Such emission has been predicted by Einstein’s General Theory of Relativity, but has never been observed,” said Spitler. “It is theorised that when stars spiral quickly around a black hole the motion will create gravitational waves – causing ripples in the space-time continuum.”

The Laser Interferometer Space Antenna (LISA) – a suite of three satellites spaced five million kilometres apart and planned for launch in 2018 – is being designed to search for such ripples as they pass through our solar system.

While the Swinburne research has greatly increased the chance of observing some of the more bizarre events in our Universe, it also has some immediate consequences for astronomers.

“As part of our study we were able to look at star clusters and black holes and determine their mass in proportion to each other and their host galaxies,” Graham said.

“This knowledge is going to affect the way astronomers develop models for galaxy formation and evolution.

“Previously evolution models only dealt with one type of nucleus per galaxy. We now have the rationale and data to develop hybrid models that can account for co-existing nuclei and hopefully explore their dynamic joint evolution.”