HIGHLY IONIZED-GAS IN THE GALACTIC HALO

We reexamine 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 less-than-or-equal-to log T less-than-or-equal-to 5.3, the data are consistent with 0.01 less-than-or-equal-to n(e) less-than-or-equal-to 0.02 cm-3 and 2200 less-than-or-equal-to P/k less-than-or-equal-to 3700 cm-3 K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. 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 less-than-or-equal-to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 10(5.3) K and process 20-40 M. yr-1 with a power of approximately 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 (approximately 6000 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 approximately 1 kpc.