It was previously believed that post-starburst galaxies scatter all of the gas and dust necessary for producing new stars in swift, explosive bursts of energy. It turns out that these galaxies don’t actually scatter all of their star-forming material after all, according to fresh data from the Atacama Large Millimeter/submillimeter Array (ALMA).
Instead, these sleeping galaxies retain and compress significant volumes of highly concentrated, turbulent gas after their alleged demise. But in contrast to what was anticipated, they are not creating stars with it.
Scientists anticipate that gas will be spread in most galaxies similarly to how sunlight is. However, this isn’t the case with post-starburst galaxies or PSBs. Because they are created during ferocious collisions or mergers between galaxies, PSBs vary from other galaxies.
Massive bursts of star formation are normally triggered by galaxy mergers, but in PSBs, this outburst slows and nearly totally ceases almost as soon as it starts. Therefore, astronomers traditionally thought that little or no star-forming fuel remained in the center star-forming factories of these galaxies.
And up until this point, it was thought that either star activity or the actions of black holes had dispersed the molecular vapors to radii far outside the galaxies. The most recent findings refute this notion.
“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,” said Adam Smercina, an astronomer at the University of Washington and the principal investigator of the study.
“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.”
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.’
J.D. Smith
The gas in the detected inert or quiescent galaxies had another surprise in store for the scientists in addition to being compact enough to produce stars: it was frequently centrally located, though not always, and was remarkably turbulent. These two traits together left researchers 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,” 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.”
Decker French, an astronomer at the University of Illinois, and a co-author of the research added, “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.”
Giving the Universe and our place in it context requires a thorough grasp of the mechanisms that control star and galaxy formation. Researchers now have another piece of the puzzle to the enigma of how galaxies in particular live, evolve, and die over the period of billions of years thanks to the discovery of turbulent, dense gas in otherwise quiescent galaxies.
And it means further studies in the future using ALMA’s 1.3mm receiver, which vividly sees the previously invisible.
J.D. Smith, an astronomer at the University of Toledo, and a co-author of the research said, “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. 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.’”
Smercina added, “It’s often the case that we as astronomers intuit the answers to our own questions ahead of observations, but this time, we learned something completely unexpected about the Universe.”
The results of the study are published today in The Astrophysical Journal.
