Optical spectroscopy of blazars for the Cherenkov Telescope Array Observatory- IV

TL;DR

This study reports spectroscopic observations of 29 blazars to determine their redshifts, crucial for gamma-ray studies, successfully measuring redshifts for 12 and setting limits for others, aiding future CTA observations.

Contribution

It provides the first detailed spectroscopic redshift measurements for a sample of blazars targeted for CTA, improving redshift knowledge for gamma-ray source studies.

Findings

Redshifts determined for 12 blazars (0.1636 to 1.1427)

Two blazars with lower limit redshifts >1.0196 and >1.4454

15 blazars with featureless spectra, no redshift

Abstract

Blazars, including BL Lacs and FSRQs, are the most luminous extragalactic {\gamma}-ray sources. They account for about 56% of the sources listed in the recent Fermi-LAT catalog (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 {\gamma}-ray BL Lacs lack reliable redshift measurements. This poses a major challenge, since redshift is crucial for studying the cosmic evolution of the blazar population and {\gamma}-ray propagation studies such as indirect evidence of EBL, placing constraints on IGMF and searches for LIV and ALPs. 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). We performed Monte Carlo simulations to select {\gamma}-ray blazars detected by Fermi-LAT with hard spectra, that lack redshift measurements. These blazars are expected to be detectable by 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 modeled its properties. We also compared the magnitudes of the targets during the observations to their long-term light curves. 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.