Optical spectroscopy of BL Lacertae objects

Broad lines, companion galaxies, and redshift lower limits

¹ Università dell'Insubria, via Valleggio 11, 22100 Como, Italy e-mail: boris.sbarufatti@uninsubria.it
² INAF, Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy
³ Tuorla Observatory, University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland

Received: 19 April 2006
Accepted: 11 May 2006

Abstract

Aims.We present optical spectroscopy for 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 3.6 m and NOT 2.5 m telescopes, obtaining spectra for the BL Lacs and for nearby sources.

Results.For seven of these 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_α 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 probably 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 the lower limits deduced from imaging.