Flat radio-spectrum galaxies and BL Lacs I. Core properties

This paper concerns the relationship of BL Lacs and flat-spectrum weak emis sion-line galaxies. We compare the weak emission-line galaxies and the BL L acs in a sample of 57 flat-spectrum objects (Marcha et al. 1996), using hig h-frequency radio and non-thermal optical flux densities, spectral indices and polarization properties. We consider whether objects which are not 'tra ditional' 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 we ak emission-line galaxies and the BL Lacs shows that, on average, the forme r 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 populati on, we indicate a number of the weak emission-line galaxies which may be 'h idden BL Lacs': relativistically boosted objects very close to the line of sight with an apparently weak AGN.