High-velocity HI Clouds Research Articles

Thermal-convective instability of interstellar gas and the origin of high-velocity HI clouds

A mechanism is proposed for the formation of high velocity HI clouds falling onto the plane of the galaxy. As a spiral shock wave passes through the interstellar gas, conditions arise under which gas can go over into a two-phase state (thermal instability). In an external gravitational field (in the given case, in the field of the disk of the galaxy) the thermal instability leads to convection -- the colder gas sinks. During the nonlinear stage the cold clouds become separated and continue to fall toward the plane of the galaxy. The cloud velocities reach the observed values of approximately 70 km/sec.

Apex of Solar Motion of Neutral Hydrogen Concentrations at Different Velocities

De Vaucouleurs and Peters1, Kerr and Sullivan2 and I3 derived the solar motion with respect to the high velocity HI clouds (HVCs) with a view to examining the suggestion by Verschuur4 that these cloud complexes and the Local Group of galaxies form part of the same extragalactic system of objects. They concluded that the solar motion solution does not seem to justify Verschuur's hypothesis. Kerr and Sullivan2 also derived the solar motion with respect to a sample of 125 intermediate velocity clouds (IVCs) and concluded that they seem to be kinematically distinct from the HVCs.

Solar Motion with Respect to the High-velocity HI Clouds and to the Local Group

WE have derived1 the solar motion with respect to galaxies at different distances and have shown how the solar apex varies systematically with the distance of the reference objects.

High Velocity HI Away from the Galactic Plane

Results of recent surveys of high velocity HI clouds with the 80-ft reflector at Dwingeloo and with the 260-ft x 80-ft standing paraboloid of the Ohio State University are combined. They show that clouds with high negative radial velocities occur mainly in the longitude range 50° to 220° from N. to S. galactic poles. However, nearly three times as many clouds are found north than south of the galactic plane. Clouds with high positive radial velocities are much less numerous than the negative velocity clouds. They have been found between longitudes 260° to 350° and scattered around lII = 75°, bII = +45°.