# Global distribution of far-ultraviolet emissions from highly ionized gas in the Milky Way

## TL;DR

This study maps and analyzes the distribution of highly ionized gas in the Milky Way using FUV emission lines, revealing a uniform temperature, a disk-like distribution, and insights into the gas's origin and structure.

## Contribution

It provides the first all-sky maps of C IV and O VI emissions, estimates the mass and filling fraction of transition-temperature gas, and evaluates models explaining its distribution.

## Key findings

- Gas temperature is approximately 1.89 x 10^5 K across the Galaxy.
- The emission measure follows a disk-like distribution with a scale height of about 6 kpc.
- The total mass of transition-temperature gas is roughly 6.4 billion solar masses.

## Abstract

We present all-sky maps of two major FUV 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 $\pm$ 0.06) $\times$ $10^5$ 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 $z_0=6_{-2}^{+3}$ kpc. The total mass of the transition-temperature gas is estimated to be approximately $6.4_{-2.8}^{+5.2}\times10^9 M_{\bigodot}$. We also calculated the volume-filling fraction of the transition-temperature gas, which was estimated to be $f=0.26\pm0.09$, and varies from $f\sim0.37$ in the inner Galaxy to $f\sim0.18$ 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.