Optical spectroscopy of blazars for the Cherenkov Telescope Array^⋆

¹ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75013 Paris, France
e-mail: goldoni@apc.univ-paris7.fr
² LUTH, Observatoire de Paris, PSL Research University, CNRS, Université Paris Diderot, Meudon, France
³ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴ Department of Physics, University of Namibia, Private Bag 13301, Windhoek, Namibia
⁵ Santa Cruz Institute for Particle Physics and Department of Physics, University of California Santa Cruz, Santa Cruz, CA, USA
⁶ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
⁷ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
⁸ Centro Brasileiro de Pesquisas Físicas (CBPF), Rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, Brazil
⁹ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
¹⁰ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹¹ University of Oxford, Oxford Astrophysics, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹² Sorbonne Université, Université Paris Diderot, Sorbonne Paris Cité, CNRS/IN2P3, Laboratoire de Physique Nucléaire et de Hautes Energies, LPNHE, 4 place Jussieu, 75252 Paris, France
¹³ Finnish Centre for Astronomy with ESO (FINCA), Quantum, University of Turku, Vesilinnantie 5, 20014 Turku, Finland
¹⁴ Universität Heidelberg, Landessternwarte Königstuhl 12, 69117 Heidelberg, Germany

Received: 8 December 2020
Accepted: 27 March 2021

Abstract

Context. Blazars are the most numerous class of high-energy (HE; E ∼ 50 MeV−100 GeV) and very high-energy (VHE; E ∼ 100 GeV−10 TeV) gamma-ray emitters. Currently, a measured spectroscopic redshift is available for only about 50% of gamma-ray BL Lacertae objects (BL Lacs), mainly due to the difficulty in measuring reliable redshifts from their nearly featureless continuum-dominated optical spectra. The knowledge of the redshift is fundamental for understanding the emission from blazars, for population studies and also for indirect studies of the extragalactic background light and searches for Lorentz invariance violation and axion-like particles using blazars.

Aims. This paper is the first in a series of papers that aim to measure the redshift of a sample of blazars likely to be detected with the upcoming Cherenkov Telescope Array (CTA), a ground-based gamma-ray observatory.

Methods. Monte Carlo simulations were performed to select those hard spectrum gamma-ray blazars detected with the Fermi-LAT telescope still lacking redshift measurements, but likely to be detected by CTA in 30 hours of observing time or less. Optical observing campaigns involving deep imaging and spectroscopic observations were organised to efficiently constrain their redshifts. We performed deep medium- to high-resolution spectroscopy of 19 blazar optical counterparts with the Keck II, SALT, and ESO NTT telescopes. We searched systematically for spectral features and, when possible, we estimated the contribution of the host galaxy to the total flux.

Results. We measured eleven firm spectroscopic redshifts with values ranging from 0.1116 to 0.482, one tentative redshift, three redshift lower limits including one at z ≥ 0.449 and another at z ≥ 0.868. Four BL Lacs show featureless spectra.