Highly Ionized Gas in the Galactic Halo

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Session 114 -- Multi-Phase Interstellar Medium
Display presentation, Saturday, January 15, 9:30-6:45, Salons I/II Room (Crystal Gateway)

[114.05] Highly Ionized Gas in the Galactic Halo

J.D.Slavin (NASA/GSFC), J.M.Shull (JILA)

We re-examine the values of electron density $n_e$ and gas pressure $P/k$ in the interstellar medium (ISM) of the Galactic halo, as inferred from C~IV emission and absorption lines and using current C~IV atomic data. In a homogeneous model with $4.7 \leq \log T \leq 5.3$, the data are consistent with $0.01 \leq n_e \leq 0.02$ cm$^{-3}$ and $2200 \leq P/k \leq 3700$ cm$^{-3}$~K, a factor of 2 -- 3 higher than advocated by Martin \& Bowyer and comparable to the thermal pressure in the disk. The volume filling factor for homogeneous models ranges from 0.5\% to 5\%. Because of the constraints arising from filling factor and radiated power, most of the C~IV must arise from gas near the peak of the cooling curve, at $\log T \leq 5.6$. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants provide significant amounts of halo gas at $\sim10^{5.3}$~K and process 20 -- 40 $M_{\odot}$~yr$^{-1}$ with a power of $\sim 10^{41}$ ergs s$^{-1}$. Since the observed C~IV and N~V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc respectively, we examine inhomogeneous models with different exponential scale heights of $T$, $P$, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C~IV. For appropriate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O~VI $\lambda1034$ diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C~IV $\lambda1549$ ($\sim6000$ photons cm$^{-2}$ s$^{-1}$ sr$^{-1}$). The ions C~IV, N~V, and O~VI are predicted to show a decreasing trend in vertical extent, with the O~VI scale height perhaps as low as $\sim1$ kpc.

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