Resolved Stars' Insights into Galaxy Physics

Thesis (Ph.D.)--University of Washington, 2016-08

The history of a galaxy is encoded in its individual stars, and resolved stellar populations in nearby galaxies provide a wealth of information about star formation history (SFH), its relation to the interstellar medium, and galactic dynamics/structures. Using the power of resolved stars, imaged in multiple wavelengths by the Hubble Space Telescope, I investigate different types of galactic physics in three nearby galaxies: (1) measuring the escape fraction of ionizing photons from NGC 4214 and discussing its implications for cosmic reionization, (2) testing the density wave theory in M81 and understanding the origin of its spiral arms, and (3) tracing stellar overdensity features in M31 using red giant branch (RGB) stars and studying potential causes of the overdensities. In each of these studies, I developed and tested new methodology. First, I develop a new approach to measure the escape fraction of ionizing photons from a galaxy, and apply the technique to the nearest starburst dwarf galaxy NGC 4214. This new technique includes inferring the intrinsic ionizing fluxes from individual stars within a galaxy as well as the amount of absorption by the intervening dust by modeling the observed individual stars’ spectral energy distributions (SEDs), constructed from multi-wavelength HST observations, using the probabilistic SED fitting tool, called BEAST (Bayesian Ex- tinction And Stellar Tool). By combining these measurements with the estimates of the amount of consumed ionizing photons by photoionization using the Hα luminosity, I present the escape fraction map for NGC 4214 with unprecedented spatial resolution. I found a significant spatial variation in the escape fraction across the galaxy, and a ∼60 times higher global escape fraction (∼12%) than the previous measurement. Second, I use resolved stellar populations to trace prominent spiral structures in M81, which are highlighted by blue, young stars, and to understand the underlying dynamical mechanisms between star formation and these galactic structures. The origin of grand-design spiral patterns has been extensively explored observationally. However, the conclusions from different studies have often conflicted even for the same galaxies. The discrepancies mainly result from estimating the age gradient using discontinuous tracers, such as star clusters and gas/dust emissions including Hi, 24μm, CO and Hα. Instead of using discontinuous tracers, I model color-magnitude diagram that are constructed from resolved stars in M81 to derive spatially resolved SFHs around one of M81’s spiral arms and com- pare with the star formation propagation predicted by the density wave theory. I find that the grand-design spiral arms in M81 are likely induced by tidal interaction with companion galaxies about 200–300 Myr ago, not by the density waves. Finally, I use resolved stellar populations to investigate the global structures of the M31’s disk. Specifically, I make use of old RGB stars, selected based on their near-IR color and magnitude from the Panchromatic Hubble Andromeda Treasury (PHAT) survey data. The selected RGB stars directly trace the stellar mass distribution in M31, whereas a standard approach (i.e., modeling two-dimensional IR surface light profile) to quantify the disk structures is biased due to variation in mass-to-light ratio among different stellar populations. From the constructed RGB stellar number density map, I constrain the disk structures of M31, trace the overdensity features at 5 and 10 kpc, and examine possible causes for this overdensity. This research project is in progress. Thus, in this thesis, I will briefly show preliminary results.