Optical spectroscopy of blazars for the Cherenkov Telescope Array – II

Active galactic nuclei (AGNs) make up about 35 per cent of the more than 250 sources detected in very-high-energy (VHE) gamma rays to date with Imaging Atmospheric Cherenkov Telescopes. Apart from four nearby radio galaxies and two AGNs unknown type, all known VHE are blazars. Knowledge cosmological redshift gamma-ray blazars is key enabling study their intrinsic emission properties, as interaction between extragalactic background light (EBL) results a spectral softening. Therefore, determination exercise crucial indirectly placing tight constraints on EBL density studying blazar population evolution across cosmic time. Due powerful relativistic jets blazars, most host galaxies' features outshined, dedicated high signal-to-noise spectroscopic observations required. Deep medium- high-resolution spectroscopy 33 optical counterparts was performed European Southern Observatory New Technology Telescope, Keck II telescope, Shane 3-meter telescope African Large Telescope. From sample, spectra 25 objects display or featureless have signal-to-noise. The other eight low quality spectra. We systematically searched for absorption estimated, when possible, fractional galaxy flux measured total flux. Our measurements yielded 14 firm redshifts, ranging 0.0838 0.8125, one tentative redshift, lower limits: at z > 0.382 0.629.