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Large scale non-thermal radio halo emission was first detected from the nearby spiral galaxy NGC 4631 in 1977. Since then, additional observations have revealed that NGC 4631's radio halo extends more than 7 kpc above the galactic plane. Recent radio surveys have determined that several other nearby spiral galaxies also have non-thermal radio halos. Of these galaxies, NGC 0891 is found to have emission that extends greater than 3 kpc above the plane, and emission from NGC 253, a nearby starburst galaxy, has been detected greater than 9 kpc above the plane. Why some spiral galaxies have extended radio emission and others do not is still unknown.
In order to study in a systematic way the differences in the size and morphology of extended radio emission from spiral galaxies we have selected a sample of eight nearby, edge-on spiral galaxies with known extended emission. Minor axis profiles were determined for each galaxy in our sample using published 20 cm data and additional 20 cm and 90 cm data obtained at the Very Large Array telescope. The profiles are compared to a two component model for the radio emission. This model is based on observations of the radio emission from our galaxy which indicate that this emission can be characterized as a combination of thin and thick disk components with characteristic exponential fall-offs. This comparison enables us to determine the exponential fall-off of the thick disk component and the relative power of each of the two components in the eight galaxies studied.
We present here the initial results of this analysis.
