Radio Properties of Narrow-Line Seyfert 1 Galaxies

TL;DR

This paper reviews the radio properties of narrow-line Seyfert 1 galaxies, revealing three main classes with diverse jet and radio emission characteristics, and discusses recent observational findings and their implications for AGN models.

Contribution

It categorizes NLSy1 galaxies based on radio properties and summarizes recent high-resolution imaging and kinematic studies, providing new insights into their jet structures and orientations.

Findings

Three main classes of NLSy1s identified based on radio properties.

Radio loud NLSy1s can have jets similar to blazars with high Lorentz factors.

Jets in radio loud NLSy1s are lower-power, smaller-scale versions of classical radio galaxies.

Abstract

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.