We present an empirical model for the halo evolution, global gas dynamics and chemical evolution of Fornax, the brightest Milky Way (MW) dwarf spheroidal galaxy (dSph). Assuming a global star formation rate ψ(t) = λ ∗ (t)[M g (t)/M ⊙ ] α consistent with observations of star formation in nearby galaxies and using the data on Fornax’s ψ(t), we derive the evolution of the total mass M g (t) and the rate ∆F (t) of net gas flow for cold gas in a growing star-forming disk with a time variable λ ∗ (t). We identify the onset of the transition in ∆F (t) from a net inflow to a net outflow as the time t sat at which the Fornax halo became an MW satellite and estimate the evolution of its total mass M h (t) at t ≤ t sat using the median halo growth history in the ΛCDM cosmology and its present mass within the half-light radius derived from observations. We find that the Fornax halo grew to M h (t sat ) ≈ 1.8 × 10 9 M ⊙ at t sat ≈ 4.8 Gyr and that its subsequent global gas dynamics was dominated by ram-pressure stripping and tidal interaction with the MW. Then we build a chemical evolution model on a 2-D mass grid, using supernovae as the enrichment source of the gaseous system. We find that the key parameter of controlling the element abundances pattern is the supernovae mixing mass. It is set differently between two types of supernovae and between two phases, before and after t sat in our model. The choice is determined based on the supernovae remnant evolution as well as the global gas dynamics. We also find the metal loss in the outflow dominated phase (t > t sat ) is severe, which is empirically implemented in our model. The data generated from the standard case can explain the observational data very well, e.g., abundance ratio of α element to Fe as a function of metallicity [α/Fe] vs. [Fe/H], metallicity evolution as a function of time [E/H] vs. t and metallicity distribution function (MDF) for Mg, Ca and Fe.
University of Minnesota Ph.D. dissertation. September 2015. Major: Physics. Advisor: Yong-Zhong Qian. 1 computer file (PDF); viii, 102 pages.
A Model for Gas Dynamics and Chemical Evolution of Fornax Dwarf Spheroidal Galaxy.
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