Astronomers catch cosmic boomerang in action for the first time

Shown here in composite view, the ALMA data (red / orange) reveals the filament structures left behind by bare dynamic pressure in an optical view of NGC4921’s Hubble Space Telescope. Scientists believe that these filaments form when the galaxy’s magnetic fields prevent the removal of matter. Credit: ALMA (ESO / NAOJ / NRAO) / S. Dagnello (NRAO), NASA / ESA / Hubble / K. Cook (LLNL), L. Shatz

For the first time, astronomers have observed a cosmic boomerang effect – streams of heavy molecular gas that are pulled from a distant galaxy only to go back and come back later.

Astronomers from Yale and Arizona State University led the research team that made the discovery, which had been theorized in simulations but not observed in detail. Their findings are published in The Journal of Astrophysics.

The observation offers new information about the life cycle of galaxies and the structural formations within galaxies, as traced by molecular gas. In particular, the research focuses on a process called dynamic pressure stripping, in which gas in galaxy clusters acts like a wind that blows away stellar material inside a galaxy – hastening its demise.

“Astronomers are interested in studying how galaxies grow, live and die, ”said lead author William Cramer, who began research as a Yale graduate student and is now a postdoctoral researcher at Arizona State. “Effects such as dynamic pressure that can accelerate the normal life cycle of galaxies are very important to understand for this reason. In addition, molecular gas in galaxies is the cradle of new stars, and therefore studying the effect of dynamic pressure on it is of paramount importance.

This side-by-side composite shows ALMA (red / orange) data layered on (optical) images from the NGC4921 Hubble Space Telescope. A new study of the barred spiral galaxy has revealed filament structures similar to the Pillars of Creation but significantly larger. These structures are caused by a process known as dynamic pressure stripping, which pushes gas out of galaxies, leaving them without the material to form new stars. Credit: ALMA (ESO / NAOJ / NRAO) / S. Dagnello (NRAO), NASA / ESA / Hubble / K. Cook (LLNL), L. Shatz

For the study, the researchers used the Atacama Large Millimeter Array (ALMA) radio telescope, located in northern Chile, to create a high-resolution map of dense molecular gas in the galaxy NGC 4921 as it undergoes dynamic pressure stripping. The map shows unusual structures forming in the dynamic pressure “wind” – long filaments of heavy gas connected to newly formed stars. This dense, heavy gas is believed to be more resistant to dynamic pressure, possibly due to the magnetic fields that hold it more firmly in place.

“When an external force such as dynamic pressure disrupts a galaxy, it provides an opportunity to learn more about the internal forces that operate in galaxies, ”said co-author Jeffrey Kenney, professor of astronomy at the faculty. of Yale Arts and Sciences. “Unusual filaments wouldn’t form without magnetic fields, so we also learn the importance of magnetic fields in galaxies from this dynamic pressure interaction.”

Magnified view of an ALMA composite (red / orange) and the Hubble Space Telescope (optical) of NGC4921. This composite highlights the filamentary structures resulting from the effects of tearing by dynamic pressure. Dynamic pressure stripping is a process known to extract gas from galaxies, leaving them without the material needed to form new stars. A new study indicates that some materials cannot be removed from the galaxy, and are instead reaccredited, potentially using magnetic fields, slowing the galaxy’s death process. Credit: ALMA (ESO / NAOJ / NRAO) / S. Dagnello (NRAO), NASA / ESA / Hubble / K. Cook (LLNL), L. Shatz

The ALMA data clearly shows molecular gas filaments connected to the galaxy NGC 4921 – the filaments indeed resist. But then the researchers saw something else: some of the gas previously extracted is coming back.

“Instead of being thrown out never to come back, some of that gas travels like a boomerang, is ejected but then spins around and falls back to its source, ”Cramer said. If this gas is recovered from the galaxy, it can form new stars.

The boomerang effect is important for several reasons, according to the researchers. It provides tangible evidence of the evolution of galaxies; this confirms a long-held theory on the development of galaxies; and it helps astronomers try to predict the birth rate of new stars.

“The interstellar medium of galaxies is complex, with many variables that are difficult to model, ”Cramer said. “This observation is important because it shows that the gas pullback can be detected and allows us to search more broadly to help characterize it. “

To learn more about this research, read Gas Reaccretion First Observed in Dying Galaxies.

Reference: “Molecular gas filaments and fallback in the ram pressure stripped Coma spiral NGC 4921” by William J. Cramer, Jeffrey DP Kenney, Stephanie Tonnesen, Rory Smith, Tony Wong, Pavel Jáchym, Juan R. Cortés, Paulo C. Cortés, Yu-Ting Wu, Accepted, The Journal of Astrophysics.
arXiv: 2107.11731