The HST view of the nuclear emission line region in low luminosity radio-galaxies

Abstract

We study the properties of the emission line regions in two samples of low luminosity radio-galaxies, while focusing on the Compact Emission Line Region (CELR) revealed to be a characteristic feature of these objects by HST narrow-band imaging. We find a strong correlation between line and optical continuum nuclear emission, which suggests that the optical cores (most likely of non-thermal origin) can be directly associated to the source of ionizing photons, i.e. that we are seeing a jet-ionized narrow line region. A photon budget argument indicates that the optical nuclear sources produce sufficient photon flux provided that the covering factor of the circum-nuclear gas is rather large, on average 0.3. Analysis of HST images and spectra suggests that the CELR may take the form of a pc-scale, high filling factor structure, possibly an optically thin torus. Estimates of the CELR mass lead to values as small as 10{-}10^3 M_⊙, and photon counting sets a limit to the Broad Line Region mass of M_BLR < 10^-2 M_⊙. When considered together with the low accretion rate and the tenuous torus structure, a general paucity of gas in the innermost regions of low luminosity radio-galaxies emerges as the main characterizing difference from more powerful Active Galactic Nuclei.