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

¹ INAF – Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy e-mail: capetti@to.astro.it
² European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: gverdoes@eso.org
³ INAF – Istituto di Radioastronomia, via Gobetti 101, 40129 Bologna, Italy e-mail: chiab@ira.cnr.it

Received: 7 July 2004
Accepted: 5 April 2005

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 , and photon counting sets a limit to the Broad Line Region mass of . 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.