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Astron. Astrophys. 361, 480-490 (2000)

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Flat radio-spectrum galaxies and BL Lacs

I. Core properties

J. Dennett-Thorpe 1,2 and M.J. Marchã 2

1 Kapteyn Institute, Postbus 800, 9700 AV Groningen, The Netherlands
2 Observatório Astronómico de Lisboa, Tapada da Ajuda, 1300 Lisbon, Portugal

Received 14 March 2000 / Accepted 14 July 2000


This paper concerns the relationship of BL Lacs and flat-spectrum weak emission-line galaxies. We compare the weak emission-line galaxies and the BL Lacs in a sample of 57 flat-spectrum objects (Marchã et al. 1996), using high-frequency radio and non-thermal optical flux densities, spectral indices and polarization properties. We consider whether objects which are not `traditional' BL Lacs - due to their larger emission line strengths, and larger Ca II spectral breaks - are simply starlight diluted BL Lacs. Their broad-band spectral properties are consistent with this interpretation, but their radio polarization may indicate more subtle effects. Comparison of the weak emission-line galaxies and the BL Lacs shows that, on average, the former have steeper spectra between 8 and 43 GHz, and are less polarized at 8.4 GHz. This is consistent with many of the weak-lined objects being at larger angles to the line of sight than the BL Lacs. In addition to this population, we indicate a number of the weak emission-line galaxies which may be `hidden BL Lacs': relativistically boosted objects very close to the line of sight with an apparently weak AGN.

Key words: galaxies: active – galaxies: BL Lacertae objects: general – galaxies: jets – galaxies: nuclei – radio continuum: galaxies

Send offprint requests to: J. Dennett-Thorpe (jdt@astro.rug.nl)

© European Southern Observatory (ESO) 2000

Online publication: October 2, 2000