Starry Fuel Tanks: What fuels the starburst phases of galaxies?

This cartoon shows how gas falling into distant starburst galaxies ends up in vast turbulent reservoirs of cool gas extending 30 000 light-years from the central regions. ALMA has been used to detect these turbulent reservoirs of cold gas surrounding similar distant starburst galaxies. By detecting CH+ for the first time in the distant Universe, this research opens up a new window of exploration into a critical epoch of star formation.

A long time ago, in galaxies far, far away, stars were churned out at unprecedented rates: over 100 solar masses were produced annually. These luminous, dusty starburst galaxies were 1000 times more common in the very early universe than they are today. But the light from these stars is only now reaching the earth, ending its multi-billion year journey through the cosmos as it reaches our telescopes. By looking out into the universe and back into the past, we gain a better understanding of how these starburst galaxies formed and how they sustained such rapid rates of star production.

In the early universe, galaxies differed in their abilities to produce stars. Many of the galaxies at this time were unable to support such rapid star formation, as the process blasted away the hydrogen gas, preventing the creation of future stars. Yet others continued to produce hundreds of stars per year for periods up to 100 million years, long after other galaxies had subsided. Why did some galaxies remain fertile while others died down? Recent research suggests that we can find the answer with the help of CH+, a rare but useful cation (a positively-charged, electron-deficient molecule).

Using the Atacama Large Millimeter Array (ALMA) radio telescopes in Northern Chile, astronomers studied six early starburst galaxies. They found that CH+ is abundantly present in all of them, providing insight into what set these galaxies apart from the others.  CH+ is very rare and was only discovered in 1941, as it can only form in extremely cold temperatures of about 20 Kelvin and with extremely high energy inputs equivalent to 4000 Kelvin.  With such a high energy requirement, CH+ must be formed in the presence of strong ultraviolet radiation or mechanical energy. It also has an extremely short lifespan, so it cannot be transported far. Therefore, its presence in these galaxies suggests that they must have recently undergone violent energy shocks.

“CH+ traces how energy flows in a galaxy. Think of plankton fluorescence which is excited by little shocks that generate turbulence in the water. When you throw a stone in, you light up trails of fluorescent plankton,” said Edith Falgarone of the École Normale Supérieure and Observatoire, where the research was conducted.

Falgarone and her team of researchers studied the emission and absorption spectral lines of this CH+ cation in samples from the six starburst galaxies. Each spectral line emitted by the interstellar gas corresponds to a different compound present in the galaxy, so the CH+ spectral lines can tell us a lot about how the CH+ was formed. The lines revealed the presence of extremely turbulent hydrogen gas surrounding the galaxies, extending far outwards from the cores where stars form.

The discovery of this turbulent gas elucidates how galaxies grow and how these star-forming engines are fueled. The width of the CH+ lines are broader than 1000 kilometers per second, suggesting that it was born in enormous shock waves. The researchers suggest that these motions are powered by energetic outflows originating in the core of the galaxy. These outflows exit the galaxy in such a way that leaves matter trapped within the galaxy’s gravitational pull. This culminates in vast, turbulent reservoirs of cool, low-density gas circling the galaxy, up to 30,000 light-years from the galaxy core.

While it may seem that these high-powered inner-galactic winds would quench the starburst phase of the galaxy, slowing the rate of star production by blowing out much of the hydrogen gas needed to create new stars, the researchers suggest that the opposite is actually true. Transforming previous models for galaxy formation, Falgarone and her team propose that the winds actually extend the star-formation phase by feeding these vast reservoirs of “fuel” for future stars.

“What we have found with CH+ is that this stellar feedback generates turbulence in the galactic environment, so energy is lost and the outward momentum of the gas is lost too.  Indeed, most of the gas expelled from the galaxy eventually falls back on it, feeding further star formation instead of quenching it,” Falgarone explains.

The study has, however, raised a few questions among relevant academic circles. “The authors suggest that turbulence in the outlying molecular gas slows down its infall and prolongs star formation. To me, this seems plausible but unproven. I don’t see how the generation of strong turbulence would contribute to the fueling of a starburst; if anything, I would expect it to inhibit gas infall,” says Dr. Richard Larson of the Yale Astronomy Department.

Yale professor Héctor Arce, who is currently researching star formation in the Milky Way galaxy, had similar questions about the data. According to Arce, the CH+ indicates massive outflows of gas from the center of the galaxies. He agrees that this likely means that these outflows feed a cool gas reservoir surrounding the galaxy but would like to see some more data before making the conclusion that this reservoir is what prolongs the starburst phase. “I don’t see the jump personally between the data and the conclusions,” Arce said. “This is not to say that the results are invalid in any way, just that the beauty of the data could have perhaps been more fully presented in a longer piece.”

Larson and Arce also both expressed excitement about what Falgarone’s work means for future research into star formation. We are currently seeing very exciting results coming from the ALMA telescopes. They observe in the millimeter and sub-millimeter wavelengths, so they’re useful for observing dust and molecular-level matter such as CH+. “This paper is an example of the great work people are doing with the ALMA telescopes,” Arce said.

The work acknowledges that the mass outflow rates caused by the winds alone do not completely account for the extreme rates of star production. Something else, still unknown, is nourishing these reservoirs. Falgarone and her team suggest that perhaps this extra mass is produced by galactic mergers or possibly accretion from streams of gas that are sucked into the gravity of the galaxy.

Falgarone’s work sheds light on questions that have been puzzling scientists for years—they have wondered, how did starburst galaxies come by their extra fuel? We may now have some answers, but the result also raises new questions. What causes the hot, violent winds at the centers of certain galaxies, powering the cool gas reservoirs? Why do some galaxies have them and others do not? Astronomers continue to scour the cosmos for answers.