Low Radiative Efficiency Accretion at Work in Active Galactic Nuclei: the Nuclear Spectral Energy Distribution of Ngc 4565

We derive the spectral energy distribution (SED) of the nucleus of the Seyfert galaxy NGC 4565. The nuclear source is substantially unabsorbed. The absorption we find from Chandra data is NH = 2.5× 10 cm is consistent with that produced by material in the galactic disk of the host galaxy. HST images show a nuclear unresolved source in all of the available observations, from the near-IR H band to the optical U band. The SED is completely different from all other Seyfert galaxies, as it appears basically “flat” in the IR-optical region, with a small drop-off in the U-band. The extremely low Eddington ratio Lo/LEdd and the location of the object in diagnostic planes for low luminosity AGNs indicate that the radiation we observe is most likely produced in a radiative inefficient accretion disk. This would make NGC 4565 the first AGN in which an ADAF-like process is directly observed in the optical. We find that the relatively high [OIII] flux observed from the ground cannot be all produced in the nucleus. Therefore, an extended NLR must exist in this object. This may imply that the nuclear source has recently “turned-off”, switching from a high-efficiency accretion regime to the present low-efficiency state. Subject headings: galaxies: active — galaxies: nuclei — accretion, accretion disks — galaxies: individual (NGC 4565)