CM – Scientists observe gas reacretion in dying galaxies for the first time

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July 29, 2021

by Amy C. Oliver, National Radio Astronomy Observatory

A new study by scientists using the Atacama Large Millimeter / Submillimeter Array (ALMA) suggests that previously displaced gases can redeposit on galaxies, potentially slowing the process of galaxy dieback caused by dynamic pressure and creates unique structures that are more impact-resistant.

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“Much of the previous work on ram pressure stripped galaxies has focused on the material that is removed from galaxies. In this new work we see some gas that, instead of being thrown out of the galaxy and never returning, instead moves like a boomerang. ejected, but then orbited and dropped back to its source, « said William Cramer, astronomer at Arizona State University and lead author of the new study. » By combining Hubble and ALMA data at very high resolution, we can prove this process « 

Ram pressure stripping refers to the process of displacing gas from galaxies, leaving them without the material needed to form new stars. As galaxies move through their clusters, hot gas acts Known as the intra-cluster medium – or the space in between – like a strong wind, pushing gases out of the wandering galaxies, over time this leads to the starvation and « death » of once active star-forming galaxies. As the stripping with dynamic pressure can accelerate the normal life cycle of galaxies and alter the amount of molecular gas in them, it is of particular interest to scientists who do this Investigate the life, maturation and death of galaxies.

“We have seen in simulations that not all of the gas pushed by dynamic pressure stripping escapes from the galaxy because it has to reach the escape speed in order to actually escape and not fall back, they believe they are from gas clouds that were pushed out of the galaxy by dynamic pressure stripping and couldn’t escape, so they’re falling behind, « said Jeff Kenney, an astronomer at Yale University and co-author on the study. » If you want to predict how fast one will go Galaxy will stop forming stars over time and turn into a red or dead galaxy, then you want to understand how effective the back pressure is in removing the gas, unless you know that gas will fall back on the galaxy and move on recycling and forming new stars, you will be overestimating star extinction, and evidence of this process means more accurate schedules for Galax’s life cycle ien. ”

The new study focuses on NGC 4921 – a barred spiral galaxy and the largest spiral galaxy in the Coma Cluster – located about 320 million light years from Earth in the constellation Coma Berenices. NGC 4921 is of particular interest to scientists studying the effects of dynamic pressure stripping as there is ample evidence of the process and its consequences.

« The dynamic pressure triggers star formation on the side on which it has 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 occurs. »

Kenney added that the back pressure stripping in NGC 4921 created a strong, visible boundary between where dust is and where it is not in the galaxy. « There is a strong line of dust and beyond that there is almost no gas in the galaxy. We believe that this part of the galaxy has been almost completely cleaned by the dynamic pressure. »

With the Band 6 receiver from ALMA, the scientists were able to dissolve carbon monoxide, the key to « seeing » both the gas-free areas of the galaxy and the areas in which it accumulates again. « We know that most of the molecular gas in galaxies is in the form of hydrogen, but molecular hydrogen is very difficult to observe directly, » says Cramer. « Carbon monoxide is widely 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 in its faintest form, revealed interesting structures that were likely formed in the process of gas displacement and remain immune to its effects. “Dynamic pressure seems to form unique structures or filaments in galaxies that provide clues as to how a galaxy develops under a dynamic pressure wind. In the case of NGC 4921, they bear a striking resemblance to the famous nebula, the Pillars of Creation, albeit on a much more massive scale, « said Cramer. » We believe they are supported by magnetic fields that prevent them from getting on with the rest of the gas. « 

Observations showed that the structures are more than just clouds of gas and dust; the filaments have bulk and a lot of it. » These filaments are heavier and stickier – they hold their material tighter than the rest of the interstellar medium of the galaxy – and they seem to be tied to this great spill of dust both in space and in speed, « said Kenney. » They are more like molasses than smoke. If you just blow on something that is smoke, there is smoke easily, and it spreads and goes in all directions.

Although a major breakthrough, the study’s results are only a starting point for Cramer and Kenney, who are a small part of a single Ga have examined laxie. « If we want to predict the death rate of galaxies and the birth rate of new stars, we need to understand whether and how much of the material that makes up stars that was originally lost through dynamic pressure is actually recycled, » said Cramer. « These observations apply to only one quadrant of NGC 4921. Even more gas is likely to fall back into other quadrants. While we’ve confirmed that some stripped gas can » rain « back, we need more observations to quantify how a lot of gas falls back and how many new stars are formed 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 NSF. “Ram pressure stripping is an important phenomenon for galaxies in clusters, and a better understanding of the process allows us to better understand galaxy evolution – and nature.

The results of the study will be published in an upcoming issue of the Astrophysical Journal.

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