How do galaxies expand while entangled in the cosmic web of the universe?

New simulations demonstrate the evolutionary process of thousands of galaxies as they traverse the intricate network of gas, dust, and stars forming the expansive “cosmic web” of the universe.

The University of Kansas is conducting a study to gain a deeper understanding of how clusters of galaxies are influenced by the cosmic web. Led by Professor Gregory Rudnick, the research involves creating a computer simulation of the cosmic web and analyzing the gas content and star-formation properties of galaxies as they navigate through this intricate network. To support their investigation, the team will utilize images of approximately 14,000 galaxies obtained from the DESI Legacy Survey, the Wide-field Infrared Survey Explorer (WISE), and NASA’s Galaxy Evolution Explorer (GALEX). Furthermore, they will gather additional data using Siena’s 0.7-m Planewave telescope.

Rudnick stated that the main goal of this project is to understand how environmental factors influence the transformation of galaxies. In the vast expanse of the universe, galaxies are not evenly distributed and vary in density. They can be found in large clusters consisting of hundreds to thousands of galaxies, as well as smaller groups with tens to hundreds of galaxies.

Rudnick highlighted that galaxies can either be located in clusters or groups, or they may exist in isolated regions of the universe with lower density, known as “the field.”

Previous studies have compared galaxies in clusters and groups to those in the field, but they have overlooked the elongated filamentary structures that connect the clustered galaxies. These structures are composed of gas, dust, and stars.

In this project, Rudnick and his colleagues took into account these filamentary structures, which act as cosmic highways. They focused on understanding the environments in which galaxies encounter these filaments, how they are guided into groupings and clusters, and how the filaments impact their evolution.

According to Rudnick, galaxies follow a trajectory into these filaments, encountering a concentrated environment for the first time before progressing into groups and clusters. By studying galaxies in filaments, we are able to analyze the initial interactions between galaxies and dense environments.

Rudnick further explained that the majority of galaxies that enter the “urban centers” of clusters do so through cosmic web “superhighways,” while only a few take alternative routes that lead them into the clusters and groups with minimal interaction with their surroundings.

Rudnick compared filaments to interstate highways, while these less-traveled routes into dense regions are similar to driving on rural roads in Kansas to reach city limits. Galaxies can either exist within filaments or be part of groups that are situated within filaments, resembling beads on a string. In fact, most galaxies in the universe are found within groups.

The team aims to gain valuable insights into the initial effects of the environment on galaxies and decipher the behavior of galaxies in filaments and groups, which are their most common habitats, through this simulation.

Trapped galaxies birthing stars

The KU team’s work involves evaluating how the cosmic web filaments impact the processing of gas in areas of high density, known as the “baryon cycle.” Disruptions in this cycle can either enhance or impede the formation of stars, thereby influencing the growth rate of galaxies.

According to Rudnick, the space between galaxies contains a significant amount of gas, with the majority of atoms in the universe being present in this gas. This intergalactic gas has the potential to accumulate onto galaxies, undergoing a transformation into stars. However, the efficiency of this process is relatively low, with only a small percentage contributing to star formation. The majority of the gas is expelled in the form of large winds.

These winds can either become outflows that blow away from galaxies into space or fall back to their original galaxies, where they are reabsorbed and recycled as part of the baryon cycle.

Rudnick provided an explanation that galaxies can be perceived as mechanisms that process baryons. They draw gas from the intergalactic medium and convert a portion of it into stars. As a result, stars go supernova and generate heavier elements. Some of the gas is expelled into space, forming a galactic fountain that eventually returns to the galaxy.

When galaxies come across a dense environment in the cosmic web, they have the ability to alter their internal pressure and disrupt the baryon cycle. This can occur through actively stripping gas from the galaxy or depriving it of its future gas supply.

Consequently, the galactic star factories, which are located at the centers of clusters, slow down their star-birthing process as their raw material for star formation is suppressed.

Rudnick stated that this disruption impacts the intake and expulsion of gas by galaxies, leading to changes in their star formation mechanisms. Although there might be a temporary increase in star formation, it typically results in a decline over time.

The team’s simulations aim to enhance scientists’ comprehension of the baryon cycle, which has been identified as a significant scientific topic for the next decade according to the Astro2020 Decadal survey.

Furthermore, the research will involve science outreach to high school students in Kansas and New Jersey until 2026. This will involve providing 11 MacBook Pros to schools, enabling students to actively engage with the research project.

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