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Publication Date

2025-5

First Advisor

Michael Petersen

Second Advisor

James Lowenthal

Document Type

Honors Project

Degree Name

Bachelor of Arts

Department

Astronomy

Keywords

astronomy, astrophysics, galaxies, Milky Way Galaxy, galactic dynamics, galactic formation and evolution, galactic bulge, galactic disk, stochastic methods, astrophysical simulation

Abstract

We examine the inner 5 kpc radius of the Milky Way, known as the bulge, to determine its formation history. We test the proposal that a non-trivial fraction of bulge stars migrated from the disk region to the bulge, or up to 10% based on Horta, Petersen, and Peñarrubia (2024) as a base significance level. Through a stochastic model that provides “kicks" in random directions in the forces experienced by a disk star, we model a channel of diffusion from the disk into the bulge by tracking the star’s motion to determine if it becomes retrograde, indicating a new orbit more characteristic of bulge stars than disk stars. These physical kicks could come from giant molecular clouds, the orbits of other stars, dark matter subhalos, or more, but our model does not depend on fully defining the perturber, only the perturbation effects. We demonstrate that this model can successfully create such a channel from the disk to the bulge, taking into account likely physical conditions from the bulge. We test the model for a range of initial phase-space conditions and kick sizes with results compared by initial position. We also test velocity kicks that produce retrograde fractions around or less than 10% of the time for some set of initial conditions and determine if this fraction is consistent for a range of initial positions and velocities as well as for different time ranges. Around this significance level, we find that this model produces bulge stars at a fraction of approximately 10% ± 5% for kick strengths 20 km/s ± 5 km/s over a period of 5 Gyr for radii of 2, 3, and 4 kpc. We then investigate the time dependence of the model and its effect on the spread of energies. Finally, we find that stars at smaller radii are easier to kick into bulge orbits (between 1 to 2 times as large a fraction from 4 kpc to 2 kpc), that larger kicks produce larger numbers of bulge stars (where increasing the kick creates a non-linearly increasing fraction), and that faster-moving stars have smaller fractions following this channel.

Rights

©2025 Emma Davis. Access limited to the Smith College community and other researchers while on campus. Smith College community members also may access from off-campus using a Smith College log-in. Other off-campus researchers may request a copy through Interlibrary Loan for personal use.

Language

English

Comments

xiv, 62 pages: color illustrations, charts. Includes bibliographical references (pages 55-58).

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