Radio images of 3C 58: Expansion and motion of its wisp

New 1.4 GHz VLA observations of the pulsar-powered supernova remnant 3C 58 have resulted in the highest quality radio images of this object to date. The images show filamentary structure over the body of the nebula. The present observations were combined with earlier ones from 1984 and 1991 to investigate the variability of the radio emission on a variety of timescales. No significant changes are seen over a 110 day interval. In particular, the upper limit on the apparent projected velocity of the wisp is 0.05c. The expansion rate of the radio nebula was determined between 1984 and 2004 and is 0.014% +/- 0.003% yr(-1), corresponding to a velocity of 630 +/- 70 km s(-1) along the major axis. If 3C 58 is the remnant of SN 1181, it must have been strongly decelerated, which is unlikely given the absence of emission from the supernova shell. Alternatively, the low expansion speed and a number of other arguments suggest that 3C 58 may be several thousand years old and not the remnant of SN 1181.