The Extension of the Transition Temperature Plasma into the Lower Galactic Halo

Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 stars and 30 extragalactic objects have been assembled to study the extensions of these species away from the Galactic plane into the Galactic halo. H I and Al III mostly trace the warm neutral and warm ionized medium, respectively, while Si IV, C IV, and O VI trace a combination of warm photoionized and collisionally ionized plasmas. The much larger object sample compared to previous studies allows us to consider and correct for the effects of the sample bias that has affected earlier but smaller surveys of the gas distributions. We find that Si IV and C IV have similar exponential scale heights of 3.2(+1.0, -0.6) and 3.6(+1.0, -0.8) kpc. The scale height of O VI is marginally smaller with h = 2.6 ± 0.6 kpc. The transition temperature gas is ~3 times more extended than the warm ionized medium traced by Al III with h = 0.90(+0.62, -0.33) kpc and ~12 times more extended than the warm neutral medium traced by H I with h = 0.24 ± 0.06 kpc. There is a factor of 2 decrease in the dispersion of the log of the column density ratios for transition temperature gas for lines of sight in the Galactic disk compared to extragalactic lines of sight through the entire halo. The observations are compared to the predictions of the various models for the production of the transition temperature gas in the halo. The appendix presents a revision to the electron scale height of Gaensler et al.'s 2008 study based on electron dispersion measures.