Distance estimation of gamma-ray-emitting BL Lac objects from imaging observations

¹ Finnish Centre for Astronomy with ESO (FINCA), Quantum, Vesilinnantie 5, 20014 University of Turku, Finland
² Aalto University Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland
³ Department of Physics and Astronomy, University of Turku, Finland
⁴ Université Paris Cité, CNRS, CEA, Astroparticule et Cosmologie, 75013 Paris, France
⁵ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁶ Instituto de Astrofísica de Canarias (IAC), 38200 La Laguna, Santa Cruz de Tenerife, Spain
⁷ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁸ Nordic Optical Telescope, Apartado 474, E-38700 Santa Cruz de La Palma, Spain
⁹ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, Université Paris Cité, CNRS, 92190 Meudon, France
¹⁰ Department of Physics, Chemistry & Material Science, University of Namibia, Private Bag, 13301 Windhoek, Namibia
¹¹ Centre for Space Research, North-West University, Potchefstroom 2520, South Africa
¹² Oxford Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
¹³ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

^⋆ Corresponding author; kani@utu.fi

Received: 26 March 2024
Accepted: 22 August 2024

Abstract

Aims. The direct redshift determination of BL Lac objects is highly challenging as the emission in the optical and near-infrared bands is largely dominated by the non-thermal emission from the relativistic jet, which points very close to our line of sight. Therefore, the optical spectra of BL Lac objects often show no spectral lines from the host galaxy. In this work, we aim to overcome this difficulty by attempting to detect the host galaxy and derive redshift constraints based on assumptions on the galaxy magnitude (‘imaging redshifts’).

Methods. Imaging redshifts were derived by obtaining deep optical images under good seeing conditions, making it possible to detect the host galaxy as a weak extension of the point-like source. We then derived the imaging redshift by using the host galaxy as a standard candle, employing two different methods.

Results. We determine the imaging redshift for 9 out of 17 blazars that we observed as part of this programme. The redshift range of these targets is 0.28–0.60, and the two methods used to derive the redshift give very consistent results within the uncertainties. We also performed a detailed comparison of the imaging redshifts with those obtained using other methods, such as direct spectroscopic constraints or looking for groups of galaxies close to the blazar. We show that the constraints from the different methods are consistent and that combining the three constraints narrows down the redshift. For example, in the case of J2156.0+1818, which is the most distant source for which we detect the host galaxy, the redshift range is narrowed to 0.63 < z < 0.71. This makes the source interesting for future studies of extragalactic background light in the Cherenkov Telescope Array Observatory era.