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.
