Intermediate BL Lac objects |
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Authors: | M Bondi MJM Marchã D Dallacasa C Stanghellini |
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Institution: | 1Istituto di Radioastronomia, Via Gobetti 101, I-40129 Bologna, Italy;2CAAUL, Observatório Astrónomico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal;3Dipartimento di Astronomia, Universitàdi Bologna, Via Ranzani 1, I-40127 Bologna, Italy;4Istituto di Radioastronomia C. da Renna Bassa, C. P. 141, I-96017 Noto, Italy |
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Abstract: | The 200-mJy sample, defined by Marchã et al., contains about 60 nearby, northern, flat-spectrum radio sources. In particular, the sample has proved effective at finding nearby radio-selected BL Lac objects with radio luminosities comparable to those of X-ray-selected objects, and low-luminosity flat-spectrum weak emission-line radio galaxies (WLRGs). The 200-mJy sample contains 23 BL Lac objects (including 6 BL Lac candidates) and 19 WLRGs. We will refer to these subsamples as the 200-mJy BL Lac sample and the 200-mJy WLRG sample, respectively. We have started a systematic analysis of the morphological pc-scale properties of the 200-mJy radio sources using VLBI observations. This paper presents VLBI observations at 5 and 1.6 GHz of 14 BL Lac objects and WLRGs selected from the 200-mJy sample. The pc-scale morphology of these objects is briefly discussed. We derive the radio beaming parameters of the 200-mJy BL Lac objects and WLRGs and compare them with those of other BL Lac samples and with a sample of FR I radio galaxies. The overall broad-band radio, optical and X-ray properties of the 200-mJy BL Lac sample are discussed and compared with those of other BL Lac samples, radio- and X-ray-selected. We find that the 200-mJy BL Lac objects fill the gap between HBL and LBL objects in the colour–colour plot, and have intermediate α XOX as expected in the spectral energy distribution unification scenario. Finally, we briefly discuss the role of the WLRGs. |
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Keywords: | galaxies: active BL Lacertae objects: general radio continuum: galaxies |
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