On the relationship between BL Lacertae objects and radio galaxies

TL;DR

This study uses deep radio imaging to explore the connection between BL Lac objects and radio galaxies, revealing that many BL Lacs are linked to FR II radio galaxies and challenging existing unified models.

Contribution

It provides new radio observations of BL Lacs from the DXRBS, identifying a significant fraction as beamed FR II radio galaxies and questioning the blazar sequence predictions.

Findings

About one-third of BL Lacs are associated with FR II radio galaxies.

Similar average extended radio power for LBL and HBL BL Lacs.

Challenges current unified schemes and the blazar sequence predictions.

Abstract

We present deep radio images at 1.4 GHz of a large and complete sample of BL Lacertae objects (BL Lacs) selected from the Deep X-ray Radio Blazar Survey (DXRBS). We have observed 24 northern sources with the Very Large Array (VLA) in both its A and C configurations and 15 southern sources with the Australia Telescope Compact Array (ATCA) in its largest configuration. We find that in the DXRBS, as in the 1-Jy survey, which has a radio flux limit roughly ten times higher than the DXRBS, a considerable number (about a third) of BL Lacs can be identified with the relativistically beamed counterparts of Fanaroff-Riley type II (FR II) radio galaxies. We attribute the existence of FR II-BL Lacs, which is not accounted for by current unified schemes, to an inconsistency in our classification scheme for radio-loud active galactic nuclei (AGN). Taking the extended radio power as a suitable measure of intrinsic jet power, we find similar average values for low- (LBL) and high-energy peaked BL Lacs (HBL), contrary to the predictions of the blazar sequence.