"Deviations from a Uniform Meta-Galactic Ionization Background: Implications for Galaxy Formation"
Ben Oppenheimer (Leiden) - April 29, 2013 at 12:10 pm
I will first motivate three reasons why metals in diffuse phases, the intergalactic and circumgalactic media, are important for problems in galaxy formation. The first poses the key test as to whether we can match the observed metal census across all phases, including the halos of galaxies, with the theoretical amount of metals produced from stellar nucleosynthesis. The second reason emphasizes how metals change the cooling dynamics of circumgalactic gas, altering processes of accretion and star formation that determine the stellar assembly
histories of galaxies. Finally, quasar metal absorption lines at z>2 can trace signatures of early dwarf galaxies, perhaps even those responsible for reionization, which cannot be observed directly via stellar light since they are too faint. A uniform meta-galactic ionizing background is usually assumed in these studies to translate the observed metal absorption columns into the underlying physical properties of gas outside galaxies. I will argue this could be a dramatically bad assumption for metals, because AGN provide ionizing flux above the level of the standard ionizing background. Of course it is well-known that quasars ionize local proximity zones, but what I will emphasize is the stage that follows when the quasar turns off and metals remain out-of-equilibrium. Quasar proximity zone fossils, where metals are over-ionized for timescales exceeding the AGN lifetime could affect a significant fraction of diffuse metals and
change the standard assumptions about the gas that typical metal absorbers trace. I will argue our proximity zone model can reproduce key observations of a metal absorber at z=0.9 with both Ne VIII and Mg II.
The seminar will be held in B-1 Hearst Field Annex.
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