High redshift BL Lac objects: spectroscopy of candidates

TL;DR

This study uses spectroscopic observations to accurately determine the redshifts of 16 candidate high-redshift BL Lac objects, revealing that only half are genuine BL Lacs and correcting previous redshift estimates.

Contribution

The paper provides new spectroscopic data that refutes previous high-redshift claims and refines redshift estimates for BL Lac candidates, improving classification accuracy.

Findings

Five objects are at z < 1, disproving previous high-redshift claims.

Two objects show broad emission lines atypical for BL Lacs.

Three objects exhibit Ly-α forest onset at z > 2.50.

Abstract

We report on 16 BL Lacertae objects that were proposed to be at z > 1. We present spectroscopic observations secured at the 10.4m GTC that allowed us to assess the redshift of these sources. In particular, for five objects we disprove the previous value of the redshift reported in literature and found that they lie at z < 1. Moreover, two of them exhibit broad emission lines that are not characteristic of BL Lacertae object. On the other hand, for eight targets we improve the tentative value of z, previously based on only one feature, by detecting a number emission lines. Finally, in three cases we detect onset of Ly-{\alpha} forest at z > 2.50. Based on the new high quality spectra we found that only half of the observed objects can be classified as bona-fine BL Lacs.