Interactions between Massive Stellar Feedback and Interstellar Gas in the Eagle Nebula

Abstract

My thesis describes multi-scale stellar feedback processes observed in the Eagle Nebula star forming region in our Milky Way galaxy. Stellar feedback from massive stars encompasses bright ultraviolet radiation which ionizes atoms and dissociates molecules in gas surrounding the stars as well as supersonic winds which impact the gas and create hot shocked layers. I study the interaction of stellar radiative and mechanical feedback with pre-existing density inhomogeneities in the molecular cloud in order to learn about the effects of the interstellar environment on the relative efficiency of various forms of feedback. This work informs our understanding of the life cycle of interstellar gas: gas forms stars and is then exposed to their winds and radiation, and we would like to know how that affects the formation of future generations of stars. The Eagle Nebula's relative proximity to us means we observe the H II region with high spatial resolution. Extra-galactic studies observe many H II regions simultaneously and at a variety of cosmic ages, but lack the resolution to understand the structure of the individual regions. High resolution studies of Galactic sources such as the Eagle serve as templates for what extra-galactic astronomers are seeing in far-away galaxies. The work also contributes to sub-grid feedback prescriptions in large-scale simulations of galaxy formation and evolution. Stars and their feedback are too small to be simulated in these contexts, so theorists require accurate approximations for the effects of stellar feedback. Massive stars form in massive molecular gas clouds and then deliver vast quantities of energy back into the clouds in the form of radiation and stellar winds. They form H II regions, 1-to-10-light-year scale areas of ionized hydrogen, which are often overpressured bubbles compared to the surrounding interstellar medium, and their supersonic winds sweep up a compressed shell of gas. Around the edge of the H II region, there lies a layer of gas which receives no >13.6 eV extreme-ultraviolet H-ionizing radiation (EUV), but is rich in 6-13.6 eV far-ultraviolet radiation (FUV) which can photodissociate molecules such as CO and H2 and ionize C. These photodissociation regions (PDRs) are heated via the photoelectric effect as FUV radiation interacts with organic molecules called polycyclic aromatic hydrocarbons (PAHs), and the regions are cooled by the collisionally excited far-infrared fine structure transitions of ionized carbon and atomic oxygen. The FEEDBACK SOFIA C+ Legacy Project (Schneider et al. 2020) studies the coupling efficiency of that energetic feedback to the gas by observing one such transition of singly ionized carbon at 158 micron referred to as C+ or [C II]. In this astrophysical context, the line is emitted primarily within PDRs. With modern heterodyne receivers and an observatory above Earth's atmosphere, we can both detect and spectroscopically resolve the [C II] line and therefore trace the morphology and kinematics of the PDR regions surrounding massive stars. We contextualize these observations with velocity-resolved observations tracing the un-illuminated molecular gas beyond the PDRs and a variety of archival data spanning the electromagnetic spectrum from radio to X-ray. I use these observations to study the Eagle Nebula, home to the iconic Pillars of Creation, and learn how pre-existing density structure evolves when exposed to stellar feedback and what that implies for the energetic coupling of the stellar feedback to the gas. My first study covers the Pillars of Creation in a detailed, multi-wavelength analysis published in the Astronomical Journal. We find that these pillars are long-lasting structures on the scale of the H II region age and that they must arise from pre-existing density structures. My second study zooms out to the greater Eagle Nebula H II region to learn how the massive stars affect the rest of the region. This analysis concludes that the primordial filamentary structure which must have led to the formation of the stellar cluster also governs the shape of the H II region and how much of the surrounding gas is affected by the feedback. Finally, I describe a software package, scoby, which I developed to aid these two studies. The software connects theoretical feedback estimates to observed star catalogs and delivers results tuned for observational studies like these. It has been used for several published analyses of other regions.