Scientists have discovered a jet of cool, dense gas – with an unusual, swirling structure – which indicates the presence of a growing supermassive black hole in the centre of a galaxy located 70 million light years from Earth.
Astronomers used the Alma telescope (Atacama Large Millimetre/submillimetre Array) in Chile to observe a remarkable structure in the centre of the galaxy NGC 1377, located in the constellation Eridanus.
“We were curious about this galaxy because of its bright, dust-enshrouded centre. What we weren’t expecting was this – a long, narrow jet streaming out from the galaxy nucleus,” said Susanne Aalto, professor at Chalmers University of Technology in Sweden, who led the research.
The observations with Alma unveil a jet which is 500 light years long and less than 60 light years across, travelling at speeds of at least 800,000 kilometres per hour.
Most galaxies have a supermassive black hole in their centres; these black holes can have masses of between a few million to a billion solar masses.
How they grew to be so massive is a long-standing mystery for scientists.
A black hole’s presence can be seen indirectly by telescopes when matter is falling into it – a process which astronomers call “accretion.”
Jets of fast-moving material are typical signatures that a black hole is growing by accreting matter. The jet in NGC 1377 unveils the presence of a supermassive black hole.
However, it has even more to tell us, said Francesco Costagliola from Chalmers.
“The jets we usually see emerging from galaxy nuclei are very narrow tubes of hot plasma. This jet is very different. Instead it’s extremely cool, and its light comes from dense gas composed of molecules,” said Costagliola.
The jet has ejected molecular gas equivalent to two million times the mass of the Sun over a period of only around half a million years – a very short time in the life of a galaxy.
During this short and dramatic phase in the galaxy’s evolution, its central, supermassive black hole must have grown fast.
“Black holes that cause powerful narrow jets can grow slowly by accreting hot plasma. The black hole in NGC 1377, on the other hand, is on a diet of cold gas and dust, and can therefore grow – at least for now – at a much faster rate,” said Jay Gallagher from University of Wisconsin-Madison.
The measurements show that the jet is precessing – swirling outwards like water from a garden sprinkler.
“The jet’s unusual swirling could be due to an uneven flow of gas towards the central black hole. Another possibility is that the galaxy’s centre contains two supermassive black holes in orbit around each other,” said Sebastien Muller from Chalmers.
The research was published in the journal Astronomy and Astrophysics.