Hydrogen river seen in space

This composite image shows the bright star-filled central region of galaxy NGC 6946 in optical light (blue), the dense hydrogen tracing out the galaxy’s sweeping spiral arms and galactic halo (orange), and the extremely diffuse and extended field of hydrogen engulfing NGC 6946 and its companions (red).

USING the Green Bank Telescope (GBT), D.J. Pisano of West Virginia University (WVU) discovered what could be a never-before-seen river of hydrogen flowing through space. This very faint, very tenuous filament of gas is streaming into the nearby galaxy NGC 6946 and may help explain how certain spiral galaxies keep up their steady pace of star formation. “We knew that the fuel for star formation had to come from somewhere. So far, however, we’ve detected only about 10 per cent of what would be necessary to explain what we observe in many galaxies,” said Pisano. “A leading theory is that rivers of hydrogen—known as cold flows—may be ferrying hydrogen through intergalactic space, clandestinely fuelling star formation. But this tenuous hydrogen has been simply too diffuse to detect until now.”

Spiral galaxies, like the Milky Way, typically maintain a rather steady pace of star formation. Others, like NGC 6946, which is located approximately 22 million light years from the earth, are much more active . This raises the question of what is fuelling the sustained star formation in this and similar spiral galaxies. Astronomers have long theorised that larger galaxies could receive a constant influx of cold hydrogen by siphoning it off other less-massive companions.

In looking at NGC 6946, the GBT detected just the sort of filamentary structure that would be present in a cold flow though there is another probable explanation for what was observed. Further studies will help confirm the nature of this observation and could shine light on the possible role that cold flows play in the evolution of galaxies.

Earlier studies of the galactic neighbourhood around NGC 6946 with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands revealed an extended halo of hydrogen (a feature commonly seen in spiral galaxies and which may be formed by hydrogen ejected from the disk of the galaxy by intense star formation and supernova explosions). A cold flow, however, would be hydrogen from a completely different source: gas from intergalactic space that has never been heated to extreme temperatures by a galaxy’s star birth or supernova processes. Using the GBT, Pisano was able to detect the glow emitted by neutral hydrogen gas connecting NGC 6946 with its cosmic neighbours. This signal was simply below the detection threshold of other telescopes. The GBT’s unique capabilities, including its immense single dish, unblocked aperture, and location in the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone, enabled it to detect this tenuous radio light. The results are published in The Astronomical Journal.