Scientists observe gas re-accretion in dying galaxies for the first time


Date:
July 29, 2021
Source:
National Radio Astronomy Observatory
Summary:
A new study suggests that previously displaced gases can re-accrete
onto galaxies, potentially slowing down the process of galaxy death
caused by ram pressure stripping, and creating unique structures
more resistant to its effects.



FULL STORY ==========================================================================
A new study from scientists using the Atacama Large
Millimeter/submillimeter Array (ALMA) suggests that previously displaced
gases can re-accrete onto galaxies, potentially slowing down the process
of galaxy death caused by ram pressure stripping, and creating unique structures more resistant to its effects.


========================================================================== "Much of the previous work on ram pressure stripped galaxies is focused
on the material that gets stripped out of galaxies. In this new work we
see some gas that rather than being thrown out of the galaxy never to
return is instead moving like a boomerang, being ejected out but then
circling around and falling back to its source," said William Cramer,
an astronomer at Arizona State University and the lead author on the new
study. "By combining Hubble and ALMA data at very high resolution, we are
able to prove that this process is happening." Ram pressure stripping
refers to the process that displaces gas from galaxies, leaving them
without the material needed to form new stars. As galaxies move through
their galaxy clusters, hot gas known as the intra-cluster medium -- or,
the space between -- acts like a forceful wind, pushing gases out of the traveling galaxies. Over time, this leads to the starvation and "death"
of once-active star-forming galaxies. Because ram pressure stripping
can speed up the normal life cycle of galaxies and alter the amount of molecular gas within them, it is of particular interest to scientists
studying the life, maturation, and death of galaxies.

"We've seen in simulations that not all of the gas being pushed by ram
pressure stripping escapes the galaxy because it has to reach escape
velocity in order to actually escape and not fall back. The re-accretion
that we're seeing, we believe is from clouds of gas that were pushed
out of the galaxy by ram pressure stripping, and didn't achieve escape velocity, so they're falling back," said Jeff Kenney, an astronomer at
Yale University, and the co-author on the study. "If you're trying to
predict how fast a galaxy is going to stop forming stars over time and transform into a red, or dead galaxy, then you want to understand how
effective ram pressure is at stripping the gas out. If you don't know
that gas can fall back onto the galaxy and continue to recycle and form
new stars, you're going to overpredict the quenching of the stars. Having
proof of this process means more accurate timelines for the lifecycle
of galaxies." The new study focuses on NGC 4921 -- a barred spiral
galaxy and the largest spiral galaxy in the Coma Cluster -- located
roughly 320 million light-years from Earth in the constellation Coma
Berenices. NGC 4921 is of particular interest to scientists studying the effects of ram pressure stripping because evidence of both the process
and its aftermath is abundant.

"Ram pressure triggers star formation on the side where it is having
the greatest impact on the galaxy," said Cramer. "It's easy to identify
in NGC 4921 because there are many young blue stars on the side of the
galaxy where it's occurring." Kenney added that ram pressure stripping
in NGC 4921 has created a strong, visible line between where dust still
exists in the galaxy and where it doesn't. "There is a strong dust line present, and beyond that there's almost no gas in the galaxy. We think
that that part of the galaxy has been almost completely cleaned out
by ram pressure." Using ALMA's Band 6 receiver, scientists were able
to resolve carbon monoxide, the key to "seeing" both those areas of the
galaxy devoid of gas, as well as those areas where it is re-accreting. "We
know that the majority of molecular gas in galaxies is in the form of
hydrogen, but molecular hydrogen is very difficult to observe directly,"
said Cramer. "Carbon monoxide is commonly used as a proxy for studying molecular gas in galaxies because it is much easier to observe."
The ability to see more of the galaxy, even at its faintest, unveiled interesting structures likely created in the process of gas displacement,
and further immune to its effects. "Ram pressure appears to form unique structures, or filaments in galaxies that are clues as to how a galaxy
evolves under a ram pressure wind. In the case of NGC 4921, they bear
a striking resemblance to the famous nebula, the Pillars of Creation,
although on a much more massive scale," said Cramer. "We think that they
are supported by magnetic fields which are preventing them from being
stripped away with the rest of the gas." Observations revealed that
the structures are more than just wisps of gas and dust; the filaments
have mass and a lot of it. "These filaments are heavier and stickier --
they hold on to their material more tightly than the rest of the galaxy's interstellar medium can do -- and they seem to be connected to that big
dust ridge both in space and in velocity," said Kenney. "They're more
like molasses than smoke. If you just blow on something that is smoke,
the smoke is light, and it disperses and goes in all directions. But
this is much heavier than that." Although a significant breakthrough,
the results of the study are only a starting point for Cramer and
Kenney, who examined one small part of just one galaxy. "If we want to
predict the death rate of galaxies, and the birthrate of new stars, we
need to understand if and how much of the material that forms stars,
originally lost to ram pressure, is actually recycled back," said
Cramer. "These observations are of just one quadrant of NGC 4921. There
is likely even more gas falling back into other quadrants. While we
have confirmed that some stripped gas can 'rain' back down, we need
more observations to quantify how much gas falls back and how many
new stars form as a result." "A fascinating study, demonstrating the
power of ALMA and the benefit of combining its observations with those
of a telescope at other wavelengths," added Joseph Pesce, NRAO/ALMA
program officer at the NSF. "Ram pressure stripping is an important
phenomenon for galaxies in clusters, and understanding the process
better allows us to understand galaxy evolution - - and nature -- better." ========================================================================== Story Source: Materials provided by
National_Radio_Astronomy_Observatory. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. William J. Cramer, Jeffrey D. P. Kenney, Stephanie Tonnesen,
Rory Smith,
Tony Wong, Pavel Ja'chym, Juan R. Corte's, Paulo C. Corte's,
Yu-Ting Wu.

Molecular gas filaments and fallback in the ram pressure stripped
Coma spiral NGC 4921. The Astrophysical Journal, 2021 [abstract] ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210729095218.htm

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