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Exploring Transformations in Different Cosmic Settings

Through studying the complex mechanisms underlying the evolution of galaxies, which move through a “cosmic web” of diverse environments over the course of their lifetimes, researchers at the University of Kansas hope to gain a deeper understanding.

A computer simulation of what the gas and stars in a galaxy cluster look like, highlighting how clusters of galaxies are embedded in the cosmic web of filaments. In the color images, the intensity and color of the image represent the density and temperature of the gas. These figures show successive zooms onto a galaxy embedded in a filament. Going counterclockwise from the top right, the scale bars represent lengths of 3.3 million light years, 3.3 million light years, 330 thousand light years, and 33 thousand light years. The image at the lower right shows the stars in the galaxies in this simulated cluster, with the scale bar corresponding to 330 thousand light years. The WISESize program will use observations to measure the spatial distribution of gas and stars in galaxies as they move through the cosmic web that permeates the nearby universe. Image Credit: Yannick Bahé

The National Science Foundation recently awarded a $375,000 grant to a team led by KU professor of physics and astronomy Gregory Rudnick to study the “gas content and star-formation properties of galaxies,” which vary depending on the galaxy's motion.

The primary objective of this project is to comprehend the impact of environmental factors on the transformation of galaxies. In the universe, galaxies are spread in a non-uniform distribution characterized by varying densities. These galaxies aggregate into large clusters, comprising hundreds to thousands of galaxies, as well as smaller groups, consisting of tens to hundreds of galaxies.

Gregory Rudnick, Professor, Physics and Astronomy, University of Kansas

Additionally, galaxies can be part of elongated filamentary structures or they can reside in an isolated state in lower-density regions of the universe, he said.

Prior research primarily compared galaxies in groups and clusters to those in “the field,” or the lowest-density regions of the universe. The highway of filaments connecting the densest regions was overlooked in these studies.

By concentrating on how galaxies respond to their surroundings in filaments that direct them toward galactic groups and into galaxy clusters, changing the evolution of galaxies along the way, Rudnick’s team will take into account the entire dynamic range of densities in the universe.

Galaxies follow a path into these filaments, experiencing a dense environment for the first time before progressing into groups and clusters. Studying galaxies in filaments allows us to examine the initial encounters of galaxies with dense environments. The majority of galaxies entering the ‘urban centers’ of clusters do so along these ‘superhighways,’ with only a minimal number taking rural routes that bring them into the clusters and groups without interacting much with their surroundings.

Gregory Rudnick, Professor, Physics and Astronomy, University of Kansas

Gregory Rudnick adds, “Whereas filaments are akin to interstate highways, these less-traveled routes into dense regions are akin to the analogy of driving on rural roads in Kansas to access city limits. Galaxies can exist in filaments or be in groups that reside in filaments like beads on a string. Indeed, most galaxies in the universe exist within groups. Therefore, with our study we will simultaneously gain insights into both the onset of environmental effects on galaxies and into how galaxies behave in the regions where they are most commonly found, filaments and groups.”

How the “baryon cycle” of gases within and surrounding galaxies is altered by conditions within these filaments, fields, groups, and clusters of galaxies will be a major area of study. Each cosmic neighborhood modifies the properties of gas in and around galaxies, and it can even have an impact on the densest molecular gas, which is the source of star formation. Therefore, perturbations to this baryon cycle may help or hinder the formation of new stars.

Understanding the baryon cycle was identified as a key science topic for the upcoming decade in the Astro2020 Decadal survey, a federal report that was recently released by the astronomical community in order to set goals for astronomical research in the 2020s.

Gregory Rudnick states, “The space between galaxies contains gas. Indeed, most of the atoms in the universe are in this gas, and that gas can accrete onto the galaxies. This intergalactic gas undergoes a transformation into stars, although the efficiency of this process is relatively low, with only a small percentage contributing to star formation. The majority is expelled in the form of large winds

Some of these winds exit into space, termed outflows, while others are recycled and return. This continuous cycle of accretion, recycling, and outflows is referred to as the baryon cycle. Galaxies can be conceptualized as baryon processing engines, drawing gas from the intergalactic medium and converting some of it into stars. Stars, in turn, go supernova, producing heavier elements. Part of the gas is blown out into space, forming a galactic fountain that eventually falls back to the galaxy.

Gregory Rudnick, Professor, Physics and Astronomy, University of Kansas

But according to Rudnick, galaxies that come into contact with dense environments may feel pressure from passing through the surrounding gas, and this pressure has the potential to either actively remove gas from the galaxy or rob the galaxy of its future gas supply, which would disrupt the baryon cycle. In fact, as their gas supply is cut off, galaxies may find that the ability to form stars is diminished in the cores of clusters.

Rudnick  says, “The disruption affects the intake and expulsion of gas by galaxies, leading to alterations in their star formation processes. While there may be a temporary increase in star formation, in nearly all cases, it eventually results in a decline in star formation.”

Undergraduate researchers and graduate students like Kim Conger, whose work influenced the grant proposal, will work with Rudnick at KU. Students will be employed and trained by co-primary investigator, Rose Finn, a Physics and Astronomy Professor at Siena College.

Astronomical datasets including the DESI Legacy Survey, WISE, and GALEX imaging of about 14,000 galaxies will be used by the researchers. Employees at both campuses will conduct additional new observations with Siena's 0.7-m Planewave telescope in order to acquire fresh images of galaxies fitted with a specially designed filter that will be funded by the grant. Due to a joint Observational Astronomy course, KU students will be able to observe remotely with the Siena telescope in 2021 and 2023.

Community Outreach

Since the grant expands Rudnick’s long-running initiative to bring university-level astronomy courses into secondary schools, it will also involve high school students from Kansas and New Jersey. The new grant expands the astronomy course already offered at Lawrence High School, which is close to KU’s Lawrence campus, and establishes a high school program associated with Siena College. Rudnick received a KU Community Engaged Scholarship Award in 2020 as a result of Rudnick’s work on this course.

Rudnick says, “These funds will extend the high school program's longevity through 2026. In collaboration with funds from KU, 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 undertake, this grant facilitated the acquisition of computers that will enable the research.”

According to Rudnick, the project now has a laptop cart specifically designated for the class, allowing students to complete the research projects. Additionally, the increase in computers has allowed organizers to increase the class size.

Rudnick concludes, “Previously, class sizes at the high school level were around 8 to 10 students. Now, at the start of the year, we have 22 students. It's a significant growth, aiming to double the class size.”

Journal Reference:

Rudnick, G., et al. (2023) Pathways to Discovery in Astronomy and Astrophysics. National Academies of Sciences, Engineering, and Medicine.

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