Optical spectroscopy of blazars for the Cherenkov Telescope Array Observatory – IV^★

¹ Departamento de Astronomiá, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
² Université Paris Cité, CNRS, CEA, Astroparticule et Cosmologie, 75013 Paris, France
³ Department of Physics, Chemistry & Material Science, University of Namibia, Private Bag 13301, Windhoek, Namibia
⁴ Santa Cruz Institute for Particle Physics and Department of Physics, University of California, Santa Cruz, CA 95064, USA
⁵ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, 92190 Meudon, France
⁶ Université Paris Cité, CNRS, Astroparticule et Cosmologie, 75013 Paris, France
⁷ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
⁸ Centro Brasileiro de Pesquisas Fisicas (CBPF), Rua Dr. Xavier Sigaud 150 – Urca, Rio de Janeiro 22290-180, Brazil
⁹ Universidad de La Laguna (ULL), Departamento de Astrofísica, E-38206 La Laguna, Tenerife, Spain
¹⁰ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Tenerife, Spain
¹¹ Oxford Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH, UK
¹² INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy
¹³ Finnish Centre for Astronomy with ESO, FINCA, University of Turku, Turku 20014, Finland
¹⁴ Astronomical Observatory of Taras Shevchenko National University of Kyiv, 3 Observatorna Street, Kyiv 04053, Ukraine
¹⁵ Instituto de Astrofísica, Facultad de Fisica, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Macul, Santiago, Chile
¹⁶ University of Bialystok, Faculty of Physics, ul. K. Ciolkowskiego 1L, 15-245 Bialystok, Poland
¹⁷ Department of Physics, University of the Free State, Bloemfontein 9300, South Africa

^★★ Corresponding author: bhoomikarjpt2@gmail.com

Received: 15 July 2025
Accepted: 29 September 2025

Abstract

Context. Blazars, comprising BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars, are the most luminous extragalactic sources that dominate the γ-ray sky. They account for approximately 56% of the sources listed in the recent Fermi-LAT (Large Area Telescope) catalogue (4FGL-DR4). The optical and UV spectra of BL Lacs are nearly featureless, making it difficult to precisely determine their redshifts. Consequently, nearly half of the γ-ray BL Lacs lack reliable redshift measurements. This poses a significant challenge, since redshift is crucial for studying the cosmic evolution of the blazar population and for understanding their intrinsic emission mechanisms. Additionally, it is vital for γ-ray propagation studies, such as indirect evidence of extragalactic background light (EBL), placing constraints on the intergalactic magnetic field (IGMF), and searches for Lorentz invariance violation (LIV) and axion-like particles (ALPs).

Aims. This paper is the fourth in a series dedicated to determining the redshift of a sample of blazars identified as key targets for future observations with the Cherenkov Telescope Array Observatory (CTAO). The precise determination of the redshifts of these objects plays a crucial role in planning future CTAO observations.

Methods. We carried out Monte Carlo simulations to identify potential γ-ray blazars with hard spectra detected by the Fermi-LAT telescope that currently lack redshift measurements. These simulations selected the blazars that are anticipated to be detectable by the CTAO within 30 hours or less of exposure assuming an average flux state. In this fourth paper, we report the results of detailed spectroscopic observations of 29 blazars using the ESO/VLT, Keck II, and SALT telescopes. Our analysis involved a thorough search for spectral lines in the spectra of each blazar, and when features of the host galaxy were identified, we modelled its properties. Moreover, we compared the magnitudes of the targets during the observations to their long-term light curves.

Results. In the sample studied, 9 of 29 sources were observed with a high signal-to-noise ratio (S/N > 100), while the remaining 20 were observed with a moderate or low S/N. We successfully determined firm redshifts for 12 blazars, ranging from 0.1636 to 1.1427, and identified two lower limit redshifts at z > 1.0196 and z > 1.4454. The remaining 15 BL Lac objects exhibited featureless spectra under the observed S/N.