SPATIALLY CORRELATED DIFFUSE H-ALPHA-CONTINUUM AND RADIO-CONTINUUM EMISSION FROM THE HALO OF NGC-891

Spatially correlated diffuse Her and radio continuum emission is obser ved in the halo of NGC 891 above the most actively star-forming region s in the disk. We use a new high-resolution, high-sensitivity radio co ntinuum map of NGC 891 to show that both Ha and radio continuum emissi on from the halo break off at a galactocentric radius of R similar or equal to 10 kpc, where the emissivity of the underlying disk drops rap idly. This indicates that only inside this radius the star formation r ate is high enough to exceed the threshold for energy input needed to allow gas and cosmic rays to leave the thin disk. Given the small scal e-length of the transition of only approximate to 1 kpc and the asymme try of the halo, convection appears to be the dominant mechanism drivi ng the disk-halo interaction for the cosmic rays. We interpret the dif fuse Ha emission as secondary emission of originally hot (10(7) K) gas which was transported into the halo and thereby cooled. This scenario is capable of explaining the large-scale inhomogeneity of the halo em ission. Other explanations for the observed H alpha halo emission, suc h as e.g. heating of H I gas in the halo by energetic photons from the underlying disk, cannot be excluded. However, our data clearly contra dict the ''decay of dark matter'' hypothesis which has also been appli ed to NGC 891 in order to explain the diffuse H alpha emission. The sm all-scale outflows (z less than or similar to 3 kpc) are apparently fe eding galactic wind which sets in at higher z-distances, as can be ded uced from a comparison of the current data with earlier radio spectral index investigations.