Global Distribution of Far-ultraviolet Emissions from Highly Ionized Gas in the Milky Way

Abstract

We present all-sky maps of two major far-ultraviolet cooling lines, C iv and O vi , of highly ionized gas to investigate the nature of the transition-temperature gas. From the extinction-corrected line intensities of C iv and O vi , we calculated the gas temperature and the emission measure of the transition-temperature gas assuming isothermal plasma in the collisional ionization equilibrium. The gas temperature was found to be more or less uniform throughout the Galaxy with a value of (1.89 ± 0.06) × 10 5 K. The emission measure of the transition-temperature gas is described well by a disk-like model in which the scale height of the electron density is kpc. The total mass of the transition-temperature gas is estimated to be approximately . We also calculated the volume-filling fraction of the transition-temperature gas, which was estimated to be f  = 0.26 ± 0.09, and it varies from f  ∼ 0.37 in the inner Galaxy to f  ∼ 0.18 in the outer Galaxy. The spatial distribution of C iv and O vi cannot be explained by a simple supernova remnant model or a three-phase model. The combined effects of supernova remnants and turbulent mixing layers can explain the intensity ratio of C iv and O vi . Thermal conduction front models and high-velocity cloud models are also consistent with our observation.