Date on Senior Honors Thesis

5-2023

Document Type

Senior Honors Thesis

Degree Name

B.S.

Department

Physics and Astronomy

Degree Program

College of Arts and Sciences

Author's Keywords

galaxies; galaxy; astronomy; astrophysics; galaxy morphology; void galaxy

Abstract

The large-scale structure (LSS) of the Universe is comprised of galaxy filaments, tendrils, and voids. The majority of the Universe’s volume is taken up by these voids, which exist as underdense, but not empty, regions. The galaxies found inside voids are void galaxies and expected to be some of the most isolated objects in the Universe. However, their standard morphology remains poorly studied. This study, using the Galaxy and Mass Assembly (GAMA) data and Galaxy Zoo survey, aims to remedy this. For completeness purposes, we use void galaxies identified by Alpaslan et al. (2014) with stellar masses (M*) of 109.35M⊙ < M∗ < 1011.5M⊙. This sample is further split by identifying a redshift-limited region, 0 < z < 0.07, in addition to the existing data, 0 < z < 0.15. To find comparable subjects in the sample of field galaxies from GAMA/Galaxy Zoo, we identify ’twins’ as galaxies within ±0.05 dex and ±0.15 dex of M∗ and sSFR. We utilize Kolmogorov-Smirnov (KS) significance testing to determine whether our samples can be considered statistically different, and to prove the extent to which these data cuts affect results. Overall, we find that the way we define the ’twins’ of void galaxies has little effect, with the biggest differences occurring between our two redshift ranges. We see that void galaxies, in contrast with field galaxies, appear to be smaller, seem to almost always have a bulge, and may have more rounded bulges.

Lay Summary

Our Universe contains billions upon billions of galaxies, as recently imaged by the James Webb Space Telescope. As hard as it is to imagine, all of these galaxies are organized into structures that look like those inside nerves, or a system of rivers and streams. This structure leaves vast expanses of "empty" space across the Universe, called "voids", but these regions are not entirely empty. Instead, the lonely galaxies found within them have been aptly named "void galaxies."

However, these galaxies are so few and far-between that we do not know much about them, and few studies currently exist. Therefore, many questions remain outstanding. Are they any different from "normal" galaxies in the Universe? If so, how? Do they produce more or less stars, or are they any bigger? Where do they get the fuel to continue growing if they are not surrounded by as much matter? Do they have a different shape?

This study aims to answer a few of these questions. Environment is thought to affect galaxy formation and evolution, as morphology encodes what has happened to a galaxy during its lifetime. Because void galaxies are so isolated, we might expect some of their shapes to be different, as they are less likely to encounter phenomena such as mergers.

As astronomical surveys become more accurate, we are able to more accurately detect void galaxies and measure their properties, such as through the use of the Galaxy And Mass Assembly (GAMA) survey. This observational survey covers a wide expanse of the sky to a high degree of precision, making it ideal to use the data for study. In addition, it can often be hard to visually classify the shape of galaxies as their numbers drastically increase. As a result, citizen science projects such as Galaxy Zoo are born, where volunteers can be shown a picture of a galaxy, respond to a series of questions, and help scientists determine their classifications. Therefore, this research combines these two very different methods, numerical analysis and citizen science, to study void galaxy morphologies for the very first time. This unique combination allows us to approach the problem from multiple perspectives.

Throughout this study, we characterize galaxy shape by utilizing a numerical method called the Sersic Index, usually represented by n. This value of n represents the curvature of the light profile, and usually can be interpreted as values around n=1 or n=2 being disky Frisbee-shaped galaxies, like our own Milky Way, and values of n=4 are more ellipsoidal or spherical galaxies, ranging from the rough shape (in American terms) of a football or a soccer ball. In addition, we calculate other basic properties of the galaxies, including their size and the rate at which they form stars, two things that can tell us a lot about their behavior and history.

We then incorporate Galaxy Zoo by analyzing the voting fractions of the galaxies. How do people typically vote on each galaxy for several questions? Using statistics such as the Kolmogorov-Smirnov and Anderson-Darling tests, we can then test our distributions to see whether we can explicitly say that void galaxies are a very different population in terms of their morphologies.

Overall, we find that both populations appear to be dominated by disky galaxies, with void galaxies being smaller, and that void galaxies typically seem to always have a bulge. It is possible that void galaxies will have rounded bulges more often than their ’twins’, but higher-resolution studies would be more effective at determining whether this is true.

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