Optical spectroscopy of BL Lac objects: TeV candidates

TL;DR

This study uses optical spectroscopy to classify and determine redshifts of gamma-ray detected BL Lac objects and BCU sources, identifying 25 redshifts and providing insights into their emission lines and intervening absorption features.

Contribution

It presents new spectroscopic classifications and redshift measurements for 55 gamma-ray sources, improving understanding of their nature and potential as TeV emitters.

Findings

Redshifts determined for 25 sources (0.05 < z < 0.91).

Detected emission lines mainly due to [O III] and [N II].

Intervening absorption lines set lower redshift limits for 5 sources.

Abstract

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.