This artist's impression highlights the compactness of molecular gas in a post-starburst galaxy and its lack of star formation.ALMA/S. Dagnello
Scientists once thought that post-starburst galaxies scattered all of their gas and dust - the fuel required for creating new stars - in violent bursts of energy, and with extraordinary speed. Now, a team led by University of Washington postdoctoral researcher Adam Smercina reports that these galaxies don't scatter all of their star-forming fuel after all. Instead, data from the Chile-based Atacama Large Millimeter/submillimeter Array, or ALMA, reveals a more complex process at work.
After their supposed end, these dormant galaxies hold onto and compress large amounts of highly concentrated, turbulent gas. But contrary to expectation, they're not using it to form stars.
In most galaxies, scientists expect gas to be distributed in a way similar to starlight. But for post-starburst galaxies, this isn't the case. Post-starburst galaxies are different from other galaxies because they are born in the aftermath of violent collisions, or mergers between galaxies. Galaxy mergers typically trigger massive bursts of star formation, but in post-starburst galaxies, this outburst slows down and near-completely stops almost as soon as it begins. As a result, scientists previously thought that little or no star-forming fuel was left in these galaxies' central star-forming factories. And until now, astronomers believed that the molecular gases had been redistributed well beyond the galaxies, either through stellar processes or by the effects of black holes. The new results, published April 25 in The Astrophysical Journal, challenge this theory.
"We've known for some time that large amounts of molecular gas remains in the vicinity of [post-starburst galaxies] but haven't been able to say where, which in turn, has prevented us from understanding why these galaxies stopped forming stars. Now, we have discovered a considerable amount of remaining gas within the galaxies and that remaining gas is very compact," said Smercina, who is lead author on the paper. "While this compact gas should be forming stars efficiently, it isn't. In fact, it is less than 10% as efficient as similarly compact gas is expected to be."
In addition to being compact enough to make stars, the gas in the observed dormant - or quiescent - galaxies had another surprise in store for the team: The gas was often centrally located, though not always, and was surprisingly turbulent. Combined, these two characteristics led to more questions than answers for researchers.
"The rates of star formation in the [post-starburst galaxies] we observed are much lower than in other galaxies, even though there appears to be plenty of fuel to sustain the process," said Smercina. "In this case, star formation may be suppressed due to turbulence in the gas, much like a strong wind can suppress a fire. However, star formation can also be enhanced by turbulence, just like wind can fan flames, so understanding what is generating this turbulent energy, and how exactly it is contributing to dormancy, is a remaining question of this work."
"These results raise the question of what energy sources are present in these galaxies to drive turbulence and prevent the gas from forming new stars," said co-author Decker French, an astronomer at the University of Illinois Urbana-Champaign. "One possibility is energy from the accretion disk of the central supermassive black holes in these galaxies."
A clear understanding of the processes that govern the formation of stars and galaxies is key to providing context to the universe and Earth's place in it. The discovery of turbulent, compact gas in otherwise dormant galaxies gives researchers one more clue to solving the mystery of how galaxies live, evolve and die over the course of billions of years.
"There is much about the evolution of a typical galaxy we don't understand, and the transition from their vibrant star-forming lives into quiescence is one of the least understood periods," said co-author J.D. Smith, an astronomer at the University of Toledo. "Although post-starbursts were very common in the early Universe, today they are quite rare. This means the nearest examples are still hundreds of millions of light-years away, but these events foreshadow the potential outcome of a collision, or merger, between the Milky Way Galaxy and the Andromeda Galaxy several billion years from now. Only with the incredible resolving power of ALMA could we peer deep into the molecular reservoirs left behind 'after the fall.'"
"It's often the case that we as astronomers intuit the answers to our own questions ahead of observations," said Smercina. "But this time, we learned something completely unexpected about the universe."
Additional co-authors are Eric Bell at the University of Michigan, Daniel Dale at the University of Wyoming, Anne Medling at the University of Toledo, Kristina Nyland at the U.S. Naval Research Laboratory, George Privon at the University of Florida, Kate Rowlands at the Space Telescope Science Institute, Fabian Walter at the Max Planck Institute for Astronomy and Ann Zabludoff at the University of Arizona. The research was funded by NASA, the Alexander von Humboldt Foundation, the Max Planck Institute for Astronomy and the National Science Foundation.
