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Astron. Astrophys. 325, 911-914 (1997)

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1. Introduction

In recent years a great effort has been spent to explain the observed properties of the various classes of radio-loud Active Galactic Nuclei (AGN) (e.g. radio galaxies, quasars and BL Lac objects) in the framework of Unified Schemes (see Urry & Padovani 1995 for a recent review).

In this context, BL Lac objects play a key role because of the evidence that extreme relativistic effects take place in them. These objects are characterized by a flat radio spectrum which steepens at higher frequencies, relatively high optical and radio polarization, rapid variability and an optical continuum with weak or absent emission lines (Urry & Padovani 1995; Kollgaard 1994).

These properties have been interpreted in terms of a relativistic jet closely aligned to the line of sight (hereafter the "beaming model"; see Blandford & Rees 1978; Ghisellini et al. 1993). This model implies the existence of a class of radio sources (hereafter the "misaligned population") intrinsically identical to BL Lac objects, but with the jets oriented at large angles to the line of sight.

Browne (1983) and Wardle et al. (1984) were the first to propose the low luminosity edge darkened FR I radio galaxies as the most likely candidate for the "misaligned population" of the core dominated BL Lac objects. A straightforward outcome of the beaming model is that all the properties not depending on orientation should be shared by the BL Lac objects and the misaligned population. Recently while some authors confirm that the majority of the BL Lac objects are projected FR I radio galaxies (Urry & Padovani 1995, and references therein), others suggest that the diffuse radio emission detected around high redshift BL Lac posseses overall properties more consistent with FR II rather than FR I radio galaxies (see e.g. Kollgaard et al. 1992; Murphy et al. 1993).

In the radio domain the luminosity of the extended radio emission and the polarization properties need to be investigated. In fact, while the morphology is distorted by projection effects and a morphological classification could be uncertain, the luminosity of the unbeamed emission is suitable for a direct comparison with the extended emission found in the candidate misaligned population.

Also the investigation of the polarization properties via deep radio observations are important to verify the FRI-BL Lac unification because the observed magnetic field orientation in the kpc-scale jets is not significantly affected by projection and relativistic effects.

To address the problem of the misaligned population of BL Lac objects we observed 28 (see Bondi et al. 1996 for a summary of the observations) out of the 34 BL Lac objects belonging to the 1 Jy sample of Stickel at al. (1991). The observations were carried out with the Very Large Array (VLA) and the Westerbork Synthesis Radio Telescope (WSRT) in order to search for extended radio emission and to obtain kpc-scale polarization images.

These observations were intended to complete the radio information on the 1 Jy RBL sample, already partially investigated by various groups at several wavelengths. Fourteen objects were observed with the VLA in A and B configuration and were chosen among those without good quality, high dynamic range, radio images in literature. All these 14 together with additional 14 objects were observed with the VLA in the D configuration and/or with the WSRT to look for very extended radio halos.

In this paper we present first promising results from the VLA data in A and B configuration.

A detailed analysis of the whole dataset and a specific discussion of the importance of these new observations in the context of the Unification Schemes for BL Lac objects will be presented in forthcoming papers.

Throughout this paper we use [FORMULA] km s-1 Mpc-1 and q [FORMULA] 0.5.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998

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