Host galaxy groups as redshift tracers for lineless BL Lac objects

ANDRUCHOW, I.; PORTILLA, J. G.; CELLONE, S. A.; BUZZONI, A.; TORRES ZAFRA, J.

Torino

Conferencia; Half a Century of Blazars and Beyond; 2018

Universita di Torino

absorption lines in their optical spectra has been a long-standingproblem. Although indirect methods (e.g.: by fitting the extragalacticbackground light absorption on the TeV gamma spectrum) may provide estimates or bounds to BL Lac's redshifts, none of these methods is completely model-independent.Using imaging and spectroscopy obtained with GMOS multi-object spectrographs at both Gemini 8-m telescopes, we have successfuly tested a method that can provide a precise redshift value -or, at least, an independent lower limit- for a lineles BL Lac. The method relays on the detection and characterization of the cluster of galaxies to which the BL Lac's host galaxy belongs.We show results for 3C 66A, PG 1553+113, and PKS 0332-403, for which we obtain z=0.340, z=0.43, and z> 0.34, respectively.