Optical spectroscopy of blazars for the Cherenkov Telescope Array – III^⋆

¹ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
e-mail: dammando@ira.inaf.it
² Université Paris-Cité, CNRS, CEA, Astroparticule et Cosmologie, 75013 Paris, France
³ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
⁶ Department of Physics, Chemistry & Material Science, University of Namibia, Private Bag 13301, Windhoek, Namibia
⁷ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
⁸ IPARCOS and Department of EMFTEL, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
¹⁰ Centro Brasileiro de Pesquisas Fisicas (CBPF), Rua Dr. Xavier Sigaud 150 – Urca, Rio de Janeiro 22290-180, Brazil
¹¹ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris-Cité, CNRS, 92190 Meudon, France
¹² Oxford Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹³ Ruhr-Universität Bochum, Fakultät für Physik und Astronomie, Astronomisches Institut (AIRUB), 44801 Bochum, Germany
¹⁴ Finnish Centre for Astronomy with ESO, FINCA, University of Turku, Turku 20014, Finland
¹⁵ Instituto de Astrofísica, Facultad de Fisíca, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago, Chile
¹⁶ Université Paris-Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
¹⁷ Department of Physics, University of the Free State, Bloemfontein 9300, South Africa

Received: 6 November 2023
Accepted: 21 December 2023

Abstract

Context. Blazars, which include BL Lacs and flat-spectrum radio quasars, represent the brightest persistent extragalactic sources in the high-energy (HE; 10 MeV–100 GeV) and very-high-energy (VHE; E > 100 GeV) γ-ray sky. Due to their almost featureless optical/UV spectra, it is challenging to measure the redshifts of BL Lacs. As a result, about 50% of γ-ray BL Lacs lack a firm measurement of this property, which is fundamental for population studies, indirect estimates of the extragalactic background light, and fundamental physics probes (e.g., searches for Lorentz-invariance violation or axion-like particles).

Aims. This paper is the third in a series of papers aimed at determining the redshift of a sample of blazars selected as prime targets for future observations with the next generation, ground-based VHE γ-ray astronomy observatory, Cherenkov Telescope Array Observatory (CTAO). The accurate determination of the redshift of these objects is an important aid in source selection and planning of future CTAO observations.

Methods. Promising targets were selected following a sample selection obtained with Monte Carlo simulations of CTAO observations. The selected targets were expected to be detectable with CTAO in observations of 30 h or less. We performed deep spectroscopic observations of 41 of these blazars using the Keck II, Lick, SALT, GTC, and ESO/VLT telescopes. We carefully searched for spectral lines in the spectra and whenever features of the host galaxy were detected, we attempted to model the properties of the host galaxy. The magnitudes of the targets at the time of the observations were also compared to their long-term light curves.

Results. Spectra from 24 objects display spectral features or a high signal-to-noise ratio (S/N). From these, 12 spectroscopic redshifts were determined, ranging from 0.2223 to 0.7018. Furthermore, 1 tentative redshift (0.6622) and 2 redshift lower limits at z > 0.6185 and z > 0.6347 were obtained. The other 9 BL Lacs showed featureless spectra, despite the high S/N (≥100) observations. Our comparisons with long-term optical light curves tentatively suggest that redshift measurements are more straightforward during an optical low state of the active galactic nucleus. Overall, we have determined 37 redshifts and 6 spectroscopic lower limits as part of our programme thus far.