Stochasticity in Nebular Emission Lines
Physical and Biological Sciences
Undergraduate Astrophysics Internship
As humanity’s observation of astronomical objects continues farther from earth, our thirst for knowledge has traveled beyond the resolution of our finest telescopes. As a result, instead of viewing single stars, our collectable data comes from large groups of stars referred to as stellar populations. The growth of these populations from areas of dense atomic gas, known as H II regions, is of great interest to astronomers as they seek to understand the nature of star formation. Although these growing galaxies contain a multitude of classes and sizes of stars, light viewed at specific frequencies can provide information on critical details of the star forming region as well as the surrounding gas.
In the past, simulations have been created to predict the light emitted by stellar populations, but they ignored the potential fluctuations that result from the birth and death of bright stars within the population. The impact of these fluctuations is exaggerated in smaller galaxies, in which the composite light can be the product of a few large stars. This unpredictable variation introduced by natural processes is described in mathematical nomenclature as stochasticity. Although by definition, the stochasticity in the light emitted from a growing galaxy is random, if studied carefully, the underlying range of probable values can be described analytically.
In order to scientifically observe the variation in light emission from growing galaxies, I wrote a Python code that couples two previously existing physical models to create a complete model for star forming regions. The first program, Slug, was created at UC Santa Cruz to simulate how stellar populations grow in size over time. The second program, Cloudy, simulates the complex interactions between light and the combination of dust and gas that surround the growing stars. By using the newly developed Python program, the variation in key wavelengths of light was calculated for a variety of star forming regions, mainly focusing on smaller galaxies where stochastic effects are more significant.
My research found that in the smallest stellar populations, stochastic effects could cause light at certain frequencies to fluctuate by large factors of up to 1000. Astronomers have used light at these frequencies to determine a key properties of stellar populations, including the rate of star formation, the mass and age of the stellar population, and the composition of surrounding gas. The findings of this paper are not only pertinent to the accuracy of stellar population models, but also these conclusions drawn from the direct observation of a galaxy's emitted light.
