2014A&A...570A.126L

We aim to determine the redshift (or stringent lower limits) of a number of bright BL Lacs objects. We secured optical and near-infrared medium-resolution spectra of four bright BL Lac objects of unknown redshift using the spectrograph X-Shooter at the ESO-VLT. In spite of the high quality of the spectra and the extended spectral range of the observations, we have not detected intrisic spectral features for these sources. However, we are able to provide strigent lower limits to their redshift. In particular, for the two TeV sources PG 1553+113 and H 1722+119, we infer z>0.30 and z>0.35 respectively. We also detect an intervening CaII absorption doublet in the spectrum of MH 2136-428 that is ascribed to the halo of a nearby giant elliptical galaxy at a projected distance of ∼100kpc. Under the hypothesis that all BL Lacs are hosted by a luminous bulge-dominated galaxies, the spectroscopic observations of bright BL Lacs presently indicate that these objects are likely sources with extremely beamed nuclear emission. We present simulations to show under which circumstances it is possible to probe this hypothesis from the detection of very weak absorptions using the next generation of extremely large optical telescopes.