Star‐forming Processes Far from the Galactic Disk: Inoperative or Indolent Where Operative

Highly supersonic collisions between gaseous clouds may effectively trigger star formation in the disk of our Galaxy, but not in the diffuse environment of the Galactic halo. This is because the observed high-velocity clouds (HVCs) are not dominated by collisions: the characteristic time between cloudlet collisions inside an HVC at an assumed distance of 10 kpc is at least 1 Gyr for collective encounters and at least 10 Gyr if a particular cloudlet is considered. In agreement with this result, we also estimate that the observed cloudlets contain smaller masses than the nonmagnetic Jeans mass that signals favorable conditions for gravitational collapse and further fragmentation in the isothermal regime. The diffuse environment observed around the Magellanic Clouds (MCs) is more difficult to understand than HVCs. Six sparse blue associations and two young B-type stars have been observed in the H I bridge between the MCs, while no stars exist in the H I cloud complexes that make up the Magellanic Stream. We discuss the conditions under which spatially sporadic star formation took place in the Magellanic Bridge during the past 16-25 Myr and the reasons for the complete absence of star formation in the Stream during its entire lifetime. We also estimate the angular resolutions that need to be achieved by follow-up radio observations of these regions that could detect cold cloudlets embedded in the gas.