We derive the spectral energy distribution ( SED ) of the nucleus of the Seyfert galaxy ( catalog NGC~4565 ) .
Despite its classification as a Seyfert 2 , the nuclear source is substantially unabsorbed .
The absorption we find from Chandra data ( N _ { H } = 2.5 \times 10 ^ { 21 } cm ^ { -2 } ) 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 that of Seyfert galaxies and QSO , as it appears basically “ flat ” in the IR-optical region , with a small drop-off in the U-band .
The location of the object in diagnostic planes for low luminosity AGNs excludes a jet origin for the optical nucleus , and its extremely low Eddington ratio L _ { o } / L _ { Edd } indicates that the radiation we observe is most likely produced in a radiatively inefficient accretion flow ( RIAF ) .
This would make NGC 4565 the first AGN in which an ADAF-like process is identified in the optical .
We find that the relatively high [ OIII ] flux observed from the ground can not be all produced in the nucleus .
Therefore , an extended NLR must exist in this object .
This may be interpreted in the framework of two different scenarios : i ) the radiation from ADAFs is sufficient to give rise to high ionization emission-line regions through photoionization , or ii ) the nuclear source has recently “ turned-off ” , switching from a high-efficiency accretion regime to the present low-efficiency state .