“Much of the previous work on ram-pressure stripped galaxies has focused on material that is stripped from galaxies. In this new work, we see some gas that, instead of being thrown out of the galaxy and never coming back, is moving like a boomerang, being ejected, but then circling back to its source,” said William Cramer, astronomer of Arizona State University and the lead author of the new study. “By combining Hubble and ALMA data at very high resolution, we can prove that this process is happening.”
Ram pressure removal 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, the hot gas known as the intracluster medium - or, the space between them - acts like a strong wind, pushing the gases out of traveling galaxies. Over time, this leads to the starvation and “death” of previously active star-forming galaxies. How removing ram pressure can accelerate the normal life cycle of galaxies and alter the amount of molecular gas within them 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 that's being pushed out by the ram pressure removal escapes the galaxy because it has to reach escape velocity to actually escape and not back down. We believe the regrowth we're seeing is from gas clouds that were pushed out of the galaxy by ram pressure reduction and didn't reach escape velocity, so they're receding,” said Jeff Kenney, an astronomer at Yale University and co- author of the study. “If you're trying to predict how quickly a galaxy will stop forming stars over time and turn into a red or dead galaxy, then you want to understand the effectiveness of ram pressure in removing gas. If you don't know that gas can fall back into the galaxy and continue to recycle and form new stars, you will overestimate star erasure. Having proof of this process means more accurate timetables for the life cycle of galaxies. ”
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 removing ram pressure because evidence of the process and its consequences is abundant.
“The ram pressure triggers the formation of stars on the side where it's having the greatest impact on the galaxy,” said Cramer. "It's easy to identify in NGC 4921 because there are so many young blue stars on the side of the galaxy where it's occurring."
Kenney added that the removal of ram pressure on NGC 4921 created a strong, visible line between where dust still exists in the galaxy and where it doesn't. “There is a strong line of dust present and, in addition, there is almost no gas in the galaxy. We think that part of the galaxy was almost completely cleared by the ram pressure. ”
Using ALMA's Band 6 receiver, scientists were able to resolve carbon monoxide, the key to "seeing" areas of the galaxy devoid of gas, as well as those areas where it is recreating itself. "We know that most 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 substitute for studying molecular gas in galaxies because it's much easier to observe."
The ability to see more of the galaxy, even in its faintest aspect, has revealed interesting structures, probably created in the process of gas displacement, and even more immune to its effects. “The ram pressure appears to form unique structures, or filaments in galaxies, which are clues to how a galaxy evolves under the pressure of the ram 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 think they are supported by magnetic fields that prevent them from being removed with the rest of the gas."
Observations revealed that structures are more than particles of gas and dust; the filaments have mass and lots of it. "These filaments are heavier and sticky - they hold their material more tightly than the rest of the galaxy's interstellar medium can - and they seem to be connected to this large ridge of dust in both space and velocity," he said. Kenney. “They are more like molasses than smoke. If you just blow on something that is smoke, the smoke is light and disperses and goes in all directions. But this is much heavier than that. ”
Although a significant advance, the study results are just a starting point for Cramer and Kenney, who examined a small part of just one galaxy. "If we are 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, originally lost by ram pressure, is actually recycled back," said Cramer. “These observations are from just one quadrant of NGC 4921. Probably even more gas is falling in other quadrants. Although we have confirmed that some extracted gas may 'rain' back, we need more observations to quantify how much gas falls 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 NSF. "The removal of Ram pressure is an important phenomenon for galaxies in clusters, and better understanding the process allows us to better understand the evolution of galaxies - and nature."
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Shown here in a composite view, ALMA data (red/orange) reveals filament structures left behind by the removal of ram pressure in an optical view of the Hubble Space Telescope from NGC4921. Scientists believe these filaments are formed as magnetic fields in the galaxy prevent some of the matter from being stripped away. Credit: ALMA (ESO / NAOJ / NRAO) / S. Dagnello (NRAO), NASA / ESA / Hubble / K. Cook (LLNL), L. Shatz
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