The detection in absorption lines of gas clouds outside the galactic plane at high velocities by Münch and Zirin (1961), high velocities then defined as velocities differing by more than 20 km/s from the LSR, showed that the space outside the Milky-Way disk contains not just stars. Of course, from a continuity argument it had been all along clear that some transition zone had to exist between the dense (relatively speaking) gas of the Milky-Way plane and the vast (almost) emptiness of intergalactic space. The presence of these clouds requires a mechanism to prevent their evaporation, and Spitzer (1956) proposed that dilute hot gas had to exist outside the Milky-Way disk reaching, in his hydrostatic-equilibrium model, temperatures of a few million K at several tens of kpc. These high temperatures led him to name these gases the Galactic Corona. Observational confirmation of the abundance of these cool clouds came from the measurements of 21-cm HI emission, but no one-to-one correspondence with clouds detected in the visual did appear (Habing 1969). For the majority of the high-velocity (HV) clouds (Hulsbosch 1978) no distances are known, and all of those are believed to exist as a gaseous halo with the halo stars. Thus our Milky Way appears to have outside the disk: a halo, a gaseous halo, and a corona.
Constraining the Milky Way Mass with Its Hot Gaseous Halo
