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

Abstract

Column densities for H I, Al III, Si IV, C IV, and O VI toward 109 stars and 30 extragalactic objects have been assembled in order to study the relative extensions of these species away from the Galactic plane into the Galactic halo. H I and Al III 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 transition temperature plasma. The large object sample allows us to consider and correct for the effects of the sample bias that has affected the results of earlier but smaller surveys of the gas distributions. We find Si IV and C IV have similar exponential scale heights of {approx}3.2 to 3.6 kpc. The scale height of O VI is marginally smaller with h{approx}2.6 kpc. The transition temperature gas is {approx}6 times more extended than the warm ionized medium traced by Al III with h{approx}0.65 kpc and {approx}14 times more extended than the warm neutral medium traced by H I with h{approx}0.21 kpc. There is a factor of 2 decrease in the dispersion of the ionic column density ratios for transition temperature gas for lines of sight inmore » the Galactic disk compared to extragalactic lines of sight through the entire halo. The logarithmic dispersions for N(C IV)/N(O VI) and N(Si IV)/N(O VI) are approximately twice that found for N(Si IV)/N(C IV)« less