On the emission lines in active galactic nuclei with relativistic jets
  The effect of the observed continuum emitted from a relativistic jet on the
measurement of the full width half maximum (FWHM) of an emission line is
analyzed. If the jet contribution is not properly subtracted, the FWHM of the
line could seem narrower than what it should be. The cases of emission line
detected in BL Lac objects and gamma-ray Narrow-Line Seyfert 1 galaxies
(gamma-NLS1s) are addressed. It is shown that the smallness of the observed
FWHM of the Ly alpha lines detected in three well-known BL Lacs, is an effect
due to the combined action of both the relativistic jet and a weak accretion
disc. Once removed the Doppler boosting of the jet continuum, the intrinsic
FWHM of the lines are found to be in the usual range. Instead, the narrow
permitted lines in gamma-NLS1s are really narrow, since the disc and the lines
are much more powerful. This also confirms that gamma-NLS1 is really a new
class of gamma-ray emitting AGN, different from blazars and radio galaxies.
