Star formation activity in spiral galaxy disks and the properties of radio halos: Observational evidence for a direct dependence

Abstract

In this article we address observationally the question how SF in the disks of galaxies leads to the creation of radio halos and which minimum energy input into the ISM is needed to facilitate this. For this investigation we use a sample of five edge-on galaxies exhibiting radio continuum emission in their halos and enhanced star formation spread over large parts of their disks. In a detailed study of the two galaxies in our sample for which we have the best data, NGC 891 and NGC 4631, we show that the radio halos break off abruptly at galactocentric radii smaller than those of the underlying thin nonthermal radio disks. Our most important result is that the halo break-offs are spatially coincident with the radii where the star formation (SF) activity in the underlying disks drops sharply. The difference in radius of the emission distributions tracing on-going SF in the disks (IRAS 50 $\mu$m, H alpha, or high-frequency radio emission), vs. that of the nonthermal radio continuum thin disks (tracing the distribution of cosmic-ray electrons) is a few kpc, typically. This difference in extent is caused by CR diffusion. We have measured the CR diffusion coefficients in the thin disks of both NGC 891 and NGC 4631. For radial diffusion of CR electrons within galactic disks the values are $D_r$ = 1.5 to 3.5 times 10<SUP>29</SUP> cm$^{-2}$ s$^{-1}$ (NGC 4631) and $D_r = 1.7 times 10<SUP>29</SUP> cm$^2$ s$^{-1}$ (NGC 891). For motions in z-direction in areas within the thin disks where no outflows occur we derive a firm upper limit of $D_z &lt;= 0.3 times 10<SUP>28</SUP> cm$^2$ s$^{-1}$ for NGC 891. The value for NGC 4631 is $D_z = 2.0 times 10<SUP>28</SUP> cm$^{-2}$ s$^{-1}$. The other three galaxies in our sample, NGC 3044, NGC 4666, and NGC 5775 show at the sensitivity of our data less extended, more filamentary radio halos. Isolated spurs or filaments of nonthermal radio continuum emission in their halos are traced only above the most actively star-forming regions in the disks. This and the results obtained for NGC 891 and NGC 4631 suggest a direct connection of the shapes and the radial extents of radio halos of normal galaxies with the level and the spatial distribution of the currently on-going SF in their disks. The existence of such filamentary structures implies that convection plays an important role already at low $z$-distances above the plane. Based on radio data we derive the disk-averaged mean energy input rates into the ISM due to supernova explosions per unit surface area in 10 galaxies, including the ones in our sample. We find a clear trend that the galaxies with the highest mean energy input rates in the underlying disks have the most extended radio halos. Below a certain threshold outflows are inhibited. Comparing ``normal'' spirals and starburst galaxies we estimate a lower and upper limit for the threshold of 0.1 and 500 times 10^{-3} ergs s$^{-1}$ cm$^{-2}$, respectively. The fact that we find radio halo emission only above the most luminous giant HII regions in late-type ``normal'' galaxies (of type Sbc or later) indicates that only these are capable of initiating disk-halo interactions by locally surpassing this threshold. The distribution of the local outflows determines the overall shapes of the CR thick disks or halos and leads to the observed filamentary structures.