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Issue A&A
Volume 439, Number 3, September I 2005
Page(s) 935 - 946
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20041609



A&A 439, 935-946 (2005)
DOI: 10.1051/0004-6361:20041609

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

A. Capetti1, G. V. Kleijn2 and M. Chiaberge3

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 $10{-}10^3~M_{\odot}$, and photon counting sets a limit to the Broad Line Region mass of $M_{\rm BLR} < 10^{-2}~M_{\odot}$. 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.


Key words: galaxies: active -- galaxies: nucleus -- galaxies: jets

SIMBAD Objects



© ESO 2005


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