Evidence for a large population of shocked interstellar clouds

Abstract

A 21 cm absorption measurement over a long path length free of the effects of differential galactic rotation indicates the existence of two distinct cloud populations in the plane. One of them consisting of cold, dense clouds has been well studied before. The newly found hot clouds appear to be at least five times more numerous. They have a spin temperature of ~ 300 K, an rms velocity of N 35 km s-1, twice the total mass, and hundred times the kinetic energy of the cold clouds. Over long path lengths, the hot clouds have NH/kpc ~ 2 x 10^21 cm2 kpc-l, and are estimated to have individual column densities ≤10^20 cm2. We propose that they are shocked clouds found only within supernova bubbles and that the cold clouds are found in the regions in between old remnants, immersed in an intercloud medium. We conclude that the solar neighbourhood must be located between old supernova remnants rather than within one.