The HST view of LINERs nuclei: evidence for a dual population?

We study a complete and 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. In order to study their physical properties, we compare the radio and optical properties of the nuclei of LINERs with those of other samples of local AGNs, namely Seyfert galaxies and low-luminosity radio galaxies (LLRG). Our results show that the LINERs population is not homogeneous, as there are two subclasses: i) the first class is similar to LLRG, as it extends the population of radioloud nuclei to lower luminosities; ii) the second is similar to Seyferts, and extends the properties of radio-quiet nuclei towards the lowest luminosities. The objects are optimally discriminated in the plane formed by the black hole mass vs. nuclear radio-loudness: all radio-loud LINERs have MBH/M ⊙ > ∼ 10 , while Seyferts and radio-quiet LINERs have MBH/M ⊙ < ∼ 10 . The different nature of the various classes of local AGN are best understood when the fraction of the Eddington luminosity they irradiate Lo/LEdd is plotted against the nuclear radio-loudness parameter: Seyferts are associated with relatively high radiative efficiencies Lo/LEdd > ∼ 10 −4 (and high accretion rates onto low mass black holes); LLRG are associated with low radiative efficiencies (and low accretion rates onto high black hole masses); all LINERs have low radiative efficiency (and accretion rates), and can be radio-loud or radio quiet depending on their black hole mass. Subject headings: galaxies: active – galaxies: nuclei – galaxies: Seyfert