Spectroscopic study of low luminosity BL Lac type objects

¹ CAAUL, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
² ESO, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany

Abstract

We have examined the spectroscopic properties of a sample of 19 optically bright, low luminosity core-dominated radio sources; this sample was expected to contain a high fraction of objects observed at small angles to the line-of-sight (blazar-like objects). Our study focuses on the properties - stellar populations and optical emission-line spectra - of such nuclei. Stellar population synthesis shows that their nuclear populations are composed, in a general way, of old stars of solar metallicity or lower; the dust content is weak. After subtraction of the stellar contribution, we are left with a set of nuclear emission-line spectra; their analysis shows that most of the objects harbour a Low Ionisation Nuclear Emission Region (or LINER), whose contribution was highly diluted by the host galaxy starlight. Such a low ionisation spectrum is in agreement with the black hole mass values and sub-Eddington accretion rates published for some BL Lacs; also, the stellar populations derived in this study are in agreement with those usually found in LINERs. Our findings give strength to a unifying picture involving low luminosity blazar-like objects and FR I radio-galaxies (which usually display LINER-type spectra).