The complex structure of low-luminosity active galactic nuclei: NGC 4579

Abstract

We have modelled the low-luminosity active galactic nucleus (AGN) NGC 4579 by explaining both the continuum and the line spectra observed with different apertures. It was found that the nuclear emission is dominated by an AGN such that the flux from the active centre (AC) is relatively low compared with that of the narrow emission-line region (NLR) of Seyfert galaxies. However, the contribution of a young starburst cannot be neglected, as well as that of shock-dominated clouds with velocities of 100, 300 and 500kms−1. A small contribution from an older starburst with an age of 4.5 Myr, probably located in the external nuclear region, is also found. Hii regions appear in the extended regions (∼1 kpc), where radiation and shock-dominated clouds with Vs= 100kms−1 prevail. The continuum SED of NGC 4579 is characterized by the strong flux from an old stellar population. Emissions in the radio range show synchrotron radiation from the base of the jet outflowing from the accretion disc within 0.1 pc from the active centre. Radio emission within intermediate distances (10–20 pc) is explained by the bremsstrahlung from gas downstream of low-velocity shocks (Vs= 100kms−1) reached by a relatively low radiation flux from the AC. In extended regions (>100 pc) the radio emission is synchrotron radiation created by the Fermi mechanism at the shock front. The shocks are created by collision of clouds with the jet. All types of emissions observed at different radius from the centre can be reconciled with the presence of the jet.