Spatially correlated diffuse Hα and radio continuum emission from the halo of NGC 891.

Spatially correlated diffuse Hα and radio continuum emission is observed in the halo of NGC 891 above the most actively star-forming regions in the disk. We use a new high-resolution, high-sensitivity radio continuum map of NGC 891 to show that both Hα and radio continuum emission from the halo break off at a galactocentric radius of R=~10kpc, where the emissivity of the underlying disk drops rapidly. This indicates that only inside this radius the star formation rate 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 scale-length of the transition of only ~1kpc and the asymmetry of the halo, convection appears to be the dominant mechanism driving the disk-halo interaction for the cosmic rays. We interpret the diffuse Hα 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 emission. Other explanations for the observed Hα halo emission, such as e.g. heating of Hi gas in the halo by energetic photons from the underlying disk, cannot be excluded. However, our data clearly contradict the "decay of dark matter" hypothesis which has also been applied to NGC 891 in order to explain the diffuse Hα emission. The small-scale outflows (z<~3kpc) are apparently feeding a galactic wind which sets in at higher z-distances, as can be deduced from a comparison of the current data with earlier radio spectral index investigations.