On the Formation of Interstellar Cloud Complexes, OB Associations and Giant H II Regions

Summary. We propose that large cloud complexes, OB associations, and giant H ii regions form as a result of the initiation of the magnetic Rayleigh-Taylor instability in the interstellar medium by the passage of a galactic shock. Total masses of about 106 M0, formation of unbound systems, and alignment along spiral arms with typical separation of about 1 kpc are natural consequences of this point of view. In addition, we argue that the enhancement of synchrotron radiation to be expected in the compression zones of spiral galaxies has hitherto been considerably overestimated. Key words: instability - spiral arms - cloud complexes - giant H ii regions