Optical spectroscopy of BL Lac objects: TeV candidates
  We investigate the spectroscopic optical properties of gamma-ray sources
detected with high significance above 50 GeV in the Third Catalog of Hard
Fermi-LAT Sources (3FHL) and that are good candidates as TeV emitters. We focus
on the 91 sources that are labelled by the Fermi team as BL Lac objects (BLL)
or Blazar candidates of uncertain type (BCU) , that are in the Northern
hemisphere, and are with unknown or uncertain redshift. We report here on GTC
spectra (in the spectral range 4100 - 7750 AA) of 13 BCU and 42 BLL. We are
able to classify the observed targets as BLL and each source is briefly
discussed. The spectra allowed us to determine the redshift of 25 objects on
the basis of emission and/or absorption lines, finding 0.05 < z < 0.91. Most of
the emission lines detected are due to forbidden transition of [O III] and [N
II]. The observed line luminosity is found lower than that of QSOs at similar
continuum and could be reconciled with the line-continuum luminosity
relationship of QSOs if a significant beaming factor is assumed. Moreover for 5
sources we found intervening absorption lines that allows to set a
spectroscopic lower limit of the redshift. For the remaining 25 sources, for
which the spectra are lineless, a lower limit to z is given, assuming that the
host galaxies are giant ellipticals.
