Southern African Large Telescope Spectroscopy of BL Lacs for the CTA project

TL;DR

This paper discusses a SALT observational program aimed at measuring redshifts of BL Lac objects, which are crucial for understanding their evolution and are challenging to determine due to their featureless spectra, in preparation for CTA detections.

Contribution

It presents new spectroscopic redshift measurements of BL Lac objects using SALT, addressing a key challenge for future CTA gamma-ray observations.

Findings

Redshift measurements for several BL Lac objects obtained.

Improved understanding of BL Lac population distribution.

Enhanced data for CTA source identification and analysis.

Abstract

In the last two decades, very-high-energy gamma-ray astronomy has reached maturity: over 200 sources have been detected, both Galactic and extragalactic, by ground-based experiments. At present, Active Galactic Nuclei (AGN) make up about 40% of the more than 200 sources detected at very high energies with ground-based telescopes, the majority of which are blazars, i.e. their jets are closely aligned with the line of sight to Earth and three quarters of which are classified as high-frequency peaked BL Lac objects. One challenge to studies of the cosmological evolution of BL Lacs is the difficulty of obtaining redshifts from their nearly featureless, continuum- dominated spectra. It is expected that a significant fraction of the AGN to be detected with the future Cherenkov Telescope Array (CTA) observatory will have no spectroscopic redshifts, compromising the reliability of BL Lac population studies, particularly of their cosmic evolution. We started an effort in 2019 to measure the redshifts of a large fraction of the AGN that are likely to be detected with CTA, using the Southern African Large Telescope (SALT). In this contribution, we present two results from an on-going SALT program focused on the determination of BL Lac object redshifts that will be relevant for the CTA observatory.