A SENSITIVE RADIO-CONTINUUM SURVEY OF LOW SURFACE BRIGHTNESS DWARF GALAXIES

A sample of 128 low-surface brightness dwarf galaxies (LSBDGs) mainly from the UGC catalogue has been observed at 4.75 GHz with the Effelsberg 100-m telescope. Only approximately 16% of the galaxies have been detected, the 2-sigma detection limit being between 2 and 3 mJy/beam area. This detection rate is much lower than that of "active" dwarf galaxies such as blue compact dwarf galaxies (BCDGs). The radio luminosity of LSBDGs appears to be correlated with, and roughly proportional to their blue luminosity; such a correlation is indistinguishable from the one observed in giant spirals. This confirms the suggestion made in previous papers that, as far as the general radio continuum properties are concerned, LSBDGs behave as scaled-down versions of spirals. Spectral indices can be derived or estimated for 11 LSDGs on the basis of the present and previous radio continuum surveys. Although the scatter is large, these spectral indices are suggestive of a nonthermal origin of the radio continuum emission. They also fall in the range of values typical for normal spirals. We have used the measured flux densities to roughly estimate total magnetic field strengths in LSBDGs, assuming energy equipartition between magnetic fields and cosmic rays. The resulting field strengths are in the range of 2-4-mu-G, substantially lower than what is derived for massive spiral galaxies. The ratios of radio-to-far-infrared flux of LSBDGs appear to be high as compared to normal spiral galaxies. These two facts together are suggestive of the possibility that the injection of relativistic particles occurred, or promptly followed, an active phase with a star-formation rate exceeding the one presently observable.