This very faint, very tenuous filament of gas is streaming into the galaxy NGC 6946 and may help explain how certain spiral galaxies keep up their steady pace of star formation, researchers said.
"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 astronomer DJ Pisano from West Virginia University.
"A leading theory is that rivers of hydrogen - known as cold flows - may be ferrying hydrogen through intergalactic space, clandestinely fueling star formation. But this tenuous hydrogen has been simply too diffuse to detect, until now," Pisano said.
Spiral galaxies, like our own Milky Way, typically maintain a rather tranquil but steady pace of star formation.
Others, like NGC 6946, which is located approximately 22 million light-years from Earth on the border of the constellations Cepheus and Cygnus, are much more active, though less-so than more extreme starburst galaxies.
This raises the question of what is fueling the sustained star formation in this and similar spiral galaxies.
Earlier studies of the galactic neighborhood around NGC 6946 with the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands have revealed an extended halo of hydrogen (a feature commonly seen in spiral galaxies, 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 US National Science Foundation's Robert C Byrd Green Bank Telescope (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.
Astronomers have long theorised that larger galaxies could receive a constant influx of cold hydrogen by siphoning it off other less-massive companions.
Looking at NGC 6946, the GBT detected just the sort of filamentary structure that would be present in a cold flow.
However, it is also possible that in the past this galaxy had a close encounter and passed by its neighbours, leaving a ribbon of neutral atomic hydrogen in its wake.
If that were the case, however, there should be a small but observable population of stars in the filaments.
Further studies will help to confirm the nature of this observation and could shine light on the possible role that cold flows play in the evolution of galaxies.
The results are published in the Astronomical Journal.