How Do Massive Stars Shape Cosmic Evolution?
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    • Outflows >
      • Outflow Velocity Scaling Relations
      • Ionization Structure of outflows
      • Mass Outflow Rates
      • Mass Loading of Galactic winds
      • Outflows shape the mass metallicity relation
      • Molecular Outflows of M 82
    • Epoch of Reionization >
      • Constraining Stellar Populations with FUV spectra
      • The escape of ionizing photons
      • Accurately predicting the escape fraction of ionizing photons
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The Different temperatures of Galactic outflows

I extended the previous analysis to 3 other transitions: O I, Si III, and Si IV. Including Si II, these four transitions probe gas at different ionization phases (between 13.6 and 45 eV), and can be used to determine the ionization structure of the outflows.

I first studied how the strength of the lines scaled with stellar mass. I then determined what drives the shallow scaling of the equivalent width with stellar mass. Finally I studied which mechanisms could create the observed Si IV/Si III, and Si IV/Si II ratios.

The paper can be found here.
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Four plots of the scaling of the equivalent width with the galactic stellar mass. The four transitions, from left to right, top to bottom are O I 1302, Si III 1206, Si IV 1393, and N V 1238. There is a strong, yet shallow, correlation between equivalent width and stellar mass for the Si III and Si IV transitions.

How does the Equivalent width scale with stellar mass?

The equivalent width is an important measurement of the properties of a given transitions. The plot on the left shows the scaling of equivalent width with stellar mass for the four different UV transitions. The neutral gas (traced by O I) does not scale significantly with the stellar mass, but the ionized tracers scale strongly with stellar mass (both Si III and Si IV). Similar to the outflow velocity, the ionized gas scales shallowly with stellar mass.

What drives the equivalent width scaling?

There is a lot of information encoded within the equivalent width: large equivalent widths correspond to large optical depths, line widths, covering fractions, or some combination of all three. To break this triple degeneracy I made measurements of the three quantities from the  Si IV doublet (figure to the right). The top left panel corresponds to the covering fraction, and has a nearly constant value at 0.82. The top right panel corresponds to the optical depth, which also has a constant value. The bottom left panel shows how the line width scales with stellar mass.
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Plot of the three components that contribute to equivalent widths: Covering Fraction (top left), optical depth (top right), and line width (bottom left).
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PhotoIonization sets the ionization structure of the outflows

To determine what sets the ionization structure I look edat the equivalent width ratios of three transitions (to the left). I used shock models and photo-ionization models to test whether shocks or photo-ionization established the observed equivalent width ratios.  The shock models under predict the high ionization gas, and over predict the low-ionization gas.

The photo-ionization models nicely match observations. To create these models we use a Starburst99 stellar continuum model to determine the number of ionizing photons. However, the models have quite a few input parameters: gas metallicity, stellar age, stellar metallicity, stellar population, ionization parameter, and hydrogen density. Mass-outflow rates require this information. Follow the button below to find out more about mass outflow rates.
Mass outflow rates

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