Revisiting FUSE O VI Emission in Galaxy Halos
  A significant fraction of baryons in galaxies are in the form of diffuse gas
of the circumgalactic medium (CGM). One critical component of the multi-phases
of CGM, the so-called "coronal" warm-hot phase gas ($\rm 10^{5}-10^{6}$ K)
traced by O VI 1031.93, 1037.62 \r{A} resonance lines, has rarely been detected
in emission from galaxy halos other than Milky Way. Here we report four
additional detections of O VI emission gas in the halos of nearby edge-on
galaxies, NGC 4631 and NGC 891, using archival Far Ultraviolet Spectroscopic
Explorer data and an updated data pipeline. We find the most intense O VI
emission to be from fields forming a vertical line near the center of NGC 4631,
despite the close proximity to the disk of two other fields. The detected O VI
emission surface brightness are about 1.1$\pm 0.3$ $\times$ $10^{-18}$ to
3.9$\pm0.8$ $\times$ $10^{-18}$ ergs s$^{-1}$ cm$^{-2}$ arcsec$^{-2}$. The
spatial distribution of the five 30" $\times$ 30" O VI detection fields in NGC
4631 can be interpreted as the existence of filamentary structures of more
intense O VI emission superimposed within a diffuse and faint O VI halo in
star-forming galaxies. Volume-filled O VI emission mapping is greatly needed to
determine the structure and prevalence of warm-hot gas and the role it plays in
the cycling of gas between the galaxy disk and the halo. Finally, we present
the sensitivity of future funded and proposed UV missions (LUVOIR-A, LUVOIR-B,
CETUS, and Aspera) to the detection of diffuse and faint O VI emission in
nearby galaxy halos.
