The Ionizing Radiation Field of NGC 4388 and Its Relation to the Extranuclear Emission-Line Regions

Abstract

New long-slit spectroscopic observations of NGC 4388 and the extranuclear emission line regions around it are used to investigate the characteristics of the ionizing source and the structure of the extended emission-line regions. The measured luminosity of the [Fe VII) λ6087 emission line detected in the nucleus of NGC 4388 is interpreted as evidence for the existence of a powerful ionizing source with N_ph_(>= 100 eV) >~ 2 x 10^50^ photons s^-1^ and for the presence of clouds of gas with electron densities N_e_~10^7^ cm^-3^. A comparison between the far-infrared luminosity and the UV- extrapolated ionizing luminosity as measured along our line of sight suggests that NGC 4388 has a hidden UV luminous source in its nucleus which is around 6 times brighter than observed; it should therefore be obscured by A_v_ = 0.31 mag. This places NGC 4388 among the intermediate luminosity Seyfert 1 galaxies with log L_UV_ 43.23 ergs s^-1^ and confirm the suggestion by Shields and Filippenko that NGC 4388 has a hidden luminous Seyfert 1 nucleus. The excitation conditions in the extranuclear emission-line regions are best accounted for by the ionization from a nonthermal central source in the nucleus of NGC 4388. Other alternative ionization sources, like star clusters or relativistic particles and shocks associated with the radio emission, are seen to play a minor role, if any.