The ultraviolet ( UV ) spectra of 16 blazars ( < z > \simeq 1 ) from the archives of the Hubble Space Telescope Faint Object Spectrograph have been analyzed in order to study in a systematic way the properties of their broad UV emission lines .
We find that the luminosities of the most prominent and intense lines , Ly \alpha and C IV \lambda 1549 , are similar to those of normal radio-loud quasars at comparable redshifts .
However , the equivalent widths of blazar lines are significantly smaller than those of radio-loud quasars .
Therefore , while the intrinsic broad line region luminosity of blazars appears to be indistinguishable from that of radio-loud quasars , their continuum must be comparatively higher , most probably due to relativistic beaming .
We have combined the UV luminosities of the de-beamed continuum with the emitting gas velocity to derive estimates of the masses of the central supermassive black holes .
The size of the broad line region was computed in two ways : 1 ) via an empirical relationship between UV continuum luminosity and broad line region size , and 2 ) through the external photon density required by blazar models to reproduce the inverse Compton components observed at gamma-rays .
The second method yields significantly different results from the first method , suggesting that it provides only a very rough estimate or a lower limit on the size of the broad line region .
We find that the average mass of the central black holes in blazars is \sim 2.8 \times 10 ^ { 8 } M _ { \odot } , with a large dispersion , comparable to those computed for other radio-loud active galactic nuclei .