Astronomy
Permanent URI for this communityhttp://hdl.handle.net/1903/2215
Browse
3 results
Search Results
Item Interactions between Massive Stellar Feedback and Interstellar Gas in the Eagle Nebula(2024) Karim, Ramsey Lee; Mundy, Lee G; Pound, Marc W; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)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.Item Molecular Gas and Star Formation at Low Metallicity in the Magellanic Clouds(2016) Jameson, Katherine Esther; Bolatto, Alberto D; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The Magellanic Clouds are two interacting, gas-rich, star-forming, low-mass, nearby satellite galaxies of the Milky Way that afford a unique view of low-metallicty star-forming regions, providing the nearest laboratories to study processes relevant to star formation in the early universe. We use the dust emission from HERITAGE Herschel data to map the molecular gas in the Magellanic Clouds, avoiding the known biases of CO emission as a tracer of H2. On small (~ few pc) scales in the Small Magellanic Cloud (SMC), we study the effect of metallicity on the structure of photodissociation regions in the outskirts of molecular clouds using [CII] and [OI] spectroscopy combined with new ALMA 7-m array maps of 12CO and 13CO. We estimate the total amount of molecular gas using [CII] to trace H2 at low-Av and 12CO to trace H2 at high-Av. We find that most of the molecular gas is traced by [CII] emission and that metallicity only affects the relationship between 12CO emission and molecular gas through changes in Av. Using mid-infrared spectroscopy from Spitzer Space Telescope in the SMC, we model the H2 rotational line emission to estimate temperatures, column densities, and fractions of warm H2 gas (T>100 K). The temperatures and column densities of warm H2 gas are similar to nearby galaxies, but the SMC shows somewhat high fractions of warm H2. The properties of the warm H2 gas indicate that it is located in photodissociation regions that are more extended in the low metallicity environment of the SMC. We use dust-based molecular gas maps data to evaluate molecular depletion time scales as a function of spatial scale. We compare galaxy-scale analytic star formation models to our observations and find that successfully predicting the trends in the low metallicity environment needs the inclusion of a diffuse neutral medium. The analytic models, however, do not capture the scatter observed, which computer simulations suggest is driven primarily by the time-averaging effect of star formation rate tracers. The averaging of the scatter in the molecular gas depletion time as a function of scale size suggests that the drivers of the star formation process in these galaxies operate on large scales.Item Probing the Multiphase Interstellar Medium and Star Formation in Nearby Galaxies through Far-infrared Spectroscopy(2015) Herrera Camus, Rodrigo; Bolatto, Alberto; Astronomy; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)We present a study of different aspects of the multi-phase interstellar medium (ISM) of nearby galaxies, including detailed analysis of the low-excitation ionized gas, the thermal pressure (Pth) of the neutral gas, the dust-to-gas mass ratio (DGR) in low-metallicity environments, and the use of far-infrared transitions as tracers of the star formation rate (SFR). We based our work on the largest sample to date of spatially-resolved, infrared observations of nearby galaxies drawn from the KINGFISH and ``Beyond the Peak'' surveys. We use deep infrared observations to study the DGR of the extremely metal-poor galaxy I Zw 18. We measure a DGR upper-limit of 8.1x10^{-5}. This value is a factor of ~8 lower than the expected DGR if a linear correlation between DGR and metallicity, as observed in spirals, were to hold. Based on the line ratio between the [NII] 122 and 205 um transitions, for 140 regions selected from 21 galaxies we measure electron densities of the singly-ionized gas in the ne~1-230 cm^{-3} range, with a median value of ne=30 cm^{-3}. We find that ne increases as a function of SFR and radiation field strength. We study the reliability of the [CII] and [NII] 122 and 205 um transitions as SFR tracers. In general, we find good correlations between the emission from these fine-structure lines and star formation activity. However, a decrease in the photoelectric heating efficiency in the case of the [CII] line, or collisional quenching of the [NII] lines, can cause calibrations based on these transitions to underestimate the SFR. Finally, for a sample of atomic-dominated regions selected from 31 galaxies, we use the [CII] and HI lines to measure the cooling rate per H atom and Pth of the cold, neutral gas. We find a \pt\ distribution that can be well described by a log-normal distribution with median Pth/k~5,500 K cm^{-3}. We find correlations of increasing Pth with radiation field intensity and SFR, which is consistent with the results from two-phase ISM models in pressure equilibrium.