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New research helps solve galactic murders

Why do galaxies stop making stars?
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UGC 6697 appears as the blueish, edge-on galaxy on the left of this image, which also contains the galaxies NGC 3837, NGC 3842, NGC 3841, NGC 3845, NGC 3844 and NGC 3840. These galaxies are all part of the Abell 1367 galaxy cluster.
NRAO
Our universe is dying. To be specific, an increasing number of galaxies are no longer able to form new stars. Just 10 billion years ago, the universe was burgeoning with plentiful star formation. So why is the health of galaxies on the decline?

New research on a massive galaxy approximately 310 million light-years away is shedding new light on one culprit responsible for killing galaxies. The results find a process, known as ram-pressure stripping, is more effective at dispatching galaxies than once thought. The process is responsible for removing large amounts of gas from galaxies, leaving little for further star formation.

“Ram-pressure stripping was known to be very effective in low mass galaxies,” says Guido Consolandi, lead author of the paper and researcher at Università degli Studi di Milano-Bicocca in Italy. “Our study, together with other recent studies, shows that ram pressure stripping appears efficient in high mass galaxies as well.”

A dynamic universe
The stars and galaxies we see in the night sky appear unchanging, but they all undergo dynamic processes, evolving throughout their lifetimes. While scientists don’t know exactly what mixture of cosmic events leads to the termination of star formation across galaxies, they have found several key players.

A leading cause of galaxy death is ram-pressure stripping. Drawn in by gravitation attraction, galaxies tend to group together in clusters. The space between galaxies in these clusters is filled with hot gas, and as the galaxies move through the cluster, the gas acts as a headwind. When the wind is strong enough, it can remove loose gas within the galaxy itself — gas that would typically be used to create new stars.

Using the MUSE instrument, which works like a compound bug’s eye, on the Very Large Telescope in the Atacama Desert of northern Chile, the scientists were able to capture images of the galaxy UGC 6697 and its smaller companion, CGCG 97087N, at multiple wavelengths simultaneously.
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UGC 6697 is seen here in a composite X-ray (blue color) and optical (red and green) image. Studying this galaxy in X-rays is helping astronomers to learn more about the processes occurring within it as a result of its plunge deeper into the galaxy cluster surrounding it.
X-ray: NASA/SAO/CXC/M.Sun et al.; Optical: GOLDMine/G. Gavazzi et al.
“Studies such as this one have begun to reveal the mechanisms that drive the star formation cycle in extraordinary detail,” says Dr. Toby Brown, former PhD. candidate at the International Centre for Radio Astronomy Research in Crawley, Australia, not involved with the study. “It's a little bit like we have been trying to understand how a car works without dismantling the engine — you would find out a lot, but a full understanding would be very difficult. What we now have, are the tools to pull it apart and take a proper look.”

Comparing the locations and motions of gas in different parts of the two galaxies, the researchers found two tails of gas connecting the galaxies that showed they had interacted in the past. Further analysis also provided evidence that both were losing material as they passed through the intergalactic gas in the cluster where the galaxies reside.  The results show that the process that removes gas from galaxies, and thus halts star formation, affects galaxies in a range of sizes, rather than only large or small ones.
 
“All of the information is included in just one observation,” says Consolandi. “Years ago it would have required different observational runs with different instruments. In the future, this will be possible at increasing sensitivity for more and more distant galaxies, allowing to gather a huge amount of details about the physics of the first galaxies of our Universe.”
 
While the gas stripped from the galaxy leaves it barren of new stars, in some cases that gas can find new life outside the galaxy. As the stripped gas becomes less turbulent after removal, magnetic fields in the intracluster medium can help confine the gas, compressing it into new stars and creating a trailing string of lights behind the dying galaxy.
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Like UGC 6697, the galaxy ESO 137-001 is also falling into a larger galaxy cluster, undergoing ram-pressure stripping along the way. This composite image, captured by the Hubble Space Telescope and the MUSE instrument on ESO’s Very Large Telescope, shows streamers of gas trailing behind ESO 137-001, removing its ability to form new stars.
ESO/M. Fumagalli
Dangerous neighborhood
Galaxies in clusters also have to be wary of their neighbors. Galaxies that pass at high speeds by one another — or even through one another — can ruffle and disturb each other with their gravitational forces. Referred to as galaxy harassment, this process often transforms the shapes of the galaxies and redistributes star-forming gas, in some cases creating a burst of star formation inside affected galaxies.

If that weren’t enough, galaxies can also undergo a process referred to by astronomers as strangulation. As a galaxy enters a cluster for the first time, the tug of gravity on the galaxy from the cluster’s dark matter halo pulls gas from the galaxy, leaving it without the ability to continue forming stars.

With all these processes stacked against galaxies, its unsurprising that so many are becoming “red and dead” — astronomer speak for a galaxy that only contains old, red stars. As instruments and telescopes become more and more sensitive, astronomers are able to look at smaller and more distant galaxies, helping us understand the evolution of galaxies through space and time.
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