1 Introduction

Despite accumulating evidence for the existence of gaseous halos around a number of nearby spiral galaxies (see, e.g., Dettmar 1992; Dahlem 1997 for overviews), it is currently a matter of controversy how many and exactly which galaxies have such halos. While the existence of gaseous halos (including radio halos, i.e. cosmic ray electrons) seems to be in general a rather rare phenomenon (Hummel et al. 1991a found that only about 5.5% of the 181 galaxies in their initial optically selected sample show signs of extraplanar radio emission), their existence appears to be quite common among nearby galaxies with high star formation rates (SFRs; e.g. Dahlem 1997). We find that this apparent discrepancy is caused by two factors.

First, different selection criteria were used to define the sample of candidate galaxies. Hummel et al.'s objects were selected by their optical (blue) magnitude. However, especially when studying edge-on systems (in searches for halos), the blue magnitude is not always a good indicator of the level of star formation (SF) because of dust extinction in the disks. The fraction of galaxies considered by Hummel et al. (1991a) as good candidates for follow-up searches of radio halos, based on extended emission found in radio synthesis images, is 10 out of 18, i.e. 56%.

In previous studies (Dahlem et al. 1995, 1998; hereafter DLG95 and D98, respectively) we have started using a far-infrared criterion, originally defined by Heckman et al. (1990) to characterise starburst galaxies, for selecting galaxies with warm dust as targets for searches of gaseous halos: their 60 $\mu$m to 100 $\mu$m far-infrared (FIR) IRAS flux ratio. Objects with have warm dust, heated by massive stars, with average dust temperatures, K. It turns out that all edge-on galaxies selected by us in this way (with 60 $\mu$m far-infrared [FIR] fluxes of $f_{60} \geq 30$ Jy) show clear signs of halo emission. Emission is detected not only in the radio, but also in other wavebands (D98). Galaxies with known luminous AGNs (that might contribute significantly to the heating of the dust) were excluded from our sample.

Second, contrary to searches of halo gas in very large samples, such as conducted by Hummel et al. (1991a), our present approach of carefully selecting target galaxies is combined with the employment of high-sensitivity observations to detect low surface brightness emission.

Irwin et al. (1999) conducted a search for radio halos in a quite heterogeneous sample of target galaxies. Despite this heterogeneity, these authors achieved a high detection rate of halo emission of >$90\%$. A discussion of the interpretation of their data in the context of the new results presented here will be provided in Sect. 4.4.

Here we present the results from our radio observations of a number of FIR-warm edge-on galaxies with 3 Jy < f₆₀ < 30 Jy FIR flux densities conducted with the Australia Telescope Compact Array (ATCA) and the Very Large Array (VLA). These galaxies are fainter than the galaxies studied by us previously, because there are too few nearby systems (with $f_{60} \geq 30$ Jy) to derive general properties of their halos. Most objects were selected to be FIR-warm, with , except NGC1055, NGC2820 and NGC7090, which have slightly lower FIR flux ratios. The primary goal is to investigate whether indeed all FIR-warm galaxies (without AGNs) do have gaseous halos and an attempt at detecting these at distances beyond that of the Virgo cluster.