Evidence of powerful relativistic jets in narrow-line Seyfert 1 galaxies

In 2008, the Fermi Gamma-ray Space Telescope has revealed - for the first time - high-energy (E > 100 MeV) gamma rays from a few Narrow-Line Seyfert 1 Galaxies (NLS1s). Later, in 2009 and 2010, two multifrequency campaigns on one of these sources, PMN J0948+0022 (z=0.585), definitely confirmed the presence, in sources of this type, of a relativistic jet very similar and with comparable power to those in blazars. However, these sources are neither blazars nor radio galaxies, as proven by their optical spectrum and by their very compact radio morphology. Moreover, since NLS1s are generally hosted in spiral galaxies, this casts a significant doubt on the paradigm of the correlation between jets and elliptical host galaxies. These findings pose intriguing challenges to the current knowledge of jet systems and on how these structures are generated. The current status of the researches in this field is reviewed and ongoing work is outlined.