Multi-Wavelength Properties of Radio-loud Narrow-Line Seyfert 1 Galaxies

Radio-loud Narrow-line Seyfert 1 (NLS1) galaxies are unique probes of the formation of powerful radio jets at extreme (near-Eddington) accretion rates and low black hole masses, in a regime very different from classical blazars and, therefore, not probed before. Further, strong thermal and non-thermal components simultaneously present in their spectral energy distributions (SEDs), shed new light on the jet-disk connection at much shorter timescales, given their low SMBH masses. While NLS1 galaxies have been studied thoroughly at optical and X-ray energies for decades, the populations of radio-loud, and gamma-ray emitting NLS1 galaxies have only emerged more recently. This contribution provides a short review of the multi-wavelength properties of this intriguing class of Active Galactic Nuclei (AGN), including X-ray variability and emission mechanisms, gamma-ray discoveries, SEDs, star formation activity, host galaxy morphologies, different methods of black hole mass estimation, (unexpected) FeII properties, and high-velocity large-scale outflows possibly driven by the radio jet and with a surprisingly pronounced ionization stratification.