Artist's impression of the HLX-1 black hole (circled) in the periphery of the edge-on spiral galaxy ESO 243-49.
Credit: NASA, ESA, and S. Farrell (Sydney Institute for Astronomy, University of Sydney).
LONDON: Detailed observations of a new kind of black hole have brought scientists one step closer to understanding how galaxies are formed.
A team of astronomers has studied the only known intermediate mass black hole - which is a type that has more mass than lightweight stellar mass black holes and less mass than supermassive black holes - to discover a very young, massive cluster of stars around it. This find helps to explain how larger, supermassive black holes and galaxies form and interact.
"Understanding how supermassive black holes form tells us a lot about the formation of galaxies like our own Milky Way," said Sean Farrell at the University of Sydney and the lead author of the paper published in the Astrophysical Journal today.
"The central black holes in galaxies have an enormous influence on the gas and stars around them, so understanding how supermassive black holes form has implications for how the galaxy itself formed, and thus how all the stars and planets came to be."
Middleweight black holes
There are two well known types of black hole, lightweight stellar mass black holes, that are a few to tens of times the mass of our Sun, and supermassive black holes that are millions to billions of times more massive.
The fact that the mass gap between these two classes is so huge led astronomers to wonder whether there might be an intermediate mass class of black holes that had previously gone undetected. In 2009, Farrell and his team discovered a middleweight, or intermediate mass, black hole called HLX-1 in the galaxy ESO 243-49.
"Up until recently the question of whether intermediate mass black holes existed was still heavily disputed, but our 2009 discovery essentially settled the debate," Farrell said.
How do supermassive black holes grow?
Next, he sought to understand how this black hole was formed and whether it might provide vital clues about the mysterious origins of supermassive black holes. Stellar mass black holes form when very massive stars run out of fuel and collapse, but the formation of supermassive black holes is not well understood.
One theory is that they might have formed at the beginning of the universe during the Big Bang. Alternatively, supermassive black holes might have formed through the merger of lower mass black holes.
If the latter theory were true, it would suggest that some point in the process would look like an intermediate mass black hole, such as HLX-1. And if that were the case, the scenario would need to play out in dense environments, where interactions between objects are likely. According to Farrell, the most suitable environments are dense, old clusters of stars called globular clusters, and the centres of low mass dwarf galaxies.
