Galaxy group-associated distances to very high energy gamma-ray emitting BL Lacs KUV 00311−1938 and S2 0109+22

ABSTRACT Blazars constitute the most numerous source class in the known extragalactic population of very high energy (VHE) gamma-ray sources. However, determining their redshifts is often challenging due to weak or non-existent emission lines in their spectra. This study focuses on two BL Lacs, KUV 00311−1938 and S2 0109+22, where previous attempts at redshift determination have faced difficulties. By combining spectroscopic observations with photometric redshift estimates, we tentatively assign a redshift of z=0. 634 to KUV 00311−1938 and a likely redshift of z=0. 49 to S2 0109+22. Establishing redshift estimates for high-redshift blazars is crucial for understanding extragalactic VHE gamma-ray sources and their interactions with the surrounding Universe.