Flat radio-spectrum galaxies and BLLacs: Part I: core properties

Abstract

This paper concerns the relationship of BLLacs and flat spectrum weak emission-line galaxies. We compare the weak emission-line galaxies and the BLLacs in a sample of 57 flat spectrum objects (Marcha 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' BLLacs-due to their larger emission line strengths, and larger CaII spectral breaks-are simply starlight diluted BLLacs. 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 BLLacs shows that, on average, the former have steeper spectra between 8 and 43GHz, and are less polarized at 8.4GHz. This is consistent with many of the weak-lined objects being at larger angles to the line of sight than the BLLacs. In addition to this population, we indicate a number of the weak emission-line galaxies which may be `hidden BLLacs': relativistically boosted objects very close to the line of sight with an apparently weak AGN.