STAR-FORMATION ACTIVITY IN SPIRAL GALAXY DISKS AND THE PROPERTIES OF RADIO HALOS - OBSERVATIONAL EVIDENCE FOR A DIRECT DEPENDENCE

In this article we address observationally the questions: how does sta r formation (SF) in the disks of galaxies lead to the creation of radi o halos, and what minimum energy input into the ISM is needed to facil itate this? For the investigation we use a sample of five edge-on gala xies exhibiting radio continuum emission in their halos and enhanced S F spread over large parts of their disks. In a detailed study of the t wo galaxies in our sample for which we have the best data, NGC 891 and NGC 4631, we show that the radio halos cut off abruptly at galactocen tric radii smaller than those of the underlying thin radio disks. Our most important result is that the halo cutoffs are spatially coinciden t with the radii where the SF activity in the underlying disks drops s harply. The difference in radius of the emission distributions tracing ongoing SF in the disks (IRAS 50 mu m, H alpha) versus that of the no nthermal radio continuum thin disks (tracing the distribution of cosmi c-ray [CR] electrons) is typically a few kpc, This difference in exten t is caused by CR diffusion. We have measured the CR diffusion coeffic ients in the thin disks of both NGC 891 and NGC 4631. For radial diffu sion of CR electrons within the galactic disks the values are D-r = 1. 1-2.5 x 10(29) cm(2) s(-1) (NGC 4631) and D-r = 1.2 x 10(29) cm(2) s(- 1) (NGC 891). For motions in the z-direction in areas within the thin disks where no outflows occur, we derive a firm upper limit of D-z les s than or equal to 0.2 x 10(28) cm(2) s(-1) for NGC 891. The value for NGC 4631 is D-z = 1.4 x 10(28) cm(2) s(-1). The other three galaxies in our sample, NGC 3044, NGC 4666, and NGC 5775 show (at the sensitivi ty of our data) less extended, more filamentary radio halos. Isolated spurs or filaments of nonthermal radio continuum emission in their hal os are traced only above the most actively star-forming regions in the disks. This, in conjunction with the results obtained for NGC 891 and NGC 4631, suggests a direct connection between the shapes and radial extents of radio halos of normal galaxies and the level and the spatia l distribution of the current SF in their disks. The existence of fila mentary structures implies that expansion and convection of CR-heated plasma plays an important role even at low z-distances above the plane . Based on radio data we derive the disk-averaged mean energy input ra tes into the ISM due to supernova explosions per unit surface area in nine galaxies, including the ones in our sample. We find a clear trend in the sense that the galaxies with the highest mean energy input rat es in the underlying disks have the most extended and pervasive radio halos. Below a certain threshold, outflows are inhibited. As a lower l imit for this threshold we derive an energy input rate of similar to 1 0(-4) ergs s(-1) cm(-2). The fact that radio halo emission is found on ly above the most luminous giant H II regions in late-type ''normal'' galaxies (of type Sbc or later) indicates that only these are capable of initiating disk-halo interactions by locally exceeding the threshol d. The distribution of the local outflows determines the overall shape s of the CR halos and leads to the observed filamentary structures.