5 Summary and future prospects

The above results prove that, after exclusion of host galaxies of AGNs, their f₆₀/f₁₀₀ FIR colour (and thus mean dust temperature) is a very successful criterion to select candidate galaxies for searches of radio halos (gaseous halos in general). Sensitive radio continuum imagery is a powerful tool to detect the halo emission.

For the 11 suitable objects which are presented here, we have a detection rate of 55%. Thus, again a substantial fraction of FIR-warm objects, although fainter than those studied by us earlier (DLG95; D98), exhibit halo emission. In some of the remaining cases, especially the galaxies with high f₆₀/f₁₀₀ ratios but no detected radio halos, the angular resolution of our data might not be sufficient to separate thin disk and halo emission. All physically small galaxies in our sample, even some with $f_{60}/f_{100}\ < 0.4$, have radio halos. The measured z scale heights of the radio halos range from about 1.4 to 3.1 kpc.

Our results suggest that the galaxies with the highest energy input rates into their disk ISM are the ones that have the most prominent radio halos. However, there is no direct relationship between the halo scale heights and the level of energy input in the underlying disks, because the halo properties far away from the disk planes are dominated by the energy losses of the CR electrons.

To confirm the results obtained here and to improve the angular resolution of the datasets where it limits our investigation, we plan to extend our VLA observations at 1.43 GHz by adding longer baselines from the C array. This will enable us to study the dependence of the intrinsic radio halo properties on the local level of SF in the underlying disks, as already done for NGC891 and NGC4631 (DLG95), in a larger sample.

Acknowledgements

We thank both the ATNF and NRAO for the generous allocation of observing time and the referee, Dr. J. Condon, for fruitful discussions, which helped improve the paper considerably. We thank J. Rossa very much for communicating to us his results of H$\alpha$ observations prior to publication. M. D. thanks Drs. R. Sault, R. Wark and H. May at the ATNF for their software and computing support and Dr. F. Israel for his kind hospitality at Sterrewacht Leiden, where most of this work was done. Thanks are also due to the VLA analysts for their support during the observations and data reduction. Many thanks to Dr. M. Gotzens-Petr, J. Krist and K. Kraiberg-Knudsen for helping us with the production of the z profiles and to Dr. G. Meurer for the calculation of the distance values. We thank Dr. J. Irwin for making available to us the VLA C array observations of NGC1421 and Dr. J. Condon for the C array data of NGC3175. Part of J. L.'s work was performed as part of an ATNF Vacation Scholarship. This research has made use of the NASA Extragalactic Database (NED), whose contributions to this paper are gratefully acknowledged. The Digitized Sky Survey was produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The National Geographic Society - Palomar Observatory Sky Atlas (POSS-I) was made by the California Institute of Technology with grants from the National Geographic Society.