Scientists find elusive gas from post-starburst galaxies hiding in plain
sight

Date:
April 25, 2022
Source:
National Radio Astronomy Observatory
Summary:
Post-starburst galaxies were previously thought to scatter all of
their gas and dust -- the fuel required for creating new stars --
in violent bursts of energy, and with extraordinary speed. Now,
new data from the Atacama Large Millimeter/submillimeter Array
(ALMA) reveals that these galaxies 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.



FULL STORY ========================================================================== Post-starburst galaxies were previously thought to scatter all of their
gas and dust -- the fuel required for creating new stars -- in violent
bursts of energy, and with extraordinary speed. Now, new data from the
Atacama Large Millimeter/submillimeter Array (ALMA) reveals that these
galaxies 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.


==========================================================================
In most galaxies, scientists expect gas to be distributed in a way similar
to starlight. But for post-starburst galaxies, or 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
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. 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." 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: it 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 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."
A clear understanding of the processes that govern the formation of
stars and galaxies is key to providing context to the Universe and our
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 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
1.3mm receiver, which sees the otherwise invisible with stark clarity.

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.


========================================================================== Story Source: Materials provided by
National_Radio_Astronomy_Observatory. Note: Content may be edited for
style and length.


========================================================================== Related Multimedia:
* Post-starburst_galaxies ========================================================================== Journal Reference:
1. Adam Smercina, John-David T. Smith, K. Decker French, Eric F. Bell,
Daniel A. Dale, Anne M. Medling, Kristina Nyland, George C. Privon,
Kate Rowlands, Fabian Walter, Ann I. Zabludoff. After The Fall:
Resolving the Molecular Gas in Post-Starburst Galaxies. The
Astrophysical Journal, 2022 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/04/220425104927.htm

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