Scientists have discovered that post-starburst galaxies compress rather than expel their gas, raising the question of what is preventing them from generating stars.
Previously, it was thought that post-starburst galaxies spread all of their gas and dust—the fuel needed to create new stars—in tremendous bursts of energy, and at incredible speeds. ALMA’s new data shows that these galaxies don’t scatter all of their star-forming material after all. Instead, these sleeping galaxies cling onto and compress vast volumes of highly concentrated, turbulent gas after their purported demise. But, contrary to popular belief, they aren’t utilizing it to create stars.
Scientists predict gas to be distributed similarly to sunlight in most galaxies. This isn’t the case with post-starburst galaxies, or PSBs. PSBs vary from other galaxies in that they form as a result of severe collisions or mergers between galaxies. Galaxy mergers usually result in huge bursts of star formation, but in PSBs, this explosion slows down and nearly terminates as soon as it starts. As a result, scientists previously assumed that the center star-forming factories of these galaxies had little or no star-forming fuel left. Until now, it was thought that the molecular gases had been redistributed to radii far beyond the galaxies, either through star processes or by black hole effects. The latest findings cast doubt on this theory.
“We’ve known for some time that large amounts of molecular gas remains in the vicinity of PSBs 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,” says Adam Smercina, the principal investigator of the study, adding “While this compact gas should be forming stars efficiently, it isn’t. In fact, it is less than 10-percent as efficient as similarly compact gas is expected to be.”
In addition to being compact enough to form stars, the gas in the detected dormant—or quiescent—galaxies surprised the researchers by being centrally placed, though not invariably, and very turbulent. When these two qualities were combined, researchers were left with more questions than answers.
“The rates of star formation in the PSBs we observed are much lower than in other galaxies, even though there appears to be plenty of fuel to sustain the process,” adds 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.”
According to Decker French, a co-author of the research, “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. One possibility is energy from the accretion disk of the central supermassive black holes in these galaxies.”
Understanding the processes that drive the development of stars and galaxies is essential for putting the Universe and our place in it into context. The discovery of turbulent, compact gas in otherwise quiescent galaxies provides astronomers with yet more clue to the puzzle of how galaxies live, evolve, and die over billions of years. And that means more future studies will be possible thanks to ALMA’s 1.3mm receiver, which sees what would otherwise be invisible with stunning clarity.
“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. Although post-starbursts were very common in the early Universe, today they are quite rare,” adds J.D. Smith, co-author of the research.
“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.”
The findings of the research were published today in The Astrophysical Journal.
Image Credit: ALMA (ESO/NAOJ/NRAO)/S. Dagnello (NRAO/AUI/NSF)
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