Fireworks Galaxy: Rivers of Hydrogen Gas Fuel Spiral Galaxies

Three distinct features are visible in this composite image of NGC 6946. 

The bright heart of the galaxy in optical light (blue), the dense hydrogen in the spirals (orange), and the extended halo surrounding the galaxy (red). 

New research also shows a faint filament that could be hydrogen flowing from the intergalactic medium into the galaxy to fuel star formation.

Credit: D.J. Pisano (WVU); B. Saxton (NRAO/AUI/NSF); Palomar Observatory – Space Telescope Science Institute 2nd Digital Sky Survey (Caltech); Westerbork Synthesis Radio Telescope

Inpouring rivers of hydrogen gas could explain how spiral galaxies maintain the constant star formation that dominates their hearts, a new study reports.

Using the Green Bank Telescope (GBT) in West Virginia, scientists observed a tenuous filament of gas streaming into the galaxy NGC 6946, known as the "Fireworks Galaxy" because of the large number of supernovae observed within it.

The find may provide insight into the source of fuel that powers the ongoing birth of young stars, researchers said.

D.J. Pisano

"We knew that the fuel for star formation had to come from somewhere," study lead author D.J. Pisano, of West Virginia University (WVU), said in a statement.

"So far, however, we've detected only about 10 percent of what would be necessary to explain what we observe in many galaxies."

Located 22 million light-years from Earth on the border of the constellations Cepheus and Cygnus, NGC 6946 is a medium-sized spiral galaxy pointed face-on toward the Milky Way.

Previous studies revealed a halo of hydrogen gas around NGC 6946 common to spiral galaxies.

Such halos are formed by hydrogen ejected from the galaxies by star formation and violent supernova explosions.

These interactions heat the gas in the halo to extreme temperatures.

When Pisano turned the GBT toward the spiral galaxy for further examination, however, he discovered a ribbon of gas too cool to have suffered the heating processes undergone by the halo gas.

On average, the Milky Way churns out between 1 to 5 new stars per year. Rich in gas, NGC 6946 is far more active. For example, it has hosted at least 9 explosive supernovae in the past century.

"A leading theory is that rivers of hydrogen — known as cold flows — may be ferrying hydrogen through intergalactic space, clandestinely fueling star formation," Pisano said. "But this tenuous hydrogen has simply been too diffuse to detect, until now."

The immense, unblocked dish of the Green Bank Telescope (GBT), combined with its location in the US National Radio Quiet Zone, where radio transmissions are limited, allow the large disk to detect the faint hydrogen signal that would be present in a cold flow.

Another possibility is that the hydrogen detected originated from a close encounter with another galaxy in the past.

The gravitational interaction between the two could have stretched out a ribbon of neutral atomic hydrogen, researchers said.

Such a ribbon would contain stars that astronomers should be able to easily observe, though none have yet been spotted. Further studies of the streamer hydrogen gas will help clarify its role.

The research was published in the Astronomical Journal.