The Hubble Space Telescope view of liner nuclei:: Evidence for a dual population?

We study a complete, distance-limited sample of 25 LINERs, 21 of which have been imaged with the Hubble Space Telescope. In nine objects we detect an unresolved nucleus. To study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local active galactic nuclei (AGNs), namely, Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our results show that the LINER population is not homogeneous, as there are two subclasses: (1) the first class is similar to the LLRG class, as it extends the population of radio-loud nuclei to lower luminosities; (2) the second is similar to Seyfert galaxies and extends the properties of radio-quiet nuclei toward the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass versus nuclear radio loudness: all radio-loud LINERs have M-BH >= 10(8) M circle dot, while Seyfert galaxies and radio-quiet LINERs have M-BH <= 10(8) M circle dot. The different nature of the various classes of local AGNs are best understood when the fraction of the Eddington luminosity they irradiate, L-0/L-Edd, is plotted against the nuclear radio-loudness parameter: Seyfert galaxies are associated with relatively high radiative efficiencies L-0/L-Edd >= 10(-4) (and high accretion rates onto low-mass black holes); LLRGs are associated with low radiative efficiencies ( and low accretion rates onto high-mass black holes); all LINERs have low radiative efficiency (and accretion rates) and can be radio-loud or radio-quiet, depending on their black hole mass.