A survey of low-luminosity compact sources and its implication for the evolution of radio-loud active galactic nuclei - II. Optical analysis

Abstract

This is the second in a series of papers concerning a new sample of low-luminosity compact (LLC) objects. Here we discuss the optical properties of the sample based on images and spectra from the Sloan Digital Sky Survey (SDSS). We have generated different diagnostic diagrams and we have classified the sources as high and low excitation galaxies (HEGs and LEGs, respectively). We have studied the jet-host interactions, the relation between radio and optical line emission and the evolution of the radio source within a larger sample that also includes the published samples of compact steep spectrum (CSS) and GHz-peaked spectrum (GPS) sources and Fanaroff-Riley type 1 and type II (FR I and FR II, respectively) objects. The optical and radio properties of the LLC sample are, in general, consistent with brighter CSS and large-scale radio sources, although the LLC objects have lower values of [O iii] luminosity than the more powerful CSS sources (L1.4 GHz > 1025 W Hz-1). However, when LLC objects are added to the other samples, HEGs and LEGs seem to follow independent, parallel evolutionary tracks. Regarding ionization mechanisms, LLC and luminous CSS objects behave like FR II sources, while FR Is seem to belong to a different group of objects. Based on our results, we propose independent, parallel evolutionary tracks for HEGs and LEGs, evolving from GPS to CSS to FR. © 2010 The Authors. Journal compilation © 2010 RAS.