A Hydrogen Gas River Seen Feeding the Fireworks Galaxy, Provides Fuel for Above-Average Star Birth Rate

February 22, 2014

A river of cold hydrogen gas has been discovered, feeding the Fireworks Galaxy with fuel it needs in order to sustain the high birth rate of its stars. Until now, it was hard to detect because it is too diffuse and cold, but it explains perfectly where the extra hydrogen comes from.

The Fireworks Galaxy surrounded by cold hydrogen gas (red) that extends to its companions and beyond in ribbon-like shape.

The Fireworks Galaxy, NGC 9645. In the centre, blue, is the core of the galaxy. Around it is an orange halo made of hot hydrogen gas. Much larger is the cold hydrogen gas that extends to the galaxy's companions to the right. 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


A river of cold hydrogen gas has been discovered, feeding the Fireworks Galaxy with fuel it needs in order to sustain the high birth rate of its stars. Until now, it was hard to detect because it is too diffuse and cold, but it explains perfectly where the extra hydrogen comes from.

There are galaxies out there that keep "giving birth" to above average number of stars, a number that was hard to explain given the age of those galaxies. After a certain time, most galaxies run out of hydrogen gas needed for formation of new stars, but some keep supplying their stars with it. Up until now, there were no observations that would confirm that these galaxies were supplied with excess hydrogen gas.

The galaxy named NGC 6946 is one such galaxy. It is also known as The Fireworks Galaxy because of the high number of its stars going supernova. Reason for the high rate of dying stars is the high birth rate of stars in the galaxy. It is a medium-sized spiral galaxy in the constellations Cepheus and Cygnus, 22.5 million light-years away.

By observing it with the Green Bank Telescope in West Virginia, USA, astronomers led by D.J. Pisano, of West Virginia University, discovered a faint, but broad stream of cold hydrogen gas between the Fireworks Galaxy and its smaller companions. "We knew that the fuel for star formation had to come from somewhere," said Pisano. "So far, however, we've detected only about 10 percent of what would be necessary to explain what we observe in many galaxies." 

Like many spiral galaxies, the Fireworks Galaxy has a halo around it, which was discovered in previous observations. It consists of hydrogen that was left over after the formation of stars or ejected by supernovae, both of which are frequent in the galaxy. As mentioned, the hydrogen in the halo isn't enough to supply new stars with fuel. The ribbon of gas that was discovered by Pisano and his colleagues is too cold to be cooled-down remnant of the halo and because of its temperature, it was really difficult to detect.

"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."

Alternative to these rivers of hydrogen is an explanation that this cold gas could have come from another galaxy that the Fireworks Galaxy had close encounter with. Due to gravitational interaction, a hydrogen gas could have been "snatched away", forming the ribbon in question. If this was true, however, there would be owner-changing stars too, none of which were detected as of yet.

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