Observations of O VI

Abstract

A useful spectroscopic tracer for a hot phase of interstellar gas is the O VI ion, which reaches its maximum concentration in collisional ionization between 10⁵ and 10⁶ K. Presently, over 70 stars have been observed for O VI absorption by the Copernicus satellite. Nearly all of the stars show broad, weak lines, but no evidence favoring a circumstellar origin for the gas can be found. An overall average for n_e of the hot gas in the galactic plane is of order 10⁻³ cm⁻³. The relative volume in space occupied by the hot gas regions (and hence their internal density) is uncertain, but a filling factor in the range 0.02 to 0.2 seems most plausible. Fluctuations in radial velocities and column densities suggest there are roughly 6 regions per kpc, each with N(0 VI) ≈ 10¹³ cm⁻². The observed rms dispersion of radial velocities for these regions is 26 km s⁻¹.