| Abstract | A magnitude-limited sample of 1063 spiral galaxies brighter than mP = +14.5 was observed in the Arecibo 2380 MHz survey. VLA maps made at 4885 MHz confirm that only eight of these have radio-to-optical luminosity ratios more than an order of magnitude above the median value for spirals. Seven of these eight galaxies contain dominant central radio sources. They show no evidence for enhanced disk emission, as might be expected if relativistic electrons in the disk were supplied by the nucleus rather than locally. Three of these seven spirals have flat radio spectra. Variability data and very-long-baseline interferometry indicate that these are compact (≤ 1 pc) synchrotron self-absorbed radio sources. All of the steep-spectrum sources were resolved by the VLA, with typical sizes ̃1 kpc. They are apparently confined to the galactic disks, and some are similar in size and shape to the associated optical forbidden-line emission regions. The strong steep-spectrum sources in spiral galaxies may result from rapid bursts of star formation induced by tidal interactions or collisions with companion galaxies. |
