Study Sheds New Light on ‘Fall of Star Formation’ in Post-Starburst Galaxies

by johnsmith

Post-starburst galaxies (PSBs) are a rare galaxy class existing between two crucial evolutionary phases — major, gas-rich galaxy mergers and gas-poor, quiescent, early-type galaxies. These galaxies were previously thought to scatter all of their gas and dust in violent bursts of energy, and with extraordinary speed. According to an analysis of new data collected by the Atacama Large Millimeter/submillimeter Array (ALMA), PSBs don’t scatter all of their star-forming fuel after all; instead, 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.

This artist’s impression highlights the compactness of molecular gas in a post-starburst galaxy and its lack of star formation. Image credit: ALMA / ESO / NAOJ / NRAO / S. Dagnello / NRAO / AUI / NSF.

This artist’s impression highlights the compactness of molecular gas in a post-starburst galaxy and its lack of star formation. Image credit: ALMA / ESO / NAOJ / NRAO / S. Dagnello / NRAO / AUI / NSF.

In most galaxies, astronomers expect gas to be distributed in a way similar to starlight. But for PSBs, this isn’t the case.

PSBs 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 PSBs, this outburst slows down and near-completely stops almost as soon as it begins.

As a result, scientists previously believed that little or no star-forming fuel was left in these galaxies’ central star-forming factories.

And until now, the belief was that the molecular gases had been redistributed to radii well beyond the galaxies, either through stellar processes or by the effects of black holes.

The new results, published this week in the Astrophysical Journal, challenge 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,” said Dr. Adam Smercina, an astronomer at the University of Washington.

“Now, we have discovered a considerable amount of remaining gas within the galaxies and that remaining gas is very compact.”

“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 quiescent galaxies had another surprise in store for the team: it was often centrally-located, though not always, and was surprisingly turbulent.

Combined, these two characteristics led to more questions than answers for the astronomers.

“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,” Dr. Smercina said.

“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 Dr. Decker French, an astronomer at the University of Illinois.

“One possibility is energy from the accretion disk of the central supermassive black holes in these galaxies.”

The discovery of turbulent, compact gas in otherwise dormant galaxies gives researchers one more clue to solving the mystery of how galaxies in particular live, evolve and die over the course of billions of years.

And that means additional future research with the help of ALMA’s 1.3-mm receiver, which sees the otherwise invisible with stark 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,” said Dr. 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, but this time, we learned something completely unexpected about the Universe,” Dr. Smercina said.


Adam Smercina et al. 202. After The Fall: Resolving the Molecular Gas in Post-starburst Galaxies. ApJ 929, 154; doi: 10.3847/1538-4357/ac5d5f

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