News Space 05 September 2017

Astronomers catch black hole shrouded in molecular cloud

A sighting of a so-called intermediate mass black hole helps to bridge the gap between star-sized black holes and the supermassive behemoths at the core of many galaxies. Andrew Masterson reports.

Artist's impression of molecular cloud swirling around an intermediate mass black hole.
Artist's impression of molecular cloud swirling around an intermediate mass black hole.
Keio University

A big black hole lurking near the centre of the Milky Way could constitute an important clue to how even bigger ones form.

In a paper published in the journal Nature Astronomy, scientists led by Tomoharu Oka from Keio University in Yokohama, Japan, report confirming a black hole with a mass one hundred thousand times that of the Sun.

The black hole was observed indirectly, following the detection of what the researchers call a “peculiar molecular cloud, [named] CO–0.40–0.22”. By carefully analysing the gas densities present in the cloud they concluded that it harboured a very large “compact object” that had to be a black hole.

The size of the hole classifies it as an intermediate-mass black hole (IMBH), considerably larger than a stellar black hole, which is formed by a collapsing star and has a mass only up to a few times that of the sun. The discovery, for astronomers, is very exciting.

It is widely accepted that extremely large galaxies contain at their centres supermassive black holes (SMBHs) with masses between a million and several billion times that of the sun – but how these form is unclear.

The mechanisms by which they grow so large is unknown, and speculation is complicated by the fact that they seem to have been in place very early in the history of the universe – fully forming only a few hundred million years after the big bang.

The leading theory is that they develop when IMBHs – which are created when multiple stars in young clusters collide – merge with others to form supermassive variants.

This idea, however, implies that IMBHs should be relatively common throughout the universe – a statement not supported by evidence.

IMBHs remain so rare, perhaps because they are challenging to detect and measure, that until recently their very existence was doubted by some astronomers. It wasn’t until 2014 that researchers from the University of Maryland and NASA definitively measured a black hole with a mass of 400 suns and thereby confirmed the existence of the whole class.

Joint Australian-US research confirmed another earlier this year, in a globular cluster known as 47 Tucanae.

There have been a handful of other confirmed or suspected IMBH discoveries over the past couple of years, but they are still so rare that the findings by Oka and colleagues are being warmly welcomed.

Theories predict, the scientists note in their paper, that the Milky Way alone should contain between 100 million and one billion black holes, but to date only 60 have been found. A proportion of the remainder should – or perhaps, must be – IMBHs.

Their eventual discovery will do more than just illuminate the process by which supermassive black holes form.

They will also “increase targets to search for evidential proof of general relativity,” the scientists note. “This would make a considerable contribution to the progress of modern physics.”

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Andrew Masterson is an author and journalist based in Melbourne, Australia.
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