March 1, 2024

How does the Cosmic Web drive the evolution of the galaxy?

Galaxies undergo a long and strange journey through the cosmic web as they grow and evolve. It turns out that the neighborhoods where they spend time on their journey change their evolution, and this affects their star formation activity and changes their gas content.

Astronomer Gregory Rudnick of the University of Kansas received a grant to follow the trail of galactic growth and discover how travel alters the evolution of a galaxy. These giant stellar cities are spread throughout the Universe, Rudnick said, explaining that they tend to group together in large conglomerates. Hundreds of thousands of them are grouped into the largest ones, while the smallest ones have only a few. In the grand scheme of things, they may also be part of the filamentary structure called the “cosmic web.” Or some may be relatively isolated in “low-density” galactic neighborhoods.

How do they group together and what do they experience when they come together? That’s what Rudnick and his colleagues want to understand. “The main objective of this project is to understand the impact of environmental factors on the transformation of galaxies,” he said.

The Evolution Process of the Galaxy

How do these giants evolve and grow? A quick look at the history of our Milky Way galaxy gives us a good idea of ​​the overall history. It began to form as a cloud of hydrogen gas about 13 billion years ago. Gravity brought the clouds together, starting the star formation process. Other structures in the galaxy – the core, the flat disk, the halo – formed in rapid succession. The infant Milky Way has experienced multiple collisions with others throughout its history. It is still cannibalizing some today. This is more or less the “executive summary” of galactic growth.

Our Galaxy is part of a larger collection called the Local Group, which is part of the Virgo Supercluster. The Virgo collection is also part of a supposedly larger group called Laniakea. And this is all part of the cosmic web that defines the large-scale cosmic structure.

A mosaic of telescopic images showing the galaxies of the Virgo Supercluster.  It is part of the cosmic web in which a galaxy can exist during part of its evolution.  Credit: NASA/Rogelio Bernal Andreo
A mosaic of telescopic images showing the Virgo Supercluster. It is part of the cosmic web in which a galaxy can exist during part of its evolution. Credit: NASA/Rogelio Bernal Andreo

How travel changes the galaxies

Rudner’s group plans to focus on the “superhighway” of filaments in the cosmic web, which connects the densest regions of the Universe. The team will focus on how galaxies react to the environment within the filaments as they are funneled into clusters. This journey seems to alter them as they traverse the web.

“Galaxies follow a path toward these filaments, first experiencing a dense environment before progressing into groups and clusters,” Rudnick said. “Studying galaxies in filaments allows us to examine the initial encounters of galaxies with dense environments. Most galaxies that enter the ‘urban centers’ of clusters do so along these ‘superhighways,’ with only a minimal number following rural routes that take them into clusters and groups without interacting much with their surroundings.”

A simulation of the
A simulation of the “cosmic web” believed to connect galaxies. A galaxy can move in and out of this web throughout its life. A void is visible in the center of the image, a place where researchers have found “tendrils” of galaxies. Credit: Cunnama, Power, Newton and Cui (ICRAR).

Tracing the Baryonic Cycle

Rudner and his team want to see how the conditions where galaxies come together alter the gases in and around them. Filaments, fields, groups and clusters have different environments. Each of these neighborhoods affects the behavior of the gas. Astronomers think of these changes in terms of a “baryonic cycle”. This is a complex phenomenon, actually. It describes all the ways in which gas is processed into the dense clumps and filaments of the cosmic web.

“The space between galaxies contains gas. In fact, most of the atoms in the universe are in this gas, and this gas can accumulate in galaxies,” said Rudnick. “This intergalactic gas undergoes transformation into stars, although the efficiency of this process is relatively low, with only a small percentage contributing to star formation. Most are expelled in the form of strong winds.”

Think of galaxies as baryon processing engines, extracting gas from the intergalactic medium and converting some of it into stars. Stellar populations produce heavier elements as they evolve. Eventually, all the material ends up in space, along with the gas, forming a fountain that eventually falls back into the galaxy. When they move into a denser environment, the pressure from their passage disrupts the baryonic cycle. It removes galactic gas or otherwise deprives it of the “stuff” needed to form new stars. This has happened in the centers of the clusters. “The disturbance affects the ingestion and expulsion of gas by galaxies, leading to changes in their star formation processes,” said Rudner. “Although there may be a temporary increase in star formation, in almost all cases this eventually results in a decline in star formation.”

Tracking Galaxies

For their work tracking the long and strange journeys that galaxies make, Rudner and his team will scour astronomical datasets from the DESI, WISE and GALEX programs. This will give them access to around 14,000 galaxies to study. They will also obtain new images of galaxies to study. Students from the university, as well as selected secondary schools, will also participate in the study.

The donation funds a high school astronomy course affiliated with Siena College and extends a course already offered at Lawrence High School, near KU’s Lawrence campus. “These funds will extend the longevity of the high school program through 2026,” Rudnick said. “In collaboration with KU funds, we were able to purchase 11 MacBook Pros for the school. Given that students only have iPads, which are not suitable for the research activities they needed to carry out, this grant facilitated the acquisition of computers that will allow their research.”

Ultimately, the expanded team’s work should uncover more about the long-term evolution of galaxies, starting in the early Universe and extending into the modern cosmos. Furthermore, it will reveal more about the cosmic web and filaments that make up the large-scale structure that defines the Universe.

For more informations

Researchers seek to understand how regions of the “Cosmic Web” influence the behavior of galaxies

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