Radio Properties of Narrow-Line Seyfert 1 Galaxies
  The last decade has witnessed a steadily increasing number of observational
studies concerning the rare class of radio loud narrow-line Seyfert 1 galaxies,
of which several hundred are currently known. According to the current AGN
paradigm, the low black hole masses and high accretion rates of narrow-line
Seyfert 1 galaxies (NLSy1) should make them unlikely to launch jets, and indeed
the vast majority ($\sim 90\%$) are very radio weak. The remainder, however,
display a wide range of radio power, from $\simeq 10^{21}$- $10^{28}$ W/Hz. In
this review I discuss recent radio imaging surveys that suggest there are three
main classes of NLSy1, which cannot be easily distinguished by the standard
radio-loudness parameter alone: (i) radio-weak NLSy1s without jets, (ii) mildly
radio-loud NLSy1s that are a mixture of star-forming and jet-dominant AGN, and
(iii) very radio loud NLSy1s with extreme properties similar to powerful
jet-dominated blazars. I present updated kinematics information from the MOJAVE
survey on six of the latter sources (all detected in gamma-rays by Fermi),
indicating high bulk Lorentz factors and small viewing angles in three cases.
Studies with the JVLA have shown that the jets of radio loud NLSy1s are likely
lower- power versions of classical radio galaxies, with typical lengths of less
than 10 kpc, although two very radio-loud NLSy1s have de-projected sizes of
several hundred kpc. I discuss the challenges of reconciling the heterogeneous
radio properties of NLSy1s with their strict optical line criteria, and
near-term prospects for the discovery of larger numbers of radio-loud NLSy1s.
