COLA. II. Radio and Spectroscopic Diagnostics of Nuclear Activity in Galaxies

Abstract

We present optical spectroscopic observations of 93 galaxies taken from the infrared-selected COLA (compact objects in low-power AGNs) sample. These are all galaxies for which we have previously obtained low-resolution radio observations and high-resolution (<0″.05) Australian Long Baseline Array snapshots. The sample spans the range of far-IR luminosities from normal galaxies to luminous infrared galaxies and contains a significant number of galaxies involved in galaxy-galaxy interactions. Of the galaxies observed, 78 (84%) exhibit emission lines indicating that they are either AGNs or actively forming stars (starburst galaxies). Using a newly developed, theoretically based, optical emission line scheme to classify the spectra, we find that 15% of the emission-line galaxies are Seyfert galaxies, 77% are starbursts, and the rest are either borderline AGN/starburst or show ambiguous characteristics. We find little evidence for an increase in the fraction of AGNs in the sample as a function of far-IR (FIR) luminosity, in contrast to previous studies, but our sample covers only a small range in infrared luminosity (1010.5 L⊙ ≤ LFIR ≤ 1011.7 L⊙), and thus a weak trend may be masked. Instead, as the infrared luminosity increases, so does the fraction of metal-rich starbursts, objects that on more traditional diagnostic diagrams would have been classified as weak, low-ionization, narrow emission line regions. As a whole the Seyfert galaxies exhibit a small, but statistically significant, radio excess on the radio-FIR correlation compared to the galaxies classified as starbursts. Compact (<0″.05) radio cores are detected in 55% of the Seyfert galaxies, and these galaxies exhibit a significantly larger radio excess than the Seyfert galaxies in which compact cores were not detected. Our results indicate that there may be two distinct populations of Seyfert galaxies, " radio-excess " Seyfert galaxies, which exhibit extended radio structures and compact radio cores, and " radio-quiet " Seyfert galaxies, in which the majority of the radio emission can be attributed to star formation in the host galaxy. No significant difference is seen between the IR and optical spectroscopic properties of Seyfert galaxies with and without radio cores.