Optical spectroscopy of BL Lacertae objects. Broad lines, companion galaxies and redshift lower limits

TL;DR

This study uses optical spectroscopy to determine redshifts, analyze emission lines, and explore the environments of BL Lac objects, revealing host galaxy properties, interactions, and establishing lower redshift limits for featureless spectra.

Contribution

It introduces a new technique to derive redshift lower limits for BL Lacs with featureless spectra, enhancing understanding of their distances and environments.

Findings

Host galaxies detected in seven BL Lacs.

Broad H_alpha luminosities similar to radio-loud quasars.

Two BL Lacs are physically associated with close companions.

Abstract

Aims: We present optical spectroscopy of a sample of BL Lac objects, to determine their redshift, to study their broad emission line properties and to characterize their close environment. Methods: Twelve objects were observed using the ESO 3.6m and the NOT 2.5m telescopes, obtaining spectra for the BL Lacs and for nearby sources. Results: For seven objects, nuclear emission lines and/or absorption lines from the host galaxy were detected. In all the four cases where absorption lines were revealed, the host galaxy has been resolved with HST or ground-based imaging. The broad H_alpha luminosities (or their upper limits) of the BL Lacs are similar to those of radio-loud quasars. For two BL Lacs, spectroscopy of close companions indicates that they are at the redshift of the BL Lacs, and therefore physically associated and likely interacting. Five BL Lacs have a featureless spectrum. In these cases, we apply a new technique to derive lower limits for their redshift. which are consistent with lower limits deduced from imaging.