Graduating Students Colloquium 2019

Jemma Walcott-Green:
Title: UV Feedback and Cooling in Protogalaxies: Challenges in Modeling the H2-Photodissociation Rate
Abstract: The abundance of molecular hydrogen (H2), the primary coolant in primordial gas, is critical for the thermodynamic evolution and star (black hole)-formation histories in early protogalaxies. Determining the photodissociation rate of H2 by an incident UV flux (in the Lyman Werner bands) is thus crucial, but prohibitively expensive to calculate on the fly in simulations. The rate is sensitive to the H2 rovibrational distribution, which in turn depends on the gas density, temperature, and incident UV radiation field. We will show updated prescriptions for modeling the optically-thick H2 photodissociation rate in simulations and briefly discuss the implications for the thermal evolution of protogalaxies irradiated by a strong UV flux.

Andrew Emerick:
Title: Stellar Feedback and Chemical Evolution in Dwarf Galaxies
Abstract: Galactic chemical evolution is driven by the complicated interplay of gas accretion, galaxy mergers, star formation, stellar feedback, mixing and turbulence in the ISM, and galactic outflows. Stellar feedback is fundamental in this evolution. How metals -- ejected in stellar winds and supernovae -- mix with a multi-phase ISM and couple to galactic winds depends sensitively on feedback physics that is poorly understood. Improving our theoretical understanding of both stellar feedback and galactic chemical evolution is becoming increasingly important as number and quality of observations of stellar and gas phase abundances in nearby galaxies continues to grow. Using these observations to understand the complex history of galactic chemical evolution requires detailed simulations capable of self-consistently resolving the processes that drive stellar abundances. We use high resolution, hydrodynamics simulations of isolated, low mass dwarf galaxies tracking individual stars to better understand the complex relationship between feedback and galactic chemical evolution. I will summarize some of the results from these simulations to-date, focusing on how metals mix within and are ejected from low mass dwarf galaxies.

Steven Mohammed:
UVGAPS: The Ultraviolet GAlactic Plane Survey
The Ultraviolet GAlactic Plane Survey (UVGAPS) produced a high resolution map of the Milky Way's Galactic plane in the NUV using the Galaxy Evolution Explorer (GALEX), an orbiting ultraviolet space telescope operated by NASA and Caltech between 2003-2013. In its last several years, the Galactic plane had only scarcely been observed by GALEX due to bright star protection limits set to protect the detectors. The full UVGAPS maps are just under 7200 square degrees (360 degrees x 20 degrees) with a full width half max resolution of 4.5-6", with 2" pixels, which is both a larger footprint at a higher resolution than previous UV all-sky surveys within the same region. Of the many astrophysical phenomena observable in ultraviolet wavelengths, we choose to focus on a few interesting objects: red clump stars, white dwarfs, bright blue objects and the Galactic dust that impacts all three. A new pipeline had to be developed to process the survey images due to a change in the GALEX observing methods from a dither mode to a long drift scan mode. We use an image source extractor to obtain the NUV photometry and several cuts to clean the data. We present a catalog of 2,986,045 objects with GALEX NUV band measurements. Despite the difference in observing strategy and analysis pipeline, we find good agreement between previously targeted GALEX observations and the UVGAPS catalog in overlapping regions. The data were cross matched to Gaia DR2 and Pan-STARRS DR1, two visible-band surveys that have considerable coverage of the Galactic Plane. We characterize matched objects in color-magnitude and color-color space to highlight a range of objects, from main sequence stars to binaries detected with these data. The data will be publicly available and of particular interest to those planning follow-up observations with HST.

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