A team of astronomers led by Susanne Aalto, professor of radio astronomy at Chalmers, has used the ALMA telescope to observe a remarkable structure in the center of galaxy NGC 1377, located 70 million light-years from Earth in the constellation Eridanus the River.
“We were curious about this galaxy because of its bright, dust-enshrouded center. What we weren’t expecting was this: a long, narrow jet streaming out from the galaxy nucleus,” said Aalto.
The observations with ALMA reveal a jet which is 500 light-years long and less than 60 light-years across, traveling at speeds of at least 500,000 miles (800,000 kilometers) per hour.
Most galaxies have a supermassive black hole in their centers; 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 reveals the presence of a supermassive black hole. But 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,” he said.
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 team member Jay Gallagher from the University of Wisconsin in Madison.
The motion of the gas in the jet also surprised the astronomers. The measurements with ALMA are consistent with a jet that 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 center contains two supermassive black holes in orbit around each other,” said Sebastien Muller from Chalmers.
The discovery of the remarkable cool swirling jet from the center of this galaxy would have been impossible without ALMA, said Aalto.
“ALMA’s unique ability to detect and measure cold gas is revolutionizing our understanding of galaxies and their central black holes. In NGC 1377, we’re witnessing a transient stage in a galaxy’s evolution which will help us understand the most rapid and important growth phases of supermassive black holes, and the life cycle of galaxies in the universe,” she said.