Astronomer Discovers Hydrogen River Flowing Into Nearby Galaxy
January 28, 2014

Astronomer Discovers Hydrogen River Flowing Into Nearby Galaxy

April Flowers for - Your Universe Online

What might be a never-before-seen river of hydrogen flowing through space has been discovered by West Virginia University astronomer, D.J. Pisano, using the National Science Foundation's (NSF) Robert C. Byrd Green Bank Telescope (GBT).

A study, published in the Astronomical Journal, reveals that this river is a faint, very tenuous filament of gas, which is streaming into the nearby galaxy, NGC 6946. Understanding this 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 percent 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 fueling star formation. But this tenuous hydrogen has been simply too diffuse to detect, until now.”

Typical spiral galaxies, such as our own Milky Way, maintain a steady, yet tranquil, pace of star formation. Galaxies like NGC 6946—located approximately 22 million light years from Earth on the border of the constellations Cepheus and Cygnus—have more active star formation. They are, however, less active than more extreme starburst galaxies. Scientists have questioned what fuels the sustained star formation in NGC 6946 and others like it.

Previous research of the galactic neighborhood surrounding NGC 6946, using the Westerbork Synthesis Radio Telescope (WSRT) in the Netherlands, revealed an extended halo of hydrogen. Such a halo is commonly seen in spiral galaxies and might be formed by hydrogen ejected from the disk of the galaxy by intense star formation and supernova explosions. A completely different source would be necessary for a cold flow of hydrogen—such as gas from intergalactic space that has never been heated to extreme temperatures by a galaxy's star formation or supernova processes.

Pisano used the GBT to detect the glow emitted by neutral hydrogen gas that connects NGC 6946 to its neighboring galaxies—a signal that was simply below the detection threshold of other telescopes. The GBT was able to detect this tenuous radio light because of its unique capabilities, including its immense single dish, unblocked aperture, and location in the National Radio Quiet Zone and the West Virginia Radio Astronomy Zone—which protect the incredibly sensitive telescope from unwanted radio interference.

Astronomers have long held the theory that larger galaxies might obtain a constant influx of cold hydrogen by siphoning it from less-massive companions. Using GBT to examine NGC 6946, Pisano detected exactly the sort of filamentary structure that would be present in a cold flow. Another possible explanation does exist, however--that sometime in the past this galaxy passed close enough to its neighbors to leave a ribbon of neutral atomic hydrogen in its wake.

If the "ships passing in the night" theory were correct, however, there should be a small but observable population of stars in the filaments. Further research is needed to confirm the nature of this observation. Pisano hopes the findings will shine light on the possible role that cold flows play in the evolution of galaxies.