Low-luminosity AGNs (LLAGNs) are a very important class of sources since they occupy a significant fraction of local galaxies. Their spectra differ significantly from the canonical luminous AGNs, most notably by the absence of the ``big blue bump''. In the present paper, taking a typical LLAGN - NGC 4258 - as an example, we investigate the origin of their spectral emission. The observational data of NGC 4258 is extremely abundant, including water maser emission, putting very strict constraints to its theoretical models. The infrared (IR) spectrum is well described by a steep power-law form fν∝ν–1.4, and may extend to the optical/UV band. Up until now there is no model which can explain such a steep spectrum, and we here propose a coupled jet plus accretion disk model for NGC 4258. The accretion disk is composed of an inner ADAF (or radiatively inefficient accretion flow) and an outer standard thin disk. A shock occurs when the accretion flow is ejected out of the ADAF to form the jet near the black hole, accelerating the electrons into a power-law energy distribution. The synchrotron and self-Comptonized emission from these electrons greatly dominates over the underlying accretion disk and can well explain the spectrum ranging from IR to X-ray bands. The further propagation of the shocked gas in the jet can explain the flat radio spectrum of NGC 4258. Several predictions of our model are presented for testing against future observations, and we briefly discuss the application of the model to other LLAGNs.
accretion, accretion disks - black hole physics - galaxies: active - galaxies: nuclei - hydrodynamics